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Abstract

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

Hafnium diboride (HfB2) powder has been synthesized via a sol–gel-based route using phenolic resin, hafnium chloride, and boric acid as the source of carbon, hafnium, and boron, respectively, though a small number of comparative experiments involved amorphous boron as boron source. The effects of heat-treatment dwell time and hafnium:carbon (Hf:C) and hafnium:boron (Hf:B) molar ratio on the purity and morphology of the final powder have been studied and the mechanism of HfB2 formation investigated using several techniques. The results showed that while temperatures as low as 1300°C could be used to produce HfB2 particles, the heat treatment needed to last for about 25 h. This in turn resulted in anisotropic particle growth along the c-axis of the HfB2 crystals yielding tube-like structures of about 10 μm long. Equiaxed particles 1–2 μm in size were obtained when the precursor was heat treated at 1600°C for 2 h. The reaction mechanism involved boro/carbothermal reduction and the indications were that the formation of HfB2 at 1300°C is through the intermediate formation of an amorphous B or boron suboxides, although at higher temperatures more than one reaction mechanism may be active.

I. Introduction

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

Many exotic materials such as Inconel x/x-750, titanium-zirconium-molybdenum (Ti–Zr–Mo) alloys, carbon fiber-silicon carbide (Cf–SiC) composites, super alloy of columbium, tantalum and, molybdenum called taz-8a cermets, Ti metal matrix composites and ultrahigh temperature ceramics (UHTC) based on the borides and carbides of Ta and Zr have been created since the 1960s to be used as the wing leading edge and propulsion components in hypersonic vehicles to withstand temperatures in excess of 2000°C and to offer ablation resistance.[1] Hafnium diboride (HfB2) has a hexagonal AlB2-type-layered structure with B atoms in 2D graphite-like rings and alternate hexagonally close-packed Hf layers.[2, 3] The strong Hf–B and B–B bonds are responsible for the very high melting point of 3250°C, high oxidation resistance, and high hardness of about 29±5 GPa, whereas the electron concentration around the B atoms gives rise to electrical conductivity in the material.[4] Due to these properties, HfB2 has several commercial and lab scale applications including electron emitters, catalysts, cutting tools, rocket nozzle inserts, nose caps, and leading edges in space craft and hypersonic vehicles.[5, 6] Research is being carried out to use HfB2 to coat SiC and carbon to improve their high-temperature oxidation resistance and ablation resistance in a range of aerospace applications. However, with HfB2 being expensive, its commercialization in the aerospace industry has not proved feasible to date. It will therefore be attractive to find a convenient and cost effective method for the large-scale synthesis of HfB2 powder.

Hafnium diboride can be synthesized by various methods, including chemical routes, reactive processes, and carbothermal reduction reactions. Chen et al.[7] synthesized nanosized HfB2 from HfCl4 and NaBH4 at 600°C using a hydrothermal reaction method. This is the lowest temperature reported for the synthesis of HfB2 in the literature. However, the use of pressure makes large-scale process, expensive. Blum et al.[8] used a Hf and B powder mixture in a non–self-propagating high-temperature synthesis (SHS) process to produce HfB2 at 1500°C. They also reported a synthesis route employing metallic Hf strips and elemental boron powders. Despite the use of elements, the authors reported the presence of a significant level of unaccounted impurities that may or may not have been core shell structures of Hf and B. HfB2 has also been prepared by the SHS route by Munir et al.[9] Chemical vapor deposition of HfB2 from Hf (BH4)4 has been reported by Yang et al.[10] and Baber et al.[11] These methods are expensive to scale up however, due to the high pressures or high temperatures involved. The solution derived precursor routes involving boro/carbothermal reduction reactions are reported to be the cheapest and most scalable processes for group IVB diborides, including HfB2.[12] Ni et al. [13] used excess B4C and carbon to reduce ultrafine HfO2 at 1500°C–1600°C and obtained <1 μm HfB2 powder. Equiaxed HfB2 particles with particle sizes ranging between 1 and 1.5 μm were synthesized through borothermal reduction reactions at 1600°C by Zhang et al.[14]

