Thermal Properties of (Zr,TM)B2 Solid Solutions with TM = Ta,Mo, Re,V, and Cr

The thermal properties were investigated for hot‐pressed zirconium diboride containing solid solution additions of tantalum, molybdenum, rhenium, vanadium, and chromium. The nominal additions were equivalent to 3 at.% of each metal with respect to zirconium. Using 0.5 wt% carbon as a sintering aid, powders were hot‐pressed to near full density at 2150°C. Rietveld refinement of X‐ray diffraction data was used to measure lattice parameters and to ensure that the additives formed solid solutions. Thermal conductivities were calculated from measured thermal diffusivities and temperature‐dependent values for density and heat capacity. Thermal conductivities at 25°C ranged from 88 W·(m·K)^?1 for nominally pure ZrB₂ down to 28 W·(m·K)^?1 for (Zr,Cr)B₂. Electron contributions to thermal conductivity were calculated from electrical resistivity measurements using the Wiedemann–Franz law. Decreases in phonon and electron conduction correlated with the size of the metallic additive, indicating that changes in atom size in the Zr lattice positions reduced thermal transport.     

Zirconium Diboride with High Thermal Conductivity

Thermal properties were characterized for zirconium diboride produced by reactive hot pressing and compared to ZrB₂ ceramics that were hot pressed from commercial powders. No sintering additives were used in either process. Thermal conductivity was calculated from measured values of heat capacity, thermal diffusivity, and density for temperatures ranging from 298 to 2273 K. ZrB₂ produced by reactive hot pressing achieved near full density, but had a small volume fraction of ZrO₂, whereas hot‐pressed ZrB₂ contained porosity and carbon inclusions. Reactive hot pressing produced a ceramic with higher thermal diffusivity and heat capacity, resulting in thermal conductivities of 127 W·(m·K)^?1 at 298 K and 80 W·(m·K)^?1 at 2273 K, which were up to ~30% higher than typically reported for hot‐pressed ZrB₂.     

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2–SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB₂ with different amounts of B₄C (0–5 wt%) and ZrB₂–SiC composites (10–30 vol% SiC) were investigated experimentally over a wide range of temperature (25–1500 °C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB₂ and ZrB₂–SiC composites. At around 200 °C, thermal conductivity of ZrB₂–SiC composites was found to be composition independent. Thermal conductivity of ZrB₂–SiC composites was also correlated with theoretical predictions of the Maxwell–Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB₂ and ZrB₂–SiC composites at room temperature were confirmed by Wiedemann–Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB₂ whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB₂–SiC composites.     

Processing and characteristics of transparent Gd3TaO7 polycrystalline ceramics

Transparent polycrystalline Gd₃TaO₇ ceramics were successfully developed. A sol‐gel process was used to synthesize Gd₃TaO₇ powder with a uniform composition and an estimated average particle size of 100 nm. Simultaneous thermal gravimetric analysis and differential thermal analysis (TGA/DTA) was used to identify the decomposition sequence as a function of temperature for the as‐synthesized sol‐gel powders. Crystallization was confirmed by X‐ray diffraction (XRD) and a single phase was achieved by calcining at 1000°C. The calcined powders were hot‐pressed at 1400°C to achieve >96% theoretical density with closed pore structure followed by a hot isostatic pressing at 1400°C at 207 MPa to achieve a fully dense structure. Microstructural characterization shows a uniform grain size distribution with an average grain size of about 7 μm. In‐line transmission measurements revealed high transparency in the red and infrared. Thermal conductivity was measured to be >1.6 W/mK at room temperature, decreasing to ~1.3 W/mK by 500°C. Dielectric properties remain stable with relative permittivity values just above 200 and loss tangents <0.005 up to 350°C.     

Transparent polycrystalline Gd2Hf2O7 ceramics

We have successfully developed transparent polycrystalline Gd₂Hf₂O₇ ceramics with high in‐line transparency. A sol–gel process was used to synthesize the Gd₂Hf₂O₇ powder. Simultaneous thermal gravimetric analysis and differential thermal analysis (TGA/DTA) was used to identify the decomposition sequence as a function of temperature for the as‐synthesized sol–gel powders. The calcined powder is single phase and was formed with an estimated average particle size of 120 nm. Crystallization was confirmed by x‐ray diffraction (XRD) and a single phase was achieved by calcining at 1000°C. The calcined powders were hot‐pressed at 1500°C to achieve >95% theoretical density with closed pore structure followed by a hot isostatic pressing at 1500°C at 207 MPa to achieve a fully dense structure. Microstructural characterization shows a uniform grain size distribution with an average grain size of about 11 μm. In‐line transmission measurements revealed high transparency in the red and infrared. Dielectric properties remain stable with relative permittivity values around 180 and loss tangents less than 0.005 up to 350°C. Thermal conductivity was measured to be ~1.8 W/m°K at room temperature, decreasing to ~1.5 W/m°K by 500°C.     

