Thermal properties of zirconium diboride ceramics

This presentation will focus on the thermal conductivity of zirconium diboride ceramics. Previous reports of thermal conductivity values for ZrB2 vary from as low as about 30 W/m•K to over 100 W/m•K without any direct evidence to identify the reasons for the variations. Our group systematically investigated the effects of transition metal impurities, which led to the discovery that the size of the dissolved impurity species was directly related to the decrease in thermal conductivity. Analysis of the electron contribution to thermal conductivity utilizing the Wiedemann-Franz methodology led to the conclusion that both the phonon and electron contributions were affected by dissolved metallic impurities. Further, the effects of some transition metals including Ti and Y were masked by other impurities in ceramics produced from commercial ZrB2 powders. Please click Additional Files below to see the full abstract

Mechanical properties of zirconium diboride ceramics

This presentation will focus on the mechanical properties of zirconium diboride ceramics. Diboride ceramics offer a combination of properties that include high elastic modulus, hardness, strength, and moderate fracture toughness to elevated temperatures. However, like all structural ceramics, their mechanical properties are controlled by microstructure wherein grain size, dispersion and size of second phases, and impurities limit their potential use at elevated temperatures, particularly for proposed extreme environment applications at temperatures exceeding 2000°C. As an example, the flexure strength of nearly phase pure ZrB2 ranges from 300 to \u3e600 MPa at room temperature but retains a strength of \u3e300 MPa at temperatures \u3e1500°C. Further, the fracture toughness of ZrB2 ceramics is generally low, typically in the range of 3 to 4 MPa-m1/2, at both room and elevated temperatures. Please click Additional Files below to see the full abstract

Mechanical andthermal properties of Zeta phase tantalum carbide atelevated temperatures

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Effects of transition metals on thermal properties of ZrB2

Nominally phase pure zirconium diboride ceramics were synthesized to study their intrinsic thermal properties. Ceramics for this study were synthesized by reaction hot pressing of reactor grade ZrH2 and B to minimize impurities commonly found in commercial powders such as the natural abundance (1-4 wt%) of Hf. Starting powders contained \u3c200 ppm Hf. Previous results showed that Hf impurities present in quantities comparable to commercial powders masked the effect of other transition metal additions. For example, additions of 3 at% Ti and Y had no apparent effect on thermal conductivity of ceramics produced from commercial ZrB2. Lowering the Hf content to 0.4 at% increased thermal conductivity from ~90 W/m•K for ZrB2 ceramics prepared from commercial powders to ~100 W/m•K for low-Hf content ZrB2 at 25 °C. Lowering the Hf content also increased the thermal conductivity at 2000°C from ~70 W/m•K to ~80 W/m•K. For the low Hf ZrB2, adding 3 at% TiB2 decreased thermal conductivity ~15 W/m•K at 25°C while adding 3 at% MoB2 decreased thermal conductivity ~45 W/m•K at 25°C. For the present study, transition metals such as Hf, Ti, Y, Ta, and W were added individually to nominally phase pure ZrB2 to study the effects on thermal diffusivity, thermal conductivity and heat capacity at temperatures from 25°C to 2000°C. These properties will be compared to values obtained for ceramics prepared from commercial ZrB2 powders, which contained the natural abundance of Hf. Most previous reports have relied on heat capacity values from the NIST-JANAF thermodynamic tables to calculate thermal conductivity of ZrB2 ceramics. However, the heat capacity of ZrB2 with low Hf content was approximately 10% greater than widely accepted values. Due to this difference, heat capacity will be measured for each composition, and these values will be used to calculate thermal conductivity. The intrinsic thermal properties of ZrB2will be discussed as well as the effect of transition metal additions on the thermal properties of ZrB2 with low and naturally abundant quantities of Hf