This study reports a sol–gel-based synthesis route for HfB2 powders at 1300°C. The sol–gel process was chosen as it forms a more homogeneous mixture and transforms to HfB2 at lower temperatures than other approaches.[15] The method is suitable to produce UHTC coatings on carbon and SiC fibers.[16] A sol–gel mixture of hafnium chloride (HfCl4), boric acid (H3BO3), and phenolic resin were used to obtain intimately mixed yet unreacted HfO2, B2O3, and C that is then subjected to boro/carbothermal reduction reaction via heat treatment. Usually, this approach results in minor impurities such as HfO2, HfC, and B4C; in this study, we have optimized the ratios of the different precursors used to obtain HfB2 with negligible impurities. The mechanism underpinning the formation of HfB2 is also discussed as is the effect of varying the stoichiometries of B and C with respect to Hf on the structure of the resultant products.

II. Experimental Procedure

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

Hafnium (IV) chloride, HfCl4, (98% purity; Sigma-Aldrich Company Ltd, Dorset, UK) was used as the hafnium source. A phenolic resin with a char yield of 51% was used as the carbon source (Cellobond J2027L; Momentive Speciality Chemicals, Louisville, KY). Boric acid, H3BO3 (99.5% purity; Fischer Scientific, Loughborough, UK) was the primary source of boron, with the exception of a few specified experiments where amorphous boron powder (≥95% purity, Sigma-Aldrich Company Ltd, Dorset, UK); was used. Ethanol (96% purity, Fischer Scientific, Loughborough, UK) was used as the solvent.

The basic reaction is shown by Eq. (1)a, b and a flow chart for the synthesis is shown in Fig. 1.

  • display math(1a)

or

  • display math(1b)

The process involved the initial dissolution of H3BO3 in ethanol kept in an oil bath at 120°C followed by adding the appropriate amounts of HfCl4 and phenolic resin, Table 11 . The stoichiometry of the reactants was varied to study their influence on the purity of HfB2 powder. The solution mixture was stirred at 120°C for 24 h in air with continuous refluxing to obtain a sol. The latter was dried at 250°C for 2 h in air and ground using a motor and pestle to obtain the HfB2 precursor powder.

Table 1. Compositions Investigated for HfB2 Synthesis
 Elemental stoichiometryRemarksEnd product after heat treatment at 1600°C for 2 h under flowing Ar
HfBC
HBC 125Exact stoichiometry, as Eq. (1a)Powders, (particles size 1–2 μm), contain HfC since boron is in deficient
HB e C 13–3.85Eq. (1a) with slight excess B to compensate for lossesSingle phase HfB2 powders, (particle size 1–2 μm)
HB e C e 1310Eq. (1a) with slight excess B and excess CNano HfB2, (particle size 1–2 μm) with free carbon impurity
HB e1 C e 1610Eq. (1a) with excess B and CHfB2 powders (particle size 1–2 μm) with B4C faceted particles
HB e 130Eq. (1a) without CNo HfB2 formation. Presence of HfO2 and B2O3 glassy phase
HB a C 122Eq. (1b) using amorphous B powderSingle-phase HfB2 powders, (particle size 1–2 μm)
B e1 C e 0610Eq. (1a) without Hf; attempt to synthesize B4CB4C formation occurs at 1450°C
HC 103Attempt to synthesize HfCHfC formation occurs at 1500°C
image

Figure 1. Flowchart for HfB2 synthesis.