Light‐weight thermal insulating fly ash cenosphere ceramics

Light weight fly ash cenosphere (FAC) ceramic composites were developed by simple slip casting method. Thermal properties, Bulk density, Microstructure, flexural strength, and phase analysis of the FAC ceramic composites were measured. The results proved that the FAC have ability to lower bulk density and thermal conductivity effectively. The lowest thermal conductivity achieved for FAC ceramic composites (0.27 W/m.K) was further reduced 0.21 W/m.K by adding combustible additives ie activated charcoal and corn starch. The flexural strength, bulk density and thermal conductivity of FAC ceramic composites reduced consistently with an increase in FAC content. The maximum flexural strength of 13.45 MPa was achieved with 50% FAC and the minimum flexural strength of 4.07 MPa was obtained with 80% FAC. The open porosity increased from 35.51% to 43.76% and 38.19% with an addition of 15% activated charcoal and corn starch, respectively, when compared to no additives. The bulk density of 699, 619, and 675 kg/m³ was achieved with 80% FAC, 80% FAC with the addition of 15% activated charcoal and corn starch, respectively. The 80% FAC ceramic composite shows low thermal expansion coefficient 6.54 × 10^?6/°C at the temperature of 50°C then it varies between 3.7 and 5 × 10^?6/°C in the temperature range above 100°C. These results prove that the developed light weight FAC ceramic are excellent low‐cost thermal insulating materials.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Thermal properties of processed lunar regolith simulant

In this paper, the thermal characterization of lunar regolith simulant, sintered using a conventional oven under ambient and vacuum conditions is presented. Additionally, thermal characterization of samples is performed for the parts manufactured using solar, laser, and microwave processing. Samples for oven sintering are prepared using regolith bulk simulant as well as simulant pressed at 255 MPa for 10 minutes. Similar experiments are performed with a mixture of Johnson Space Center (JSC)-2A + 20 wt% of ilmenite, a common lunar mineral. Samples are characterized regarding their thermal capacity, thermal diffusivity, density, and thermal conductivity. Furthermore, sample morphology is studied using scanning electron microscopy. Lastly, processing of regolith for lunar thermal energy storage is discussed.     

Synthesis and thermophysical properties of RETa3O9 (RE = Ce,Nd, Sm,Eu, Gd,Dy, Er) as promising thermal barrier coatings

Thermal barrier coatings (TBCs) are one of the most important materials in gas turbine to protect the high temperature components. RETa₃O₉ compounds have a defect‐perovskite structure, indicating that they have low thermal conductivity, which is the critical property of TBCs. Herein, dense RETa₃O₉ bulk ceramics were fabricated via solid‐state reaction. The crystal structure was characterized by X‐ray diffraction (XRD) and Raman Spectroscope. Scanning electron microscope (SEM) was used to observe the microstructure. The thermophysical properties of RETa₃O₉ were studied systematically, including specific heat, thermal diffusivity, thermal conductivity, thermal expansion coefficients, and high‐temperature phase stability. The thermal conductivities of RETa₃O₉ are very low (1.33‐2.37 W/m·K, 373‐1073 K), which are much lower than YSZ and La₂Zr₂O₇; and the thermal expansion coefficients range from 4.0 × 10^?6 K^?1 to 10.2×10^?6 K^?1 (1273 K), which is close to La₂Zr₂O₇ and YSZ. According to the differential scanning calorimetry (DSC) curve there is not phase transition at the test temperature. Due to the high melting point and excellent high‐temperature phase stability with these oxides, RETa₃O₉ ceramics were promising candidate materials for TBCs.     

Novel SiC/C composite targets for the production of radioisotopes for nuclear applications

A partially porous SiC ceramic, reinforced with 30 vol% short carbon fibers, was hot pressed and characterized as potential ISOL target for nuclear applications. Powder milling and hot pressing were effective for the realization of a ceramic with about 40% interconnected porosity in the 0.6–0.8 µm size range. A fiber-free porous SiC material was also synthesized for the sake of comparison. Compression strength of the fiber-rich SiC passed from about 200 MPa at room temperature to about 120 MPa upon testing at 1200 °C. The thermal conductivity was higher than the fiber-free SiC and other state-of-art ISOL target materials and was 48 W/m·K at 600 °C and decreased to 17 W/m·K at 1400 °C, owing to the porosity. Remarkably, this fiber-rich ceramic in form of thin disk, possessed suitable thermo-mechanical behavior to successfully withstand a 350 °C thermal gradient without failure.     

