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Abstract

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Reported Advantages of PECS
  5. III. The PECS Process
  6. IV. PECS Parameters and their Effect on Processing
  7. V. Simulation Studies on the PECS Process
  8. VI. Consolidation of Functional Materials
  9. References

The phenomenal increase during the past decade in research utilizing pulsed electric current to activate sintering is attributed generally to the intrinsic advantages of the method relative to conventional sintering methods and to the observations of the enhanced properties of materials consolidated by this method. This review focuses on the fundamental aspects of the process, discussing the reported observations and simulation studies in terms of the basic aspects of the process and identifying the intrinsic benefits of the use of the parameters of current (and pulsing), pressure, and heating rate.


I. Introduction

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Reported Advantages of PECS
  5. III. The PECS Process
  6. IV. PECS Parameters and their Effect on Processing
  7. V. Simulation Studies on the PECS Process
  8. VI. Consolidation of Functional Materials
  9. References

Sintering as a process to consolidate powders is an ancient art that has been practiced for more than 6000 years in the making of bricks and pottery1 and in the consolidation of precious metals in pre-Columbian South America.2 The ability to achieve consolidation without melting is made possible by the thermal activation of mass transport processes driven by reduction of surface and grain boundary energies. To optimize thermal activation and attain high density with concomitant strength, sintering is carried out at high temperatures, relative to the melting point of the material. For practical as well as economic reasons, significant efforts have been, and continue to be directed, toward other means of activation to achieve high density at lower temperatures or in shorter times. Among these is the use of an electric current to activate sintering. The recent widespread use of this form of activation has been referred to variously as spark plasma sintering (SPS), pulsed electric current sintering (PECS), field-activated sintering technique, and current-activated pressure-assisted densification. Research using field activation in sintering has increased dramatically in the past decade and has drawn attention to this process at both the fundamental and the applied levels.

In a previous review, we provided a historical perspective for the use of a current to activate sintering.3 A recent review of patents on activated sintering attributes the first use of current in sintering to Bloxam, who obtained a patent in 1906.4 However, little and typically unnoticed work on current activated sintering was carried out during the next eight decades. Between 1900 and the first half of 2008, more than 640 patents were issued worldwide4; the majority of these (86%) were issued since 1990. The topics of these patents cover a wide range of properties and utility of materials, as can be seen in Fig. 1.4 The most dominant coverage in these patents deals with the functional aspect of materials, including magnetic, thermoelectric, and electronic properties, Fig. 1(a), while coverage for structural properties and use is dominated by utilization as cutting tools and composites, Fig. 1(b). The marked increase in the number of patents issued since 1990 is mirrored by a corresponding increase in the number of publications on field-activated sintering. Figure 2(a) shows the number of published papers since 1993; statistics for earlier years are reported in the previous review.3 The increase in the number of published papers worldwide has an exponential trend, reflecting the strong interest in and the utilization of this method of sintering. Nearly 450 papers were published in 2009. Initially, most of the publications came from Japanese investigators, a reflection of the fact that until relatively recently the equipment for field-activated sintering was manufactured exclusively in Japan. However, as Fig. 2(b) shows, other countries have become more active in this area, with the largest number of recent publications now coming from China.

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Figure 1.  Number of pulsed electric current sintering patents published from 1900 to 2008 applied to (a) functional and (b) structural materials.4

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Figure 2.  Number of published papers (a) from 1994 to 2009 and (b) by country.

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Several reviews of this process, with different emphasis on aspects of the process or on specific materials, have been published previously.3–9 In this review, we will attempt to emphasize the fundamental aspects related to the PECS. In doing so, it will neither be possible nor advisable to provide a comprehensive discussion of all published papers on PECS. Aside from being a daunting undertaking, inclusion of all papers on PECS would add little to the emphasis on fundamentals, which is the aim of this review.

The rapid increase in the use of PECS can be attributed largely to two broad considerations: (a) the intrinsic advantages of the method relative to conventional sintering methods and (b) the observations of enhanced properties of materials consolidated by this method. Here, we refer to selected examples of both categories and plan to discuss some in more detail in the subsequent sections of this paper.

II. Reported Advantages of PECS

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Reported Advantages of PECS
  5. III. The PECS Process
  6. IV. PECS Parameters and their Effect on Processing
  7. V. Simulation Studies on the PECS Process
  8. VI. Consolidation of Functional Materials
  9. References

The literature contains numerous publications demonstrating the advantages of PECS over other methods of consolidation. A common advantage is the shorter time needed to consolidate powders relative to conventional methods, including hot pressing. For example, to obtain a density of ∼95% for ultrafine Ni powders (100 nm), it took 150 min at 700°C when hot pressing was used, while it took only 1 min at an even lower temperature (500°C), using approximately the same pressure.10 Similar observations of a shorter sintering time and lower sintering temperatures have been made in other studies.11,12 Attainment of higher densities at the same temperature has also been reported.13,14 Thus, the difference between PECS and other methods has ramifications in process efficiency and energy savings as well as microstructural and compositional implications. The energy efficiency of PECS relative to hot pressing has been demonstrated in the sintering of composites.15 With respect to compositional and microstructural changes, sintering at lower temperatures and for shorter times minimizes material loss due to vaporization,16–19 undesirable phase transformation,20 and suppression of grain growth.21,22

While the benefits cited above provided a significant impetus for the increased interest in PECS, the claims of better or improved properties of materials sintered by this method have generated an even stronger push for its use. Improvements in a variety of properties have been reported,23–29 including cleaner grain boundaries in sintered ceramic materials,30 a remarkable increase in the superplasticity of ceramics,31,32 higher permittivity in ferroelectrics,33 improved magnetic properties,34 reduced impurity segregation at grain boundaries,32 higher chemical stability,25 higher hydrogen storage capacity in BCC solid metallic solutions,35 better thermoelectric properties,36,37 improved mechanical properties,38 and better optical properties.39 In the most recent work on impurity segregation, Mitzuguchi et al.12 showed that ZrB2 sintered by PECS had lower levels of impurities in the grain boundaries and in the grains than samples sintered by hot pressing. Furthermore, PECS samples had a higher density and a lower grain size than those consolidated by a hot press.