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The precursor powders were characterized using Fourier transform infrared spectroscopy (FTIR 8400S, Shimadzu, Milton Keynes, UK), and thermo gravimetric analysis (TGA) coupled with differential thermal analysis (DTA) (F1 Jupiter; Netzch TGA/DTA, Selb, Germany). The TGA/DTA runs were carried out from room temperature up to 1600°C in a flowing argon atmosphere. The heating rates were maintained at 5°C/min up to 1000°C and 3°C/min above 1000°C. Differential scanning calorimetry (DSC) coupled with TGA (2920 Modulated DSC; TA instruments, Zellik, Belgium) was used to analyze the decomposition of phenolic resin and boric acid. The TGA/DSC runs were carried out from room temperature to 1000°C in flowing argon atmosphere at a heating rate of 5°C/min.

The samples contained in a 99.7% pure alumina boat were subjected to boro/carbothermal reduction reaction in a flowing argon atmosphere at temperatures in the range 1300°C–1600°C in a horizontal tube furnace (TSH17/75/450; Elite Thermal Systems Ltd, Market Harborough, UK) fitted with a 99.7% pure alumina tube. The heating and cooling rates were maintained at 5°C/min from room temperature up to 1000°C and 3°C/min above 1000°C in all cases. An argon—1% hydrogen mixture was passed through the tube for the first 500°C for all of the runs. This helped to remove both the Cl ions[17] trapped in the system as HCl vapors2 and also the oxygen from the tube. An estimation of the primary particle size of the powders was obtained by sprinkling a small amount of powder on a sticky carbon pad attached to an aluminum stub and using field-emission gun scanning electron microscopy (FEGSEM 1530 VP; Carl Zeiss (Leo), Oberkochen, Germany). Phase analysis of the powders was carried out by room-temperature X-ray diffraction (XRD) using CuKα radiation (Bruker D8 X-Ray Diffractometer; Bruker, Coventry, UK). The d spacings were calculated from the 2θ values and were compared with the standard values from the JCPDS powder diffraction files to identify the phases. The percentage of carbon content in the powders was also analyzed (CE-440 Elemental Analyser; Exeter Analytical Inc., Coventry, UK). High-resolution XPS spectra for HfB2 powders were recorded on a K α surface analyzer (Thermo Scientific, West Sussex, UK) operating with an un-monochromatized AlKα X-ray source (1486.6 eV). The samples were placed under a vacuum environment and then irradiated with photons in the X-ray energy range. A Focused dual ion beam (FIB–FEI Nova 600 Nanolab Dual Beam system, Eindhoven, the Netherlands) was used to prepare the sample for electron backscattered diffraction (EBSD) imaging (Hikari hi-speed camera fitted with the FIB). Thermodynamic calculations were carried out using Factsage 6.1 (CRCT-ThermoFact Inc. Montreal, Canada & GTT-Technologies, Herzogenrath, Germany).

III. Results and Discussion

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

(1) Powder Synthesis

The Fourier-transform infrared spectrometry (FTIR) absorption spectra for all the HfB2 precursors formed with different Hf:C:B molar ratios and dried at 250°C for 2 h were very similar; one of the spectra across the 4000–400 cm−1 range is shown in Fig. 2. The broad peak at around 3400 cm−1 is attributed to the O–H stretching vibrations associated with the hydroxyl groups in the phenolic resin, boric acid, and the Hf–OH stretching vibrations. The peaks at 418, 1392, 2841, and 2914 cm−1 are attributed to the aliphatic C–H stretching vibrations from the phenolic resin, whereas those at 543, 1099 and 1622 cm−1 represent the aromatic C–H deformation and C=C deformation from the phenolic resin. The peaks at 2330 and 2357 cm−1 are due to C=O stretching arising from the CO2 and C3O2+ groups present in the system. The O–B–O stretching vibrations peaks are present at 449, 520, and 675 cm−1 and these are associated with boric acid present in the system. The peaks at 651 and 748 cm−1 are attributed to the OHfO-asymmetric stretching.[18] The FTIR result confirms that there was no complex formation between the reactants and that at the end of the sol–gel process and subsequent drying step, the precursor powder consisted of H3BO3, Hf–OH, resulting from the hydrolysis of HfCl4, and aromatic and aliphatic chains resulting from the cross-linking of the phenolic resin.