Mechanical properties of borothermally synthesized ZrB2

Mechanical properties of borothermally synthesized, highly pure ZrB2 were tested at room and elevated temperatures. Commercially available ZrB2 powder typically contains 1 to 4 wt % hafnium which has been shown to lower thermal properties of dense ZrB2 ceramics. Further, commercial grade ZrB2 contains other impurities (0.6 wt% O, 0.11 wt% N, 0.04 wt% Fe and others) which are also known to decrease its high-temperature mechanical strength. Purer grades of zirconia and boron powders, containing \u3c 75 ppm hafnium and \u3c0.5 wt% of other metal impurities, were reacted to produce ZrB2 for room and elevated temperature mechanical property studies. The zirconia and boron powders were reacted at 1000°C in a graphite vacuum furnace for two hours. The synthesized ZrB2 powder was then rinsed with methanol to remove boria from its surfaces, sieved with a #45 mesh, and hot pressed to near full density with 32 MPa applied pressure in a flowing argon atmosphere at 2100°C. The hot pressed billets were machined to ASTM standard test bars with the flexure surface polished to 1 um. Young’s modulus, Vickers Hardness, fracture toughness, and four-point bend strength were measured, and the results will be discussed

Influence of Nb coating on the oxidation behavior of ZrB2

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Plasticity of ZrB2 grains during micropillar compression: The effect of anisotropy, temperature and dislocations

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Mechanical Activation and Cation Site Disorder in MgAl2O4

The synthesis and crystallographic site occupancy were investigated for MgAl2O4 with and without mechanical activation of the precursor powders. Heating to 1200 °C or higher resulted in the formation of a single spinel phase regardless of whether the powders were mechanically activated or not. Neutron diffraction analysis was used to determine cation site occupancy and revealed that mechanical activation resulted in a lower degree of cation site inversion compared to the nonactivated materials, which indicated that the powders were closer to thermodynamic equilibrium. This is the first study to characterize the effects of mechanical activation on crystallographic site occupancy in magnesium aluminate spinel using neutron diffraction. © 2022 by the authors

Synthesis, densification, and cation inversion in high entropy (Co,Cu,Mg,Ni,Zn)Al2O4 spinel

The synthesis, densification behavior, and crystallographic site occupancy were investigated for four different spinel-based ceramics, including a high-entropy spinel (Co0.2Cu0.2Mg0.2Ni0.2 Zn0.2)Al2O4. Each composition was reacted to form a single phase, but analysis of X-ray diffraction patterns revealed differences in cation site occupancy with the high-entropy spinel being nearly fully normal. Densification behavior was investigated and showed that fully dense ceramics could be produced by hot pressing at temperatures as low as 1375°C for all compositions. Vickers’ hardness values were at least 10 GPa for all compositions. The cations present in the high-entropy spinel appear to have a stabilizing effect that led to nearly normal site occupancy compared to full cation inversion behavior of nickel aluminate spinel. This is the first report that compares cation site occupancy of a high-entropy spinel to conventional spinel ceramics

Super-strong materials for temperatures exceeding 2000 °C

Ceramics based on group IV-V transition metal borides and carbides possess melting points above 3000 °C, are ablation resistant and are, therefore, candidates for the design of components of next generation space vehicles, rocket nozzle inserts, and nose cones or leading edges for hypersonic aerospace vehicles. As such, they will have to bear high thermo-mechanical loads, which makes strength at high temperature of great importance. While testing of these materials above 2000 °C is necessary to prove their capabilities at anticipated operating temperatures, literature reports are quite limited. Reported strength values for zirconium diboride (ZrB₂) ceramics can exceed 1 GPa at room temperature, but these values rapidly decrease, with all previously reported strengths being less than 340 MPa at 1500 °C or above. Here, we show how the strength of ZrB₂ ceramics can be increased to more than 800 MPa at temperatures in the range of 1500–2100 °C. These exceptional strengths are due to a core-shell microstructure, which leads to in-situ toughening and sub-grain refinement at elevated temperatures. Our findings promise to open a new avenue to designing materials that are super-strong at ultra-high temperatures