In addition to the above, many reports have indicated unusual accomplishments in materials processing using the PECS method. The consolidation of mechanically alloyed amorphous Al-based Al–Ni–Ti intermetallic was investigated recently using three methods: PECS, hot pressing, and pressureless sintering.40 The work demonstrated that the consolidation was best accomplished with PECS to produce a uniform distribution of intermetallic nanoparticles in an amorphous matrix. As will be discussed in a later section in this paper, a related observation was made earlier in a study on the effect of an electric field on the crystallization of bulk metallic glass.41 Also, recently, Ericksson et al.16 succeeded in consolidating lead-free ferroelectric niobate ceramics using the PECS method avoiding volatilization, and demonstrated that the ceramic had improved ferroelectric properties with an unusually high remnant polarization. In a recent investigation, Wang et al.42 produced a glass phase of zeolite from crystalline powders using field-activated sintering. The approach is based on an order–disorder transformation under a pulsed-current heating and a uniaxial pressure. In a series of related studies, disorder–order transformation was shown to enhance the densification of SiC under similar conditions.43 Similar observations were also made in the consolidation of carbon with an amorphous-graphite transformation.44 Under PECS conditions, the transformation resulted in an unconventional alignment of lattice planes in the resulting graphite (normally the c-axis of sintered graphite aligns parallel with the pressure direction, but that resulting from the transformation aligns with the c-axis perpendicular to the pressure direction).

The combination of prior mechanical activation (high energy milling) on powders with a subsequent sintering or reactive sintering by the PECS process has been utilized to simultaneously synthesize and densify nanostructured, intermetallic, and composite materials.45–47 For example, using this approach, it was shown that MoSi2 could be microalloyed successfully with Mg,48 a long sought after goal, facilitating a reduction in the ductile-brittle transition temperature in accordance with theoretical predictions.49 Accomplishments were also made in the formation of functionally graded materials (FGMs)50–52 and in the joining (bonding) of materials.53–55

At the end of the “Section I,” we pointed that increased interest in the PECS process was generally motivated by the intrinsic advantages of the method relative to conventional sintering methods and by the observations of enhanced properties of materials consolidated by this method. It should be pointed out, however, that these considerations are not independent of each other and that the observed property enhancement is most likely (in a large segment of these observations) related to the reduction in the sintering temperature. Lower sintering temperatures affect composition and microstructure and these changes in turn result in different properties. However, it should be emphasized that not all the observations of property enhancement could be attributed solely to the effect of lower temperature. As we will discuss in more detail in a subsequent section, it is likely that other parameters of the process can play a role, specifically the current and pressure.

III. The PECS Process

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Reported Advantages of PECS
  5. III. The PECS Process
  6. IV. PECS Parameters and their Effect on Processing
  7. V. Simulation Studies on the PECS Process
  8. VI. Consolidation of Functional Materials
  9. References

In its simplest form, the PECS process involves the consolidation of powders under the simultaneous action of a current and a unixial pressure. The current provides the heat to achieve the desired sintering temperature and its application constitutes the main difference between hot pressing and the PECS process. As will be seen later, the application of the current gives rise to the high heating rates that can be accomplished in the PECS and to other nonthermal contributions including current effect on mass transport. Figure 3 shows a schematic of the PECS apparatus. Typically, a pulsed DC current is applied with a relatively low voltage (∼10 V). The pulsing pattern is made up of a sequence of pulses with the current, followed by the absence of a current. Thus, a pulse pattern of 12–2 means that 12 pulses are applied, followed by a duration of two pulses where the current is not applied. Pulses in a typical PECS apparatus are 3.3 ms in duration.

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Figure 3.  Schematic of a pulsed electric current sintering apparatus.

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The simultaneous output of temperature and displacement (shrinkage) makes it possible to gain an insight into sintering kinetics or the reaction mechanism in the PECS method. However, the observed displacement represents an overall characterization of shrinkage as it also includes contributions from the die and system. With calibration (to obtain baseline displacement) and accurate measurements of temperature, it is possible to obtain valid shrinkage data, as is demonstrated in Fig. 4(a) for the densification of zirconia.56 Using such an analysis, the PECS method can provide information on reactivity, as has been demonstrated in Anselmi-Tamburini et al.56 This is shown in Fig. 4(b), where the small increase in temperature represents the exothermic reaction of the formation of B4C from elemental powders. As the measuring thermocouple also provides feedback to the system, the dip in the power curve represents the control step taken by the system to account for the small, albeit transient, increase in temperature due to the reaction. Similarly, the decrease in the displacement (i.e., shrinkage) signifies the decrease in the molar volume accompanying the reaction.

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Figure 4.  (a) Corrected displacement during the densification of nanometric zirconia under a pressure of 106 MPa and (b) temperature, power, and displacement profiles during the synthesis of B4C from the elements in PECS (pressure=50 MPa and heating rate=100°C/min).56

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It is the application of the pulsed current that has been claimed to be the main advantage of the PECS process. A more specific discussion on this will be presented below. But the concept of using an electric spark in the sintering of powders is not new. As was detailed in a previous review paper,3 in a recent comprehensive review,8 and as has been discussed in a paper on PECS patents,4 the concept of electric spark was utilized in various forms for more than a century. However, the recent surge in the use of this approach is, in large part, the consequence of the availability of commercial units, manufactured initially by companies in Japan. More recently, PECS equipment has been manufactured in Germany, the U.S., Korea, and China.

(1) Nature and Influence of Pulsing

A persisting source of controversy regarding the benefits of the PECS process is the oft-repeated claim that the pulsing of the current creates a plasma that activates the surfaces of the powder particles, through the removal of surface layers (e.g., oxides). Conflicting results have been provided to argue for the existence or absence of the plasma,57–61 but the more convincing evidence points to its absence under PECS conditions.