Comparison of Two‐Phase Thermal Conductivity Models with Experiments on Dilute Ceramic Composites

Thermal shock resistance of cubic 8 mol% yttria‐stabilized zirconia (YSZ) can be increased by the addition of dilute second phases. This study addresses how these dilute second phases affect the thermal conductivity for two‐phase ceramic composites of 8 mol% YSZ with 10–20 vol% alumina (Al₂O₃) or 10–20 vol% mullite (3Al₂O₃·2SiO₂). Thermal conductivity measurements from 310 K (37°C) to 475 K (202°C) were made using the 3ω method and compared with results from 3D analytical models and a 2D computational microstructure‐based model (Object‐Oriented Finite Element Analysis, OOF2). The linear Rule of Mixtures was the least accurate and significantly overestimated the measured thermal conductivity at low temperatures, with errors in some cases exceeding 100%. Calculations using the Bruggeman and OOF2 models were both much better, and the deviation of less than ±2.5% across all compositions and temperatures is within the range of experimental and modeling uncertainty. The Maxwell Garnett equation was a close third in accuracy (±8%). A sensitivity analysis for each model quantifies how small perturbations in the thermal conductivity of the dispersed second phase influence the effective thermal conductivity of the composite, and reveals that the linear Rule of Mixtures model is physically unrealistic and oversensitive to the thermal conductivity of the dispersed phase.     

Investigation on low-temperature sintered AlN nanoceramics with high thermal conductivity

The aim of this research was to investigate the effect of sintering additive and relatively low-sintering temperature on the thermal conductivity of aluminum nitride nanoceramic. While using nanosized AlN powder and liquid-phase sintering additives, the various sintering processes were performed at temperatures 1400 and 1500°C for several hours. In the analysis methods, thermal conductivity (K) and thermal diffusivity (α) were measured using thermal conductivity analyzer (Hot Disk), scanning electron microscope (SEM) was used to observe the surface morphology of the microstructure, x-ray diffraction analyzer (XRD) to analyze the grain size and crystal structure, Raman spectroscopy (Raman) emission spectrum was analyzed to identify the material microstructure and the densities of AlN specimens were measured by Archimedes method. It was found that the thermal conductivity is related to the densification of nanosize low-temperature sintered material, which can be controlled by additives and sintering temperature. With Y₂O₃ sintering add, the densification of AlN for low-temperature sintering increased by the factor of ～5% to ～12%, and the thermal conductivity was enhanced by 25%. The relative density observed in this research is about 78%-84%, and the thermal conductivity measured is in the range of 9-14 W/mK.     

Microstructure and Thermal Conductivity of Silicon Carbide with Yttria and Scandia

A fully dense SiC ceramic with high thermal conductivity was obtained by conventional hot pressing, with 1 vol% Y₂O₃–Sc₂O₃ additives. The ceramic had a bimodal microstructure consisting of large and small equiaxed SiC grains. Observation with high‐resolution transmission electron microscopy (HRTEM) showed two kinds of homophase (SiC/SiC) boundaries, that is crystallized and clean boundaries, and a fully crystallized junction phase. The thermal conductivity of the SiC ceramic was 234 W (m·K)^?1 at room temperature. The high thermal conductivity was attributed to a clean SiC lattice and good contiguity between SiC grains.     

An Evaluation of the Thermophysical Properties of Stoichiometric CeO2 in Comparison to UO2 and PuO2

The thermal conductivity of stoichiometric CeO₂ was determined through measurement of thermal expansion from 313 to 1723 K, thermal diffusivity from 298 to 1473 K, and specific heat capacity from 313 to 1373 K. The thermal conductivity was then calculated as the product of the density, thermal diffusivity, and specific heat capacity. The thermal conductivity was found to obey an (A + BT)^?1 relationship with A = 6.776×10^?2 m·K·W^?1 and B = 2.793 × 10^?4 m·W^?1. Extrapolations of applied models were made to provide suggested data for the specific heat capacity, thermal diffusivity, and thermal conductivity data up to 1723 K. Results of thermal expansion and heat capacity measurements agreed well with the limited low‐temperature data available in the literature. The thermal conductivity values provided in the current study are significantly higher than the only high‐temperature data located for CeO₂. This is attributed to the tendency of CeO₂ to rapidly reduce at elevated temperatures given the available partial pressure of O₂ in air at ambient pressure. The CeO₂ data are compared to literature values for UO₂ and PuO₂ to evaluate its suitability as a surrogate in nuclear fuel systems where thermal transport is a primary criterion for performance     

Thermal Properties of Transparent Yb‐Doped YAG Ceramics at Elevated Temperatures

This work presents the thermal properties of ytterbium‐doped yttrium aluminium garnet (Yb:YAG) transparent ceramics at elevated temperatures in dependence on the dopant concentration and on temperature. Transparent polycrystalline Yb:YAG ceramics were prepared by solid‐state reaction of oxide powders sintered under high vacuum. The dopant amount varied from 0 to 20 at.% of Yb. Thermal diffusivity of the sintered samples was measured by the laser and xenon flash methods at temperatures ranging from room temperature to 900°C. Both the thermal diffusivity and thermal conductivity values decreased with increasing dopant content, and until 500°C a decrease was observed also with increasing temperature. When available, the measured values were compared to data published in literature, and were found to be in good agreement. Based on the measured values, empirical relations in the form of shifted power laws are proposed for the temperature dependence of thermal diffusivity.     

