Electronic, mechanical, and thermal properties of [Ca24Al28O64]4+(4e−) electride ceramic

The compound [Ca24Al28O64](4e-) named as C12A7:ehas outstanding properties because of a low work function and high electron conductivity due to a cage like crystal structure. For the application of the material as hollow cathodes in small satellite propulsion systems, its preparation as sinterable glass ceramics via the powder route is purposeful. However, the mechanical and thermal properties of the C12A7:eglass ceramics have only been insufficiently measured so far. In this study, the measured Vickers hardness (HV0.5) is 6.7 ± 0.2 GPa. The ceramic has a fracture toughness of 1.6 MPa m0.5 (calculated by Shetty model) and a 4-pt. bending strength of σ = 75 ± 12 MPa (σ0 = 90 MPa). The coefficient of thermal expansion CTE is between 4.2 and 6.0 × 10-6 K-1 (RT-1000 °C) and the thermal conductivity ranges between 2.3 W m-1 K-1 (20 °C) and 1.7 W m-1 K-1 (1000 °C). For calculating the thermal conductivity, the heat capacity and the thermal diffusivity are measured.


Introduction
The compound 12CaO . 7Al 2 O 3 (C12A7, Ca 12 Al 14 O 33 ) is formed near the eutectic composition in the CaO-Al 2 O 3 system in the temperature range between ~850 °C and the melting point of ~1410 °C [1] (Fig. 1). This rather rare circumstance makes it possible to synthesize this composition via the melt as a glass or glass frit.
Powders prepared from such glass frits, respectively, powder compacts made thereof can be transferred into dense polycrystalline glass ceramic microstructures by a combined sintering and crystallization step [2]. Other methods for the preparation of the crystalline compound can be either the growth as a single crystal [3,4], sintering as polycrystalline ceramic [5], plasma arc melting [6] or deposition as thin film [7]. By the cage structure a positively charged framework [Ca 24 Al 28 O 64 ] 4+ is built, in which the charge balance can be given by two O 2-(oxide), four e -(electride), four H -(hydride) or other anions such as Cl - [8,9,10].
In case of the electron-occupied composition [Ca 24 Al 28 O 64 ] 4+ (4e -) -also named as C12A7:e-in the followingthe known electronic properties are characterized for several different preparation routes (Tab. 1). For C12A7:e-single crystals highest electronic conductivity is ranging between 100 and 1500 S cm -1 and also highest electron concentrations (electron density) of about 2 x 10 21 cm -3 are determined by optical absorption [11,12,13,14]. In comparison to the single crystalline C12A7:e-, the polycrystalline ceramics seem to indicate a lower conductivity along with a decreased electron concentration [15,16]. But the differences in the single and polycrystalline structure do not exclusively explain these lower values but also the measuring method used for determining the electron concentration. Using electron paramagnetic resonance (EPR) for determination of the electron concentration is not comparable to optical absorption, because the saturated diamagnetic electron pairs are not distributing on the measured signal [17,18]. Similar is the situation with Hall measurement, which detects only the mobile electrons [16]. So, the reported data on the electronic properties summarized in table 1 do not allow a direct comparison or a reliable characterization but give a profound base for the explanation of the unique properties of this material. Even less reported than data on the electronic properties are those on the thermal and mechanical properties of C12A7:e-ceramic. For the oxide C12A7, a thermal conductivity of 1.7-1.3 W m -1 K -1 (RT-250 °C) was measured by Rudradawong et al. [19]. A slightly higher value of 3.1-1.9 W m -1 K -1 (RT-600 °C) is reported by Mackey et al. [20] for the C12A7:e-electride ceramic. In the same paper, the coefficient of thermal expansion (CTE) is given as 7 x 10 -6 K -1 .
In this study, the C12A7:e-material was prepared by a glass ceramic route and samples were prepared by sintering under nitrogen in a graphite crucible. Details of the synthesis procedure have been published earlier [2]. For measuring the electronic, thermal, and mechanical properties, different specimen geometries were cut and dry ground. With this the electron concentration, the thermal conductivity, diffusivity, and the coefficient of thermal expansion as well as the hardness, fracture toughness and bending strength were determined.

Experimental
To prepare C12A7 oxide powder, CaCO 3 (VWR Chemicals) and Al 2 O 3 (NABALOX®, Nabaltec) were first mixed with stoichiometric ratio of 12:7 in a tumbling mixer. This mixture was melted in a platinum crucible by heating in a muffle furnace to a temperature above 1450 °C. The melt was quenched on a brass block and the glass splinters were ground in a rotating disk mill (RS 200, Retsch), in a planetary ball mill (Pulverisette 5, Fritsch) for 12 h and in a vertical ball mill (NETZSCH Feinmahltechnik). The last milling step was performed as wet milling process with ethanol and alumina balls for 2 h.
For measuring the electron density of the resulting electride, the samples must be thinner than 1 mm.
Therefore, the prepared oxide powder was casted as slurry with binder, dispersant and plasticizer to a tape.
The flexible tape was cutted by Laser into samples with 40 mm x 40 mm dimension. The samples were presintered by debinding and sintering in one step by heating with 0.5 K/min to a temperature of 600 °C and