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Figure 2. Fourier-transform infrared spectrometry of precursor powder HBC.

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Figure 3 shows the TGA/DSC curves of boric acid and phenolic resin. TG analysis shows that the main weight loss occurred below 320°C for H3BO3, due to the evaporation of the bound water, and below 640°C for phenolic resin, due to the decomposition of the phenolic resin. H3BO3 loses 3 molecules of water at 150°C, 189°C, and 210°C forming metaboric, tetraboric, and pyroboric acid. Further heating above 300°C led to the formation of boron trioxide and the weight stabilizes.[19] The phenolic resin, however, continues to lose weight until 640°C, due to the decomposition resulting in evaporation of physically and chemically bound water. The percentage of the mass remaining at the end of the decomposition process is assumed to be the carbon content of the resin and was used for stoichiometric calculations.

image

Figure 3. Thermo gravimetric analysis/differential scanning calorimetry of phenolic resin and boric acid.

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Figure 4 shows the XRD patterns for the HBeC precursor powders heated from 600°C to 1500°C with a 0.1 h dwell in a flowing Ar atmosphere. The results show that monoclinic HfO2 was formed at 650°C. At this temperature, the degradation of phenolic resin to carbon and dehydration of boric acid to boron trioxide will also have been complete, Fig. 3.[20] Therefore, at 650°C the powder mixture consisted of unreacted but intimately mixed B2O3, HfO2, and C. By 1300°C HfB2 peaks formed, indicating the onset formation of HfB2.

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Figure 4. XRD patterns of the HBeC precursor powder heated from 600°C to 1500°C with a 0.1 h dwell.

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Figure 5 shows the indexed XRD patterns of HBC and HBeC heat treated at 1600°C for 2 h. The patterns were indexed according to the JCPDS cards 00-039-1491 and 00-038-1398 for HfC and HfB2, respectively. It can be seen that the pattern corresponding to the HBC precursor, which was made using the stoichiometric mix of the 3 elements, includes significant amounts of HfC impurity3, whereas the HBeC XRD pattern shows only HfB2 peaks. This confirms the need to include excess B in the initial mix to allow for B2O3 losses by vaporization, as observed by Ni et al.[6] The boron loss depended on various factors like humidity, flow rate of the inert gases, and substrate (99.9% alumina boat that contained the powder sample) used. For this reason, the ratio of B:Hf was kept between 3:1 and 3.8:1 instead of 2:1 for subsequent compositions.

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Figure 5. XRD patterns of the HBC and HBeC precursor powders heat treated at 1600°C for 2 h.

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The isothermal TGA showed that 25 h was needed at 1300°C to complete the boro/carbothermal reaction and achieve hexagonal HfB2 with no secondary phases, this was confirmed by a heat treatment in tube furnace at the same temperature and dwell time, Fig. 6; only 2 h was required at 1600°C.

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Figure 6. XRD of HBeC precursor powder heat treated at 1300°C for 25 h.

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As seen from Fig. 7, the products from HBC and HBeC have a very similar particle size of about 1–2 μm indicating that an increase in boron oxide content does not have any effect on the particle size of the end product. The HfB2 powder obtained from HBeC precursor powder was subjected to XPS analysis, Figs. 8(a) and (b). As seen from the spectra, the peaks at 17.7 and 185 eV correspond to the Hf–B bond in HfB2.[21] No obvious peaks corresponding to free boron or HfO2 were detected.[22, 23] The weight percentage of carbon in the same powder as determined by elemental CHN analysis was around 0.19 wt%.

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Figure 7. Field-emission gun scanning electron microscopy micrographs showing the particle morphology for (a) HBeC, (b) HBC, heat treated at 1600°C for 2 h (c) HBeC heat treated at 1300°C for 25 h.

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Figure 8. XPS of HBeC precursor powder heat treated at 1600°C for 2 h a) Hf 4f5/2 orbital and b) B1s orbital.