Measurements of the thermal conductivity and thermal diffusivity of polymers

In this paper, the thermal conductivity and thermal diffusivity of nine polymers were measured by using the transient short‐hot‐wire method. The corresponding specific heat was measured with a commercial Differential Scanning Calorimeter (DSC). The effects of temperature on the thermal conductivity, thermal diffusivity, and the product of density and specific heat are further discussed. The results show that the transient short‐hot‐wire method can be used to measure the thermal conductivity, thermal diffusivity, and the product of density and specific heat of polymers within uncertainties of 3%, 6%, and 9%, respectively.     

Thermal properties of HfN-MoW surrogate cermet fuel for nuclear thermal propulsion

The thermal properties of MoW-HfN, a surrogate cermet for MoW-UN nuclear thermal propulsion fuel, were characterized over a wide range of elevated temperatures. Thermal diffusivity, coefficient of thermal expansion (CTE), and heat capacity were measured. Optical and scanning electron microscopy were performed to characterize the microstructure and draw structure–property correlations. The thermal diffusivity was obtained using the laser flash method. Diffusivity values ranged from about 0.18 cm²/s at 200°C to 0.15 cm²/s at 1800°C. The CTE was measured using push-rod dilatometry up to 1600°C, giving values between 6.0 and 9.0 μm/m. A scientific rationalization of the effective material properties is made using the rule-of-mixtures and other effective properties models.     

Thermal and electrical properties of a high entropy carbide (Ta,Hf, Nb,Zr) at elevated temperatures

The thermal and electrical properties were measured for a high entropy carbide ceramic, consisting of (Hf, Ta, Zr, Nb)C. The ceramic was produced by spark plasma sintering a mixture of the monocarbides and had a relative density of more than 97.6%. The resulting ceramic was chemically homogeneous as a single-phase solid solution formed from the constituent carbides. The thermal diffusivity (0.045–0.087 cm²/s) and heat capacity (0.23–0.44 J/g•K) were measured from room temperature up to 2000°C. The thermal conductivity increased from 10.7 W/m•K at room temperature to 39.9 W/m•K at 2000°C. The phonon and electron contributions to the thermal conductivity were investigated, which showed that the increase in thermal conductivity was predominantly due to the electron contribution, while the phonon contribution was independent of temperature. The electrical resistivity increased from 80.9 μΩ•cm at room temperature to 114.1 μΩ•cm at 800°C.     

Colloidal Processing of Zirconium Diboride Ultra‐High Temperature Ceramics

Colloidal processing of the Ultra‐High Temperature Ceramic (UHTC) zirconium diboride (ZrB₂) to develop near?net‐shaping techniques has been investigated. The use of the colloidal processing technique produces higher particle packing that ultimately enables achieving greater densification at lower temperatures and pressures, even pressureless sintering. ZrB₂ suspension formulations have been optimized in terms of rheological behavior. Suspensions were shaped into green bodies (63% relative density) using slip casting. The densification was carried out at 1900°C, 2000°C, and 2100°C, using both hot pressing at 40 MPa and pressureless sintering. The colloidally processed materials were compared with materials prepared by a conventional dry processing route (cold pressed at 50 MPa) and subjected to the same densification procedures. Sintered densities for samples produced by the colloidal route are higher than produced by the dry route (up to 99.5% relative density by hot pressing), even when pressureless sintering is performed (more than 90% relative density). The promising results are considered as a starting point for the fabrication of complex‐shaped components that can be densified at lower sintering temperatures without pressure.     

Photoacoustic thermal characterization of electrical porcelains: effect of alumina additions on thermal diffusivity and elastic constants

The open-photoacoustic-cell technique to measure thermal diffusivity is described in detail. We have applied it to the measurement of thermal diffusivity of porcelain samples with four different alumina additions in the range 0–15 wt% and fired at four temperatures in the range 1270–1350°C. Thermal diffusivity is shown to vary from 4·1×10⁻⁷ m² s⁻¹ for a classical triaxial porcelain to 6·4×10⁻⁷ m² s⁻¹ for 15 wt% alumina addition made mainly at the expense of quartz when the firing temperature was 1325°C. The values of thermal diffusivity are found to be well correlated with the values of the shear and Young's modulus. Good correlation was also observed among the measured values for thermal diffusivity and those for density and mullite to quartz ratio. We conclude that the open-photoacoustic-cell technique has enough sensitivity to detect small changes in composition and microstructure of materials as complex as porcelains.