Accepted Article
This article is protected by copyright. All rights reserved with 3 K/min to the final temperature of 1200 °C in air. A thermal treatment step under reductive atmosphere, described later in detail, was performed to transform the oxide into the electride.
For specimen with other dimensions for thermal and mechanical testing, the powder compacts were drypressed into shape with a pressure of 50 MPa and afterwards cold-isostatically compacted with 700 MPa. All samples made of the tape and after compaction were sintered under reductive atmosphere with 5 K/min heating ramp at a temperature of 1340 °C for 10 h dwell time.
The phase composition of the sintered glass ceramic was characterized using a powder X-ray diffractometer D8 Advance (Ltd.Bruker AXS) using Cu Ka radiation and a LynxEye position sensitive detector. The phase composition was determined by TOPAS software (V5, Ltd., BrukerAXS). The microstructure was visualized after ion beam polishing with a field assisted electron beam source microscope (NVision 40, Zeiss GmbH).
The specific resistance and the electron density (electron concentration) were determined using a Hall measurement system (RH2035, PhysTech GmbH) with van-der-Pauw geometry of the contacted specimen.
For contacting the electride ceramic, a silver-based paste was used, and the electron density was measured in a magnetic field of 0.8 T at a temperature of 22 °C.
The electron concentration was measured with a second method by determination of mass changes during oxidation. Therefore, thermogravimetric equipment (STA 449 C, NETZSCH-Gerätebau GmbH) was used for measuring the mass increase in air of a C12A7:e-sample during heating with 5 K/min to a temperature of 1200 °C and electron concentration was calculated. For the determination of the strength, more than 10 bending bars with 2.5 mm x 2.5 mm x >25 mm were cut and water-free ground with oil. The 4-point-bending tests were performed using an Inspekt

Accepted Article
This article is protected by copyright. All rights reserved  … thermal conductivity,  … thermal diffusivity,  … density (C12A7: 2.68 g cm -3 ), c p … heat capacity

Phase composition and microstructure
The phase analysis and the microstructure of the sintered C12A7:e-ceramic are presented in Figure 2. due to the sensitivity to moisture of the etching polish. Thermal etching was also not successful. Therefore, no information of the grain size distribution can be given.

Electronic properties
The C12A7:e-electride with 600 µm thickness was characterized by Hall method (Fig. 3). A specific resistance of 0.14 Ohm cm (conductivity σ of 7.1 S cm -1 ) and an electron concentration of 4.9 x 10 18 cm -3 are determined.

Accepted Article
This article is protected by copyright. All rights reserved On the first view, the measured values seem to be low in comparison to single crystals (2 x 10 21 cm -3 ; σ = 100 S cm -1 [11]). Kim et al. [15] measured a comparable conductivity (~4 S cm -1 ) but a higher electron concentration of 8 x 10 19 cm -3 by ESR method for sintered ceramics. Li and Xiao et al. [16,18]

Mechanical properties
For the first time, C12A7:e-electride ceramic were characterized in terms of mechanical properties. For measuring the hardness with Vickers indentation methods, the surface of the ceramics was dry-polished.
With a testing load of 49.1 N (HV5), the edge lengths of the indentation were not clearly measurable (Fig. 4a), because of brittle chipping of the ceramic. Therefore, the load was reduced to 9.81 N (HV1) without measurable indentation (Fig. 4b). After reducing the load to 4.91 N (HV0.5), the indentation was measurable

Accepted Article
This article is protected by copyright. All rights reserved we assume that after crystallization and densification of the parent glass powder compacts into a dense glass ceramic microstructure the glassy properties still predominate the fracture behavior. In a previous study it has been demonstrated that the parent glass crystallizes (accompanied by a volume increase) before the sintering process starts at 1050°C [2]. A strengthening effect caused by the crystalline phases, which is known from other types of glass ceramic materials is not observed.
To determine the 4-point bending strength, 20 bending bars were machined by cutting and dry grinding. Of these, only 10 were tested as they did not show any obvious damage due to chipping. The bending strength of σ = 75 ± 12 MPa (σ 0 = 90 MPa) is also lower than expected for a ceramic material (Fig. 5). Also, this low bending strength indicates a behavior of the C12A7:e-ceramic which is more typical for an oxide glass.

Thermal properties
The coefficient of thermal expansion (CTE) of the C12A7:e-electride ceramic was measured in a temperature range between 25 and 1000 °C in one cycle (Tab. 2, Fig. 6). Starting with 4.2 x 10 -6 K -1 at room temperature, the CTE is increasing to 6.0 x 10 -6 K -1 at T = 1000 °C. Mackey et al. [20] measured a CTE of 6 x 10 -6 K -1 (RT-600 °C) in the 1 st cycle and 7 x 10 -6 K -1 (RT-600 °C) in the 2 nd cycle for the C12A7:e-electride ceramic. To calculate the thermal conductivity, the thermal diffusivity and the heat capacity were determined. Figure 7 is presenting the thermal diffusivity of 1.0 -0.5 mm²s -1 and heat capacity of 0.8 -1.1 J g -1 K -1 for the C12A7:eceramic in the temperature range between 25 and 1000 °C.

Accepted Article
This article is protected by copyright. All rights reserved In comparison to Mackey et al. [20], our data of CTE and thermal conductivity are slightly lower. Independent of measurement inaccuracies, the composition and the electronic state of the samples could explain the differences in the measurements. Three main differences could be reasonable for influencing the thermal properties: (1) Mackey et al. [20] reported the contamination of the C12A7 with secondary phases ( normalized intensity (arb. unit) 2theta (°)

Accepted Article
This article is protected by copyright. All rights reserved

Accepted Article
This article is protected by copyright. All rights reserved

Accepted Article
This article is protected by copyright. All rights reserved