Theoretical predictions on intrinsic lattice thermal conductivity of ZrB2

As a most important thermal management material, high thermal conductivity of ZrB₂ is expected. However, the reported values of thermal conductivity κ of ZrB₂ are quite scattering, and no consensus has been reached. The contribution from lattice separated by Wiedemann-Franz law is low and the relationship between electron and phonon contributions is still blurry. To explore the intrinsic κ of ZrB₂, in this work, two approaches, i.e. analytical Debye-Callaway model and iterative solution to the Boltzmann transport equation (BTE), are used to simulate the temperature-dependent theoretical lattice κ of ZrB₂. Our work demonstrates that the lattice thermal conductivity of ZrB₂ has been underestimated. The intrinsic lattice thermal conductivity of ZrB₂ is estimated to be 91 and 88 W m^?1 K^?1 at 300 K, by two different models, respectively. The effects of low lying optical phonon modes and grain boundary on the thermal conductivity of ZrB₂ are discussed. The thermal conductivity of ZrB₂ is controllable by designing effective grain size and microstructure. By casting light on the micro mechanism on lattice heat conduction of ZrB₂, our work will be constructive to the application of ZrB₂ as thermal management material.     

Effect of ball milling on reactive spark plasma sintering of B4C–TiB2 composites

The influence of mechanical activation by ball milling (BM) of Ti, B and graphite powders mixture on the synthesis of dense B₄C-41% vol. TiB₂ composite by Spark Plasma Sintering (SPS) is investigated. BM treatment produces grains size refinement (50–150 nm) in the processing powders and the formation of TiB and TiB₂, when milling times are longer than 6 h.     

High heterogeneous compatibility of HfB2-SiC ceramic and Zr-4 alloy with in-situ assembled interface

HfB₂-based ceramics, such as HfB₂-SiC, are promising materials of neutron control rods. Under nuclear conditions, the interfacial compatibility between the HfB₂-SiC control rod and the guide tube (made of Zr-4 alloy) is critical in the operation. The present study demonstrated the high compatibility of the two heterogeneous materials even at 1400 °C. The formation of an in-situ assembled triple diffusion layer with stable phases and dense structures largely improved the interface compatibility. In addition, the as-produced phases with unique microstructures were organized at the interface. ZrSi formed typical columnar crystals with strong [001] fiber texture, which extended in the direction favorable to the interface. ZrB₂ grains grew as needle shapes and entered the Zr-4 alloy substrate. These unique morphologies clearly revealed an interface with strong stability and high compatibility. The results provide the basis for the application of HfB₂-based ceramics in nuclear infrastructures.     

Electrical discharge machining of B4C-TiB2 composites

The interrelationships between the microstructure and electrical discharge machining (EDM) behaviour of B₄C-TiB₂ composites with respectively 30, 40 and 60 vol.% TiB₂ are investigated. Special attention was given to the influence of the grain size on the EDM behaviour by producing composites with an ultrafine TiB₂ phase using in situ synthesis during PECS. The experimental work revealed that 40 vol.% of TiB₂ results in an optimal material removal rate while the surface roughness for rough cut EDM decreases with increasing TiB₂ content. The finer microstructure of the ultrafine composite shows higher MRR's and lower R_a values than the commercial powder based composites. The major material removal mechanism for the PECS based composites was melting. The 3 point bending strength of all composites after grinding, EDM rough cut and EDM finish cut was not statistically different and about 800 MPa. The EDM recast layer was analysed by X-ray photoelectron spectroscopy.     

Phonon engineering in tuning the thermal conductivity of alkaline-earth hexaborides

Controlling the thermal conductivity of alkaline-earth hexaborides is of great importance for their applications as thermoelectric materials and ultrahigh temperature thermal insulation materials. However, no consensus has been reached on intrinsic κ, and the mechanism behind thermal reduction due to metal element incorporation is still blurry. In this work, the intrinsic thermal conductivity of three alkaline-earth hexaborides has been investigated theoretically. The simulated thermal conductivity of three borides shows good agreement with experiments. By examining mode contributions to total thermal conductivity, important role of low-lying optical modes in shaping the thermal transport property of hexaborides through controlling the interaction between acoustic and optical modes is uncovered. Our results demonstrated that tuning the thermal conductivity relies on shortening of mean free path of contributive phonon either by grain boundary scattering or by phonon-phonon interaction. The choice criterion on the doping atom to efficiently decrease thermal conductivity of alkaline-earth hexaborides is proposed.     

Characterization of fusion welded ceramics in the SiC-ZrB2-ZrC system

Various SiC-ZrB₂-ZrC ceramics were joined by fusion welding to determine the maximum silicon carbide content that could be joined. Commercial powders were hot pressed, machined, and preheated to 1450 °C before joining with a tungsten inert gas welding