However, aside from the concept of plasma, the role of pulsing pattern in sintering and reactivity in the PECS has been the subject of several investigations. Nanko and coinvestigators reported earlier an absence of pulse current effects on the sintering of cast iron and Ni-20Cr powders.61,62 Xie et al.63 investigated the effect of the frequency of sintering of Al powders. They densified the powder in the PECS under different pulse frequencies: 0 and 300 Hz, and 10 and 40 kHz, with patterns shown in Fig. 5, and concluded that pulse frequency had no effect on the densification and microstructure of the sintered powders. Similarly, Dang et al.64 studied the effect of the waveform of the pulsed current on the sintering of α-Al2O3 using 300 Hz and 16 kHz, with pulse patterns (on-off) of 12–2 and 2–6 for the former frequency and 40–10 and 10–20 for the latter. Their results are summarized in Figs. 6(a) and (b), which show the effect on density and grain size, respectively. As the figures show, neither the frequency nor the pulse pattern had an effect on the densification or the grain growth of alumina.

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Figure 5.  Measured waveforms during the pulsed electric current sintering process of aluminum powder with different pulse frequencies.63

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Figure 6.  Relative density (a) and grain size (b) of alumina pulsed electric current sinteringed at different temperatures with various frequencies and pulse patterns.64

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As part of a series of investigations on the fundamental aspects of the SPS process, we studied the nature and effect of the pulsing pattern.65,66Figure 7(a) shows a pulse pattern of 8–2, i.e., eight pulses of 3.3 ms on, followed by two pulses off. As can be seen from this figure, the peaks do not correspond to one voltage and in fact the exact pulse number is not always followed, as is evident in the second sequence of on pulses, where there are nine instead of eight pulses. A Fourier transform of the pattern of Fig. 7(a) is shown in Fig. 7(b). The transform exhibits a peak at about 350 Hz and smaller ones at higher frequencies. However, it is seen that the bulk of the power in the PECS is provided by the component at zero frequency, i.e. DC power. As the contribution of a given frequency to the heating is proportional to the square of its amplitude, we plot this as a function of frequency in Fig. 7(c). From this, it can be seen that most of the heating in the PECS is generated by frequencies of <100 Hz.

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Figure 7.  (a) Typical pulse pattern of 8:2 (on:off) in the PECS; (b) Fourier transform of the pattern of (a); (c) Fourier transform plotted as the square of the magnitude versus frequency.66

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As we have seen above, various studies have shown that pulse pattern had no effect on densification or grain growth.61–64 To investigate the effect of pulse pattern on reactivity (and hence mass transport) in the PECS, a study was carried out using a three-layer sample to determine the effect on product formation at the interfaces between the layers. A wafer of p-type Si was placed between two foils of Mo and annealed at constant temperatures under different pulsing patterns.65 The use of a three-layer system was planned to determine the possible effect of the direction of the DC current. As can be seen from Fig. 8, the pulse pattern had no effect on the thickness of the product (primarily MoSi2) formed at the two interfaces. Moreover, the direction of the current also had no effect. As will be seen later in this paper, this latter finding is not surprising in interactions where a compound is formed.

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Figure 8.  The growth of MoSi2 layer in the pulsed electric current sintering at different temperatures under different pulse patterns.65

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The results discussed above lead to the conclusion that pulsing pattern and pulse frequency have no measurable effect on densification, grain growth, and mass transport.

IV. PECS Parameters and their Effect on Processing

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Reported Advantages of PECS
  5. III. The PECS Process
  6. IV. PECS Parameters and their Effect on Processing
  7. V. Simulation Studies on the PECS Process
  8. VI. Consolidation of Functional Materials
  9. References

The parameters that are typically associated with the PECS process include the current, the applied uniaxial pressure, and heating rate. Typically, the current and sintering temperature are dependent parameters as Joule heating is the source of thermal activation, whether in the graphite die only (when the sample is nonelectrically conducting) or in the die and sample (when the sample is electrically conducting). The maximum (DC) current available depends on the specific PECS apparatus; in the case of the Sumitomo Model 2050 apparatus, for example, the maximum current is 5000 A. As seen later, it is possible to make the current and temperature independent parameters in the SPS with certain design modifications. While the current is commonly (but not accurately) associated with Joule heating only, it can also play another intrinsic role in enhancing mass transport, as will be shown subsequently.

The pressure has been shown recently to play a crucial role in the consolidation of materials, particularly nanostructured powders. Until recently, the maximum pressure that can be uniaxially applied in the PECS was limited by the mechanical property of the graphite die. A theoretical upper limit of about 140 MPa is used as a guide, but in practice, the limit may be lower. Recent modifications of die design (Fig. 9) have made it possible to achieve much higher pressures.67 With this design, it is possible to apply pressures as high as 1 GPa. The importance of this will be seen when we discuss the role of the pressure, below.

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Figure 9.  Schematic of a double acting die that allows the use of much higher pressures.67

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Heating rate is another parameter that distinguishes the PECS process from hot pressing. Because of the nature of heating, much higher heating rates can be achieved in the PECS, as high as about 2000°C/min. The advantage of higher heating rates is the bypassing of the nondensifying sintering mechanisms, e.g., surface diffusion. However, for nonelectrically conducting samples, the heating rate can play a role in the occurrence of thermal gradients, depending on the thermal conductivity and the size of the sample.68

(1) Influence of the Current

(A) General Observations of Current Effect in Materials Processing: As was stated above, the notion of the presence of plasma (and hence the common name of the process) has not been demonstrated adequately and thus the (nonthermal) role of the current is likely to be in its effect on mass transport. That the current could have an influence on mass transport has been shown clearly by numerous investigations. The current enhances mass transport through electromigration,69 point defect generation,70 and enhanced defect mobility.71

Electromigration studies on multilayer systems have shown that the current increases the rate of product layer formation and decreases the incubation time for the nucleation of the new phase.72–75 The imposition of a current has also been shown to have other effects. It was shown to increase the solubility in liquid metals and influence the resulting microstructure of the solidified product.76,77 In a study on the crystallization of bulk metallic glasses, the imposition of a current was shown to influence the grain size and fraction of the resulting nanocrystallites.41 Conrad and colleagues have carried out extensive investigations on the effect of an electric field (current) on a variety of materials-related processes. In a recent investigation, they showed that the imposition of a modest electric field reduced the tensile flow stress of MgO, Al2O3, and tetragonal ZrO2.78 They attributed the decrease to a reduction in the electrochemical potential for the formation of vacancies. They also reported a retardation of grain growth as a consequence of the field. It was proposed that grain growth retardation might be attributed to either a field effect on solute ion segregation (Y in the case of ZrO2), a decrease in grain boundary energy, or a decrease in ion mobility. The grain growth retardation was consistent with earlier observations on the influence of a field on copper and tetragonal zirconia, as can be seen in Figs. 10 and 11, respectively.79,80 Similar observations of grain growth retardation in tetragonal yttria-stabilized zirconia (YSZ) were recently reported by Ghosh et al.81 with the application of an electric field of about 4 V/cm. While these observations of field effects on grain growth have important implications in the processing of materials, their underlying cause is not yet clear, a circumstance that does not diminish their importance but clearly suggests that more research needs to be carried out.