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Reducing the synthesis temperature from 1600°C to 1300°C and increasing the dwell time from 2 to 25 h for the HBeC precursor powder had the effect of slightly decreasing the particle size from 1–2 μm to 0.25–2 μm, however, the latter heat-treatment conditions also yielded a high proportion of rod shaped particles, Fig. 9(a). It is believed that the particles grow into rods due to the long heating time involved with the growth occurring along the c-axis, Fig. 9(b). Similar structures were reported by Begin et al.[23] when they synthesized HfB2 through mechanical activation.

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Figure 9. (a) Field-emission gun scanning electron microscopy of HBeC heat treated at 1300°C for 25 h showing rod shaped particles, (b) Focused dual ion beam and electron backscattered diffraction image of “a.”

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(2) Effect of Varying the Hf:B and Hf:C Molar Ratio

No HfB2 formation was observed after heat treating HBe to 1600°C for 5 h; the end product consisted of HfO2 and B2O3, Fig. 10(a); as identified by EDX. This confirms that, without carbon, no reduction reaction occurs in the system, i.e., carbon plays an important role in creating the reducing conditions required for HfB2 formation. In the presence of excess carbon and slight excess boron, HBeCe, the end product contained both HfB2 and ~0.19%, free carbon, as determined by elemental CHN analysis, Fig. 10(b). Interestingly, the size of the HfB2 particles was only 20–30 nm. On the other hand, if both excess boron and carbon were used, HBe1Ce, then boron carbide was formed; Fig. 10(c) and the HfB2 particles were 1–2 μm in size. The presence of the free carbon in the HBeCe system thus appears to act as a physical barrier, preventing particle growth and yielding much finer HfB2 compared with the other powders synthesized, though attempts to remove the residual carbon without chemically degrading the nano HfB2 failed. Without free carbon, the particles are free to grow in size during the heat treatment. Similar results were reported by Krishnarao et. al.[24] during TiB2 synthesis. HfC formation occurred only when the boron source, i.e., Hf:B < 3.8, was present in deficient amounts, see Fig. 5. The XRD of HB and HBeCe powders, Fig. 11, do not give any valuable information regarding the presence of amorphous B2O3 and amorphous carbon. Reactions (2) and (3) represent the excess C and B scenarios, respectively:With slight excess boron excess carbon

  • display math(2)

With excess boron and excess carbon

  • display math(3)
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Figure 10. (a) HB, (b) HBCe and (c) HBeCe precursor powders, all heat treated at 1600°C for 2 h.

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Figure 11. XRD of HB and HBeCe precursor powder heat treated at 1600°C for 2 h.

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(3) Reaction Mechanism

For any reaction to be thermodynamically favorable and spontaneous the Gibbs free energy (ΔG) of the reaction should be negative. For reaction (1a),

  • display math

ΔG is negative only at temperature >1523°C, as calculated using Factsage 6.1, however, it has been shown that HfB2 forms at a temperature as low as 1300°C. To explain this difference the mechanism by which HfO2, B2O3, and C react to form HfB2 was studied in detail.

There are three possible routes by which these three reactants could form HfB2; they are indicated below as reactions (4)(6). Note that the latter has some variations within it.

  • display math(4)
  • display math(5)
  • display math(6a)

or

  • display math(6b)

The intermediate reactions (4i)(6i) are given below along with their ΔG values.

  • display math(4i)
  • display math(5i)
  • display math(6i)

Hence, based on thermodynamic calculations, the only reaction that has a realistic chance of leading to HfB2 formation at ~1300°C is reaction (6). Note that at higher temperatures, e.g., above ~1450°C, there may be more than one reaction path. The difference in the experimental and theoretical temperatures was due to the fact that the calculations assumed steady-state conditions and standard state values, whereas the actual reactions took place in a dynamic state.

