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

Abstract

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.

Synthesis of ultra-refractory transition metal diboride compounds

“…Dopants have been shown to modify the morphology of the crystalline phase in the oxide scale and reduce the oxygen permeation rate. [187][188][189] Dopants such as Ti, Nb, and Ta, form continuous solid solutions with ZrB2, 190 , 191 with solid solution formation limited by diffusion of TM atoms. Hence, activation energies for metal diffusion correlate to the ease of forming a solid solution.…”

Factorial design to minimize residual oxygen in reaction hot‐pressed zirconium diboride

“…Further, dissolved Group IVB, Group VB, or Group VIB transition metal diborides reduce the thermal conductivity of ZrB 2 . For example, the addition of W (commonly introduced during powder processing from milling media wear) reduces the room temperature thermal conductivity from ~90 W/(m·K) for nominally pure ZrB 2 to ~30 W/(m·K) for ZrB 2 containing ~3 at% W dissolved into the lattice . Similarly, hafnium impurities, which are ubiquitous in zirconium, also exhibit a drastic effect on thermal conductivity of ZrB 2 , reducing the thermal conductivity from 141 W/(m·K) for a Hf content of 0.01 at% to 88 W/(m·K) for an Hf content of 0.37 at% …”

Solute distributions in tantalum‐containing zirconium diboride ceramics

“…Lonergan et al showed that Hf impurity contents as low as 0.04 at% decreased thermal conductivity . Similarly, McClane et al showed that various (Zr,TM)B 2 solid solutions had lower thermal diffusivity and thermal conductivity values and higher electrical resistivity values as a result of many different TM additions …”

“…13,14 Similarly, McClane et al showed that various (Zr,TM)B 2 solid solutions had lower thermal diffusivity and thermal conductivity values and higher electrical resistivity values as a result of many different TM additions. 15,16 The effects of TM solid solution formation on the mechanical properties of ZrB 2 have also been examined. Silvestroni et al demonstrated that ZrB 2 -based ceramics with core-shell microstructures had strengths as high as 840 MPa at 1800°C.…”

“…Numerous studies of TM additions to ZrB 2 -based ceramics have used X-ray diffraction (XRD) as the main technique for characterizing solid solution formation. 15,16,21 Analysis of lattice parameter changes has been used to confirm dissolution of transition metal additions into the ZrB 2 lattice as well as the presence of any secondary phases. The magnitude of the changes in the lattice parameters can be used to estimate the TM content of solid solutions using Vegard's law.…”