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Figure 10.  Log grain size D versus log time t for annealing of a Cu foil (thickness=18 μm) at 150°–195°C with and without an electric field: (a) top side of the foil and (b) bottom side.79

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Figure 11.  Mean linear intercept grain size inline image of tetragonal zirconia versus temperature in the grip tab (ɛ≈0) and near the fracture surface (ɛ≈1.0) with and without an electric field.80

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(B) Current Effects in PECS Processing: Various observations made while processing materials on the PECS have been directly or indirectly attributed to the role of the pulsed current. Using a starting powder of Pb(Mg1/3Nb2/3)O3-PbTiO3 with a grain size of 1–10 μm, Chen et al.82 obtained a sintered body with a grain size in the range 20–100 nm. This unique observation of making a nanostructured ceramic from microstructured powders during sintering in the PECS was attributed to the role of the current. It is proposed that the pulsed current induced thermo-mechanical fatigue, which resulted in the breakup of the microstructured grains into nanograins. This observation is of significant interest and its application to other materials is needed before it can be considered as a general phenomenon in PECS processing. Nagae et al.83 studied the sintering of aluminum powders by the PECS and hot-pressing methods and reported a lower electrical resistivity of samples sintered by the PECS, which they attributed to the effect of the pulsed current on the destruction of the surface oxide through localized Joule heating at the contact points between particles. Another observation, which was attributed to the role of the field (current), was made during the sintering of the spinel MgAl2O4. Mussi et al.84 observed a different (inversion) occupation of the tetrahedral and octahedral sites when powders were sintered in the PECS relative to observations when the spinel was sintered by pressureless sintering and hot-isostatic pressing. They explained their finding on the basis of the effect of the PECS process (presumably the electric field) on the space charge. While there is no direct evidence of changes in the space charge layer during SPS sintering, the occurrence of space charge and its associated distribution of defects in magnesium aluminum spinel and alumina have been demonstrated.85,86 As in the case of the observation of grain growth retardation, the concept of a change in the space charge in ceramics due to exposure to PECS conditions remains to be demonstrated directly.

The effect of current on reactivity in the PECS has been investigated.56,87–89 The reaction between Mo foils and Si wafers was investigated under PECS conditions with and without a current passing through the multilayer ensemble.56 As was stated above,65 the reaction between these elements was not affected by the pulse pattern. However, the reaction rate to produce the interface product, primarily MoSi2 with minor amounts of Mo5Si3, was markedly influenced by the presence of a current, as can be seen in Fig. 12. The kinetics of growth of MoSi2 were determined from the results under both conditions, as can be seen in Fig. 13, in which the rate constant is plotted against the reciprocal of the absolute temperature. The calculated activation energies for the growth of MoSi2 for the cases with and without a current (∼600 A/cm2) are 168 and 175 kJ/mol, respectively. The effect of the direction of the DC current on the growth of the product was made possible by the three-layer geometry (Si is sandwiched between two Mo foils). The results, depicted in Fig. 14, show that there is no effect of current direction on the growth of the product layer. This finding is not surprising as the growth of the product, MoSi2 in this case, requires the diffusion of both Mo and Si. Thus, if one is enhanced by electromigration, the growth rate will still depend on the slower diffusing species. A validation of this is seen in cases where no product forms. This was demonstrated clearly by the electromigration study on the Cu–Ni system that forms continuous solid solutions.75 In that study, a clear dependence on the direction of the current on interdiffusivity was seen.

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Figure 12.  Growth rates of the MoSi2 layer at different temperatures in the presence and absence of current flowing through the sample.65

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Figure 13.  Arrhenius plot of the temperature dependence of the growth rate of MoSi2 in the presence and absence of current flowing through the sample.65

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Figure 14.  Comparison of MoSi2 layer thickness at the two (Mo–Si and Si–Mo) interfaces relative to the direction of the current.65

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Other observations on the effect of current on reactivity under PECS conditions include reactions between carbon and Nb,87 Mo,88 and Ti and Zr.89Figure 15 shows the time dependence of the growth of the product layer thickness (β-Mo2C) on current density for samples annealed at 1842 K.88 The effect of the current density is best shown in Fig. 16, in which the annealing time is constant at 20 min. The figure shows an apparent threshold value of current before an effect is observed; in this case, the value is approximately 500 A/cm2. In contrast to the case of the reaction between Mo and Si,56 the activation energy for the growth of the product has a strong dependence on current density, as can be seen in Fig. 17. Relative to annealing without a current, a decrease of about 44% was seen when the samples were annealed at 1476 A/cm2. In the cases of the growth of TiC and ZrC layers, the activation energy was basically unaltered with the application of a current, although the growth rate was enhanced in the presence of a current.89 Similar results were obtained with the system Nb–C, except that in this case, two carbide phases were formed: NbC and Nb2C.87 Again, growth enhancement was observed, but no change in activation energy. The difference in behavior between the Mo–C case and the others (Mo–Si, Ti–C, Zr–C, and Nb–C) is not well understood. It may be the consequence of the kinetics of the α–β phase transformation of Mo2C relative to similar transformations in the other binary systems.

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Figure 15.  Influence of current density on the growth of the β-Mo2C layer at 1842 K.88

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Figure 16.  Influence of current density on the thickness of the β-Mo2C layer (T=1842 K; t=20 min).88

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Figure 17.  Calculated values of the activation energy for the growth of β-Mo2C as a function of current density.88

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The results presented above clearly show the effect of the current on reactivity (mass transport) under PECS conditions. However, as the PECS method is used primarily for the consolidation of powders, a more convincing investigation would be the direct demonstration of current effect on sintering. Such an investigation was carried out using the sintering of copper spheres to copper plates as the model.90Figure 18 shows a schematic of the arrangement used in the SPS. In order to demonstrate the effect of the current, it was necessary to devise an experimental setup in which the current can be varied while the temperature is held constant.