If the mechanism of formation of HfB2 were to proceed through reaction (4), then HfC should form at a much lower temperature than HfB2. From Fig. 4, it can be seen that HfC peaks did not appear even by 1500°C, though in theory this could be due to the HfC being consumed immediately as it is formed. To understand this better precursor powder HC, made from Hf and C sources, was prepared and heated from 600°C to 1500°C with 0.1 h dwell and subjected to XRD analysis. As seen from Fig. 12, HfO2 formation occurred at 650°C, Fig. 4, but HfC peaks only began to appear at 1500°C. Blum et al.[25] observed that there was a kinetic preference and a lower threshold temperature for the onset of the reaction of Hf with B powder compared with the reaction with C powder. The present work shows that the same also occurs for the reaction between HfO2 and B or C. Hence, it is concluded that HfC is not an intermediate product during HfB2 synthesis, suggesting that reaction (4) is not the correct formation mechanism.

image

Figure 12. XRD patterns of the HC precursor powder heated from 600°C to 1500°C with a 0.1 h dwell.

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When the HBe1Ce precursor powder was heat treated there was clear evidence of HfB2 formation at 1300°C, as for the HBeC. However, although no boron carbide phases were detected by XRD, even after heat treatment at 1600°C for 2 h, Fig. 13, it was detected by electron microscopy, Fig. 10(c), and EDX analysis revealed the presence of both boron and carbon in the faceted phase. The amount of the boron carbide phase was therefore below the level for XRD detection, especially given the low X-ray density of boron carbide in comparison with hafnium-based compounds.

image

Figure 13. XRD of the HBe1Ce precursor powder heat treated at 1300°C for 0.1 h and 1600°C for 2 h.

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Reaction (5) requires that carbon reacts with B2O3 to form B4C as an intermediate product, which subsequently reacts with HfO2 to form HfB2. If this is the case then B4C has to form below 1300°C, i.e., at a temperature lower than that required for HfB2 formation. As potential boron phases were not detected by XRD when they occurred in combination with a heavy-metal compound, a composition containing only boron and carbon was synthesized (Be1Ce). The carbothermic reduction sequence of B2O3 is presented by reactions (7)(10)[26, 27].

  • display math(7)
  • display math(8)
  • display math(9)
  • display math(10)

Figure 14 shows the XRD pattern of the Be1Ce precursor powder heat treated at different temperatures in a scan range 2θ = 30°–40°. It is evident that a boron carbide peak does not appear until 1450°C and hence it is very unlikely to be an intermediate product in HfB2 formation, unless there are some very significant, and unknown, catalytic effects occurring when Hf-based compounds are present.

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Figure 14. XRD patterns of the Be1Ce precursor powder heat treated at different temperatures for 0.1 h.

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This may possibly suggest that, at the end of reactions (7)(9), the products react with HfO2 to form HfB2 before reaction (10) can occur. This is the basis of reaction (6), according to which HfB2 is formed by the reaction between HfO2 and boron suboxides and/or amorphous boron. Due to the limitations of experimental conditions, it was difficult to detect B2O2, B2O, or amorphous B in the intermediate reaction products. Hence, to shed further light on the issue, the experiments were performed involving amorphous boron powder as the source of boron.

High-temperature DTA curves for the precursors HBeC and HBaC, showed similar trends, Fig. 15. The total weight loss for HBaC was 34 wt% whilst that for HBeC was 51 wt%, as mentioned earlier. The difference in the total weight loss may be due to the loss of B2O3 in the HBeC system. The weight loss for the HBaC system occurred up to 500°C due to the loss of physisorbed and chemisorbed water and degradation of the phenolic resin, whilst that for HBeC occurred up to 700°C due to the presence of B2O3 and excess resin. The weight losses remained stable up to 1275°C in both cases and then rapidly decreased and stabilized toward 1600°C.

image

Figure 15. Thermo gravimetric analysis/differential scanning calorimetry of HBeC and HBaC precursor powders.

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The endothermic peaks depicting the boro/carbothermal reduction reactions at 1275°C and 1470°C are identical for both precursor powders, indicating that the reaction paths were similar for the two. This could only mean that either the B2O3 had dissociated into B during the course of the HfB2 formation in the HBeC precursor powder or the amorphous B in the HBaC precursor had oxidized into B2O3. As the reaction was carried out in a reducing atmosphere in the presence of carbon, the likelihood for the latter is extremely small.