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Figure 18.  Schematic of a sample of a Cu sphere to plate sintering geometry in the pulsed electric current sintering apparatus.90

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This is important as under a normal PECS operation, the current and temperature are interdependent parameters. The use of carbon foil layers made it possible to achieve the desired goal. The current (I) dependence on the number of graphite foil layers, x, is derived as90:

  • image(1)

where P is the power and R is the resistance, where the subscript “co” refers to contact resistance and “gf” refers to graphite foil resistance. Figure 19 shows the experimentally determined current as a function of the number of graphite foil layers for T=900°C and 15 min. The figure also shows the predicted relationship from Eq. (1) as a solid line. The effect of current on the sintering of the copper spheres to copper plates is shown qualitatively in Fig. 20. The figure shows SEM images of the fracture surface of the necks that formed between them on sintering at 900°C for 60 min under different current values, ranging from 0 to1040 A. These images clearly show the effect of the current by the increase in the diameter of the neck with an increase in current. Quantitative interpretations of these results were made from measurements of the radii of the neck, x, and expressing them in the form,

  • image(2)

where R is the radius of the sphere, B is a constant that contains the diffusion coefficient, and the exponents n and m are mechanism-dependent constants. Eq. (2) is for the initial stages of sintering with x/R≤0.3. A plot of (x/R) versus t is shown in Fig. 21, in which the effect of the current is clearly demonstrated.

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Figure 19.  Dependence of total current on the number of graphite foil layers at 900°C.90

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Figure 20.  SEM images showing the effect of current on the neck formation between copper spheres and copper plates sintered at 900°C for 60 min: (a) zero current, (b) 700 A, (c) 850 A, and (d) 1040 A.90

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Figure 21.  Time dependence of neck growth between copper spheres and copper plates at 900°C under different currents. The neck size at zero time refers to the value obtained during ramp up to temperature.90

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An important observation made in that study is the presence of features on the surface of the copper plates indicative of evaporation. The features appear as white rings surrounding neck regions under optical microscopy (Fig. 22) but are shown to be surface ledge structures when observed by scanning electron microscopy, Fig. 23. The extent of these areas is directly related to the strength of the imposed current, as can be seen from Fig. 22. The occurrence of these ledge structures and their proximity to the neck regions suggest an effect of the current on evaporation. While there are no published accounts on the effect of a current on evaporation, there is indirect evidence from previous electromigration studies on the system Ag–Zn.91

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Figure 22.  Optical micrographs of neck images on a copper plate showing the “halo” formation around the perimeters of necks: (a) zero current, (b) 700 A, and (c) 1040 A.90

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Figure 23.  SEM image near the edge of a neck showing the formation of ledges (AIP).90

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A comparative study showing the contribution of the PECS conditions to sintering relative to hot pressing was carried out by Fu et al.92 In this study, the sintering between spheres (Cu–Ni and Fe–Cu) was investigated under the two conditions. In contrast to the study on copper spheres and copper plates,90 in this study, the difference was not related directly to the level of the current, but only to a specific temperature and hold time of annealing. However, the results unambiguously show an effect of the PECS conditions on sintering, as can be seen from Table I.92 The table shows the effect of temperature and hold time on the (x/R) ratio for samples sintered in PECS and in the hot press. Also shown in Table I are calculated diffusion coefficients based on neck formation.93 The results show a significant sintering enhancement under PECS conditions relative to hot pressing, both in terms of neck formation and in terms of the diffusion coefficient. The diffusion coefficients of Ni, calculated from concentration profiles across necks between nickel and copper spheres for samples sintered under the two conditions, are shown in Fig. 24.92 The values obtained when sintering in PECS are greater by a factor of 2–3 than those obtained during hot pressing.

Table I.   Neck Ratios (x/R) and Calculated Diffusion Coefficients of Ni in the Pulsed Electric Current Sintering (PECS) and Hot-Press Sintering of Ni/Cu Spheres92
Sintering processT (°C)Hold time (s)x/RDNi (× 108) (m2/s)
PECS10003000.5953.564
11003000.7059.239
Hot press100027000.5481.111
110027000.6262.142
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Figure 24.  Temperature dependence of the diffusion coefficients of Ni at the interface for pulsed electric current sintering (PECS) and hot pressing (HP).92

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More recently, N. Toyofuku, T. Kuramoto, T. Imai, M. Ohyanagi and Z. A. Munir (unpublished results) investigated the effect of current on the sintering of W wires to W plates, in an arrangement similar to that described above for the copper spheres and plates. The time dependence of the neck radius is shown in Fig. 25 for the cases of sintering with and without current at 1700°C. After 30 min of sintering, the neck radius with a current is 1.5 times that of the neck formed in the absence of a current. Evidence of a possible effect of the current on evaporation is also provided. However, in this case, the effect is likely due to the reduction of surface oxide, as has been observed in the sintering of tungsten.94,95

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Figure 25.  Time dependence of neck growth between W wires and plates annealed at 1700°C with and without current (N. Toyofuku, T. Kuramoto, T. Imai, M. Ohyanagi and Z. A. Munir unpublished results).

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The investigations discussed briefly above (N. Toyofuku, T. Kuramoto, T. Imai, M. Ohyanagi and Z. A. Munir unpublished results)90,92 provide evidence of sintering enhancement under PECS conditions, with a direct correlation to the level of the current, as was shown in the sintering of copper spheres to plates. The enhancement of mass transport is believed to be the consequence of electromigration. The increase in the flux, Ji, of a diffusing species, “i,” is a result of the momentum transfer from the “electron wind” effect, as can be seen from the following relationship96:

  • image(3)

where Ji  is the flux of the diffusing ith species, Di is the diffusivity of the species, Ci is the concentration of the species, F is Faraday's constant, inline image is the effective charge on the diffusing species, E is the field, R is the gas constant, and T is the temperature.

The above studies show the effect of current on neck formation, i.e. during the initial stage of sintering. We are not aware of any study showing such an effect directly during the intermediate and final stages of sintering.

(2) The Effect of Pressure

Experimental observations showing the benefit of pressure in densification are numerous, with hot pressing being a common example.97 The role of pressure in sintering has been investigated extensively. The effect of pressure on various applicable mechanisms in sintering has been discussed in a review by German.98 In the present review, we will focus primarily on the effects of pressure in the PECS process. In typical PECS experiments, using graphite dies, there is a practical upper limit for the applied pressure (∼140 MPa) dictated by the mechanical properties of graphite. Higher pressures were possible through a modification of die design.66 As was indicated in a previous review,3 the pressure has intrinsic and extrinsic effects on sintering; at a fundamental level, the former involves an increase in the chemical potential, as indicated by the following99:

  • image(4)

where μI is the chemical potential at a particle interface under stress, μio is the standard chemical potential, σn is the normal stress at the interface, and ΩI is the atomic volume of the diffusing species. In addition to influencing diffusion-related mass transport, the pressure influences other processes intrinsically, including viscous flow, plastic flow, and creep. Extrinsically, the pressure influences particle rearrangement and the destruction of agglomerates in powders, the latter playing an important role in the consolidation of nanopowders, as will be seen below.

Makino et al.100 investigated the effect of pressure on the sintering of ultrafine α-alumina powders under PECS conditions. They showed that densification under a low pressure (30 MPa) was influenced by the grain size of the starting powder (100 and 230 nm powders) but that such an influence was suppressed when densification was carried out under a high pressure (100 MPa). In addition, the authors reported an influence of the pressure on crystallite growth; grain growth suppression increased when the powders were sintered at the high pressure, as can be seen in Fig. 26.

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Figure 26.  Dependence of crystallite size on pulsed electric current sintering (PECS) temperature and applied pressure for two different commercial powders of alumina (TM-alumina and AA-alumina).100

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Guillard et al.101 investigated the role of the PECS parameters in the densification of SiC and showed that the effect of pressure depended on the temperature at which it is applied. They sintered SiC under two different conditions: in the first, they applied the pressure (75 MPa) at the ultimate temperature of sintering (1800°C), and in the second case, the pressure was applied at a lower temperature (1000°C). Their results show that samples in which the pressure was applied at the lower temperature (i.e., case 2) had lower densities, which they attributed to the difficulty in removing closed porosity after the application of the pressure. In a similar investigation, Chaim and Shen102 showed no effect on the density of the temperature at which the pressure was applied in the sintering of Nd–yttrium–aluminum garnet (YAG) nanopowders. However, the effect on grain size was more complex: the grain size was lower when the pressure was applied at the sintering temperature if the latter (T) was less than about 1375°C but it was larger when sintering was carried out at higher temperatures. The grain size was independent of the sintering temperature when the pressure was applied at a lower temperature (1200°C). The authors explain their results in terms of the role of particle coarsening. They suggest that the process of coarsening during the heating up period has a strong influence on grain growth. This process may lead to a variation in the particle size distribution, which, in turn, affects the grain boundary curvature before pressure application. They conclude that application of pressure before significant coarsening of the nanoparticles would be beneficial for the suppression of grain growth in the dense compact.

To demonstrate the effect of the applied pressure (as well as other sintering parameters), Chaim and Margulis103 developed SPS densification maps for nanocrystalline MgO using hot-isostatic pressing as a model. As can be seen in Fig. 27, an increase in pressure at a constant relative density changes the mechanism from diffusion to plastic flow control. Or for any given pressures, the sintering is dominated initially by plastic flow and finally by diffusion for the case of MgO. Agreement with experimental observations was taken as support for the validity of the assumed model.

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Figure 27.  Densification map for 20 nm particle size nanocrystalline MgO at 750°C in pulsed electric current sintering (PECS).103

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The effect of pressure on the densification of nanopowders was demonstrated in the case of sintering of 8 mol% YSZ.104Figure 28 shows the effect of the pressure on the temperature needed to obtain 95% dense YSZ in 5 min. The figure shows an exponential decrease in temperature from about 1350° to about 890°C as the uniaxial pressure is increased from 30 to 800 MPa. The grain size decreased initially from about 200 to 15 nm and then remained constant for pressures higher than 150 MPa. As was pointed out above, in order to reach high pressures in the PECS, we have designed a die configuration, as shown in Fig. 9. The results of a more detailed determination of the effect of pressure on density and grain size for YSZ are depicted in Fig. 29.105 When sintering was carried out at a relatively low temperature (980°C), the pressure had a marked effect on density; the density increased from about 78% to 96% as the pressure was increased from 150 to 700 MPa. However, when sintering was carried out at a relatively high temperature (1180°C), the pressure had a marginal effect.

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Figure 28.  Relationship between hold temperature and the applied pressure required to obtain samples with a relative density of 95% in the case of nanometric fully stabilized zirconia (8% YSZ). Hold time: 5 min. The grain size of the materials is also shown.104

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Figure 29.  Effect of the applied pressure on the final density of c-YSZ samples pulsed electric current sinteringed at 980° and 1180°C for 5 min.105

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The difference in behavior seen in these results reflects the role of temperature relative to that of the pressure, as can be seen in the following relationship:

  • image(5)

where ρ is the fractional density, B is a parameter that includes the diffusion coefficient (of the slowest species) and temperature, g is a geometric constant, γ is the surface energy, x is a parameter representing a size scale that is related to particle size, t is the time, and P is the effective pressure. The effective pressure exerted on pores varies according to the pore geometry and the stage of sintering, but we can assume its value to be proportional to the macroscopic applied pressure. At the lower temperature, mass transport through diffusion is less significant (B in Eq. (1) is relatively small) and the pressure plays a dominant role. At the higher temperature, the relative contribution of the pressure becomes less significant. This was demonstrated by Quach et al.105 through simple comparative calculations.