This result, combined with the Gibbs free energy value, indicates that the formation of HfB2 at 1300°C is most likely to occur through the formation of amorphous boron or boron suboxides as the intermediate product [reaction (6)], though, as indicated earlier, at higher temperatures there could be more than one reaction mechanism taking place simultaneously.

IV. Conclusions

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

HfB2 powder has been prepared using a simple sol–gel approach which does not involve any complicated processing steps such as milling and which has a potential for commercialization. The ratio of Hf:B:C required for achieving this purity was 1:3–3.8:5. The effect of changing the stoichiometry of the precursors on the purity of the final powders was studied. If the carbon source was in excess then the final product contained free carbon whilst if the both boron (Hf:B ≥ 6) and carbon source were in excess then the final product consisted of boron carbide in addition to the HfB2 particles. If free carbon was present in the system, it acted as a capping agent for the HfB2 particles and lead to a much finer final particle size of 20–80 nm. However, it was not possible to remove the carbon without degrading the HfB2. Boro/carbothermal reduction temperatures as low as 1300°C could be used to form HfB2, however, it required 25 h to form the pure phase. This long dwell time gave rise to a significant fraction of rod shaped particles, with growth occurring along the c-axis.

During boro/carbothermal reduction, the formation of HfB2 occurred before HfC and B4C formation, so it was concluded that neither HfC nor B4C were the intermediates. The TGA/DTA results of the HfB2 precursor powders made from B2O3 and amorphous B, however, showed identical endothermic peaks for the boro/carbothermal reduction reactions and hence it is believed that the main thermodynamically favorable path for HfO2/B2O3/C mix to react and form HfB2 at 1300°C is through the intermediate formation of amorphous B or boron suboxides [reaction (6)a, b], although at higher temperatures more than one reaction mechanism could be active.

Acknowledgments

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

The authors thank Dr Luc Vandeperre, senior lecturer at Imperial College, London for help with thermodynamic calculations and for analyzing our samples using high-temperature TGA/DTA analysis. Funding from Defence Science and Technology Laboratories (DSTL), UK, is acknowledged.

Notes
  1. 1

    According to reaction (1), the stoichiometry of the Hf:B:C should be 1:2:5, but excess boron was added to account for the loss that occurs during high-temperature heat treatments.[7]

  2. 2

    HfCl4 being highly hygroscopic and oxygen sensitive reacts with moisture in both the air and the ethanol during the synthesis stage to release HCl vapors. Any reaction with H2 gas only triggers this reaction and helps remove Cl ions from the precursor powder.

  3. 3

    The lattice parameter at 2θ = 33.44 (major peak for HfC) a33.44 = 0.46372 nm and d33.44 = 0.26773 nm; n = 1 and λ = 0.154056 nm. According to the JCPDS card number 00-039-1491 the Hf:C was equal to 1, indicating stoichiometric hafnium carbide.

References

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References
  • 1
    M. M. Opeka, I. G. Talmy, and J. A. Zaykoski, “Oxidation-Based Materials Selection for 2000°C+ Hypersonic Aerosurfaces: Theoretical Considerations and Historical Experience,” J. Mater. Sci., 39 [588] 7904 (2004).
  • 2
    T. Lundstroem, “Structure, Defects, and Properties of Some Refractory Borides,” Pure Appl. Chem., 57 [10] 138390 (1985).
  • 3
    Y. B. Kuz'ma and S. I. Mikhalenko, “Crystal Chemistry of Refractory Compounds,” Zh. Vses. Khim. Obshch. Mendeleeva, 30 [6] 50914 (1985).
  • 4
    B. Post, F. W. Glaser, and D. Moskowitz, “Transition Metal Diborides,” Acta Metall., 2 [1] 205 (1954).
  • 5
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