An important, extrinsic, contribution of the pressure relates to its effect on agglomerates in powders, especially for nanopowders. Nanopowders are susceptible to the formation of agglomerates due to van der Waal bonds between particles.106 When compacted, agglomerates produce an inhomogeneous structure in the green body and this leads to a low green density.107 The role of the pressure in the particle rearrangements and the breakup of agglomerates is illustrated schematically in Fig. 30.108 The effect of pressure on the pore size distribution of a reactive mixture of ZrO2 and Y2O3 is shown in Fig. 31.108 As will be discussed below, the destruction of agglomerates through the application of high pressure was the key to producing, for the first time, dense bulk cubic YSZ with a grain size <20 nm.104

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Figure 30.  Schematic on the role of applied pressure in particle re-arrangements and breakup of agglomerates in the reactive mixture of ZrO2 and Y2O3 when the pressure is (a) 4 MPa (b) 8 MPa (c) 95 MPa (d) 400 MPa.108

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Figure 31.  Pore size distribution curves determined from gas desorption measurements for a reactive mixture of ZrO2 and Y2O3 obtained by compaction under (a) 4 MPa (b) 8 MPa (c) 95 MPa and (d) 400 MPa.108

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Another aspect of the influence of pressure relates to the temperature difference between the sample and the die.109 Grasso et al.109 investigated the effect of the applied pressure on the difference in the temperature at these two locations experimentally and through simulation for a graphite die and sample. They showed that an increase in pressure resulted in a markedly lower difference in temperature at the two locations, and attributed this to a decrease in electrical and thermal contact resistances at the punch/die interface due to Poisson deformation of the punch with a higher pressure. Contact resistance has been identified previously as a reason for the temperature differences between the surface of the graphite die and the center of the sample.110,111 Another aspect of the pressure, the rate at which it is applied, was investigated by Xu et al.112 in the densification of YSZ. They found that the displacement rates affected the densification rates and the final density of the samples significantly. Higher rates resulted in higher densification rates, as can be seen from Fig. 32.

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Figure 32.  Time evolution of the relative density of eight YSZ samples processed using varying displacement control rates at 1200°C.112

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(3) The Effect of Heating Rate

One of the main differences between the PECS process and hot pressing is the heating rate. Heating rates as high as about 2000°C/min can be achieved in the PECS. High heating rates reduce the time that powders dwell at the lower temperatures, where nondensifying (grain coarsening) mechanisms (e.g., surface diffusion) are dominant. In addition, higher heating rates create an additional driving force due to large thermal gradients.113 However, despite theoretical expectations and the results of simulations,114 experimental observations on the effect of the heating rate in the SPS have provided conflicting results.

For example, in the case of alumina, in one study, it was found that the heating rate (50–700°C/min) had no effect on the final density,115 and in another study, the effect was negative (i.e., the density decreased with an increase in the heating rate) when the heating rates were high (>350°C/min).116 In both of these studies, the heating rate had an effect on grain size; higher heating rates resulted in smaller grain sizes. Similar observations were made more recently by other investigators in a study on alumina117 and on cubic zirconia.118Figure 33 shows the effect of the heating rate on the densification and grain size of 8 mol% YSZ sintered under a high pressure (500 MPa).105 As can be seen in the figure, the heating rate had no effect on density but had an effect on grain size, consistent with earlier observations.

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Figure 33.  Effect of the heating rate on the density and grain size of eight YSZ heated up to 1180°C with no holding time under 500 MPa.105

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The inconsistency in some of the results on the effect of heating rate on densification in the PECS is likely the consequence of differences in the materials' properties and also experimental uncertainties. The latter include differences in the effective thermal and electrical conductivities of the samples (and thus temperature gradients) on the contact resistances between the sample and the die and between parts of the die assembly, and the timing and rate of pressure application. And the effect of the heating rate on grain size relates to the bypassing of grain coarsening processes and on the effective time for sintering: higher heating rates have a shorter dwell time and are thus expected to result in smaller grain growth. Nevertheless, this area of investigation has not received adequate attention experimentally. Simulations studies have been performed with clear predictions, as will be discussed below.114

V. Simulation Studies on the PECS Process

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Reported Advantages of PECS
  5. III. The PECS Process
  6. IV. PECS Parameters and their Effect on Processing
  7. V. Simulation Studies on the PECS Process
  8. VI. Consolidation of Functional Materials
  9. References

Simulation studies on the PECS process have been carried out generally for two purposes: (1) to verify the role of the assumed parameters of the process and to make predictions on their effects and (2) to explain observations made in the processing of various materials by this method. Thus, an important contribution of all these studies is providing a basic understanding of the PECS process, and in doing so, removing the “black box stigma” that, we believe, has contributed to the slow establishment of this method in the United States.

A significant number of simulation investigations have been conducted in recent years. Matsugi and colleagues investigated the voltage, temperature, and density distributions of titanium. They found that the largest heat source was in the punch of the die and thus heat flow was mainly from the punch to the sample.119 McWilliams and Zavaliangos120 investigated the density evolution in relation to the conduction path of the current and showed that variation in the local density in sample during the PECS process can influence its sintering behavior.

Olevsky and Froyen have made significant contributions in the area of simulations. They examined the role of the Ludwig–Soret effect of thermal diffusion during SPS sintering and reported it to be significant, especially for small particle sizes.121 Using a model that incorporates thermal diffusion, they found their predicted results on the sintering of Al2O3 to be in qualitative agreement with experimental observations,116 as can be seen in Fig. 34. Olevsky et al.114 also investigated the effect of heating rate on densification and, as was pointed out above, their results show an enhancement of consolidation with an increase in the heating rate. Figure 35 shows the predicted effect of heating rate on shrinkage of aluminum powders. In another study, Olevsky and Froyen122 incorporated electromigration into a constitutive model for PECS sintering and concluded that mass transport due to this effect can provide a significant contribution to the sintering process. This provides analytical support for the experimental observations discussed above: (a) the direct sintering enhancement due to the action of a current for the case of copper spheres to copper plates90 and tungsten wires to tungsten plates (N. Toyofuku, T. Kuramoto, T. Imai, M. Ohyanagi and Z. A. Munir unpublished results) and (b) for the observed reactivity enhancements in the PECS by the action of the current.87–89

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Figure 34.  Porosity kinetics during pulsed electric current sintering of alumina powder.121 Comparison of the developed model taking into account the impact of thermal diffusion with the experimental data of Shen et al.116

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Figure 35.  Simulations on the shrinkage kinetics of aluminum powder.114

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Numerous other simulation studies have focused on the temperature and current distributions for conducting and nonconducting materials under PECS conditions.65,110,111,123–129 In addition to elucidating the temperature and electrical potential distributions, Wang et al.130 also simulated the stress distribution in SPS experiments and found that stress gradients, which depended on materials' properties (coefficient of thermal expansion, CTE, and modulus), were larger than thermal gradients. Stress gradients, both in the vertical and in the radial directions, were found to be significant, especially for materials with a high CTE, which indicates that simple calculations of stress on the basis of the force and sample area may not be valid. The influence of percolation on the sintering of ZrO2–TiN composites by SPS was investigated experimentally and by simulation by Vanmeensel et al.131,132 The influence of adding TiN to zirconia on the temperature and current distribution was simulated.

The effect of pressure on the homogeneity of SPS-sintered tungsten carbide was investigated experimentally and by simulation by Grasso et al.133 Taking into account the electrothermal contact resistance change due to sample shrinkage and the concomitant punch sliding, and using a moving-mesh finite element model,129 the authors showed the effect of pressure on grain growth, residual porosity, and hardness distributions along the sample radius. Increasing sintering pressure resulted in a reduction in the sintering temperature, a conclusion consistent with previously reported observations on oxide ceramics.104

VI. Consolidation of Functional Materials

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Reported Advantages of PECS
  5. III. The PECS Process
  6. IV. PECS Parameters and their Effect on Processing
  7. V. Simulation Studies on the PECS Process
  8. VI. Consolidation of Functional Materials
  9. References

In “Section II,”, we provided examples of the reported advantages of the PECS process. In this section, we highlight selected accomplishments focusing on functional materials, including transparent materials, electroceramics, and porous materials.

(1) Transparent Materials

Efforts aimed at obtaining transparent materials focus on the parameters of density and grain size. Pores and grain boundaries are light-scattering regions, but as has been shown, porosity plays a more determining role.134 A decrease in pore size (to nanometric scale) leads to a decrease in scattering, a circumstance that has led to the motivation to prepare nanostructures as a means of obtaining transparency.134 Success in obtaining transparent nanostructured tetragonal and cubic zirconia was demonstrated, Fig. 36.135 The effect of sintering pressure on transparency for both modifications of zirconia is shown in Fig. 37; for any given temperature, increasing the pressure changed the samples from translucent to transparent.

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Figure 36.  (a) Sample of 1-mm-thick YSZ 8% densified at 1000°C under a pressure of 600 MPa; (b) sample of 1-mm-thick YSZ 3% densified at 1000°C under a pressure of 800 MPa. For both samples, the hold time was 5 min.135

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Figure 37.  Combined temperature and pressure conditions required for optical transparency in (a) YSZ 3% and (b) YSZ 8% sintered powders. Transparency limit set at 10% of transmittance at a wavelength of 600 nm for a 1-mm-thick sample.135

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Success has been shown in the preparation of a variety of transparent materials by PECS processing. A transparent MgAl2O4 spinel was prepared by reducing porosity and grain size by controlling the heating rate (<10°C/min).39 Morita and colleagues found that high heating rates enhanced the formation of closed porosity during the heating process, with the closed pores remaining at grain–boundary junctions. Figure 38 depicts the SEM images showing the effect of the heating rate on the microstructure of samples heated at 100 and 10°C/min.39 Other transparent materials have also been prepared successfully by using the PECS method. These include alumina, AlN ceramics, mullite, YAG, cubic zirconia, spinel, and others.136–144

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Figure 38.  SEM images of MgAl2O4 spinel pulsed electric current sintered at 1300°C with no hold time under different heating rates of (a) 100°C/min and (b) 10°C/min.39

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(2) Porous Materials

Typically, the use of the PECS method is directed toward obtaining dense materials; however, the method has also been used to obtain porous materials. Kun et al.145 obtained porous stainless steel and found it to have compressive strength superior to that of samples prepared by hot pressing. Through modification of die design, it is possible to obtain a temperature gradient along the vertical axis of the die, and with this, obtain FGMs. Suk et al.146 used this approach to obtain porous structures of tungsten FGM with porosity and pore size distribution, as shown in Fig. 39. Using TiH2 powder as an additive to form pores upon decomposition, Zhao and Taya147 prepared porous Ni–Ti alloys by PECS. Other nanocrystalline alloys of Ti were prepared in porous form for biomedical applications,148 and other porous materials for biomedical (implant) applications have also been prepared successfully using the PECS methods.149,150 Porous alumina, boron nitride, and other materials have also been prepared successfully.151,152

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Figure 39.  Porosity and pore size at various locations in the pure W sample146

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(3) Electroceramics

The concept of the size effect in electroceramics has been the focus of a number of investigations. The aim is to demonstrate the effect of grain size on the electrical properties of various oxide ceramics. Okamoto et al.22 investigated the effect of grain size in suppressing the cubic-rhombic phase transformation in (12 mol%) scandia-stabilized zirconia, and thus the elimination of a discontinuity in electrical conductivity. Their work showed that fine grain size, which can be obtained by the PECS, lowered the rhombic-cubic transformation point to room temperature and thus prevented the formation of the less conductive rhombohedral phase (Zr7Sc2O17).

In a more recent series of investigations on YSZ and samaria and gadolinia-doped ceria, new and unexpected behavior was discovered. For the first time, through the application of high pressure in the PECS, these ceramics were prepared in a dense form (98%+) with a grain size of <20 nm.104 It was discovered that in this grain size, YSZ is a protonic conductor in the presence of moisture.153 This can be seen in Fig. 40, in which the decrease in the oxygen ion conductivity with decreasing temperature is reversed at low temperature due to the contribution of protonic conductivity.

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Figure 40.  Temperature dependence of conductivity of YSZ for nanometric samples in the range 13–100 nm (inline image=23 000 ppm).153

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The examples briefly discussed above highlight the success of the PECS process in the preparation of highly dense materials with a very small grain size, an accomplishment that opens the door for application in fuel cells at low temperatures.

References

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Reported Advantages of PECS
  5. III. The PECS Process
  6. IV. PECS Parameters and their Effect on Processing
  7. V. Simulation Studies on the PECS Process
  8. VI. Consolidation of Functional Materials
  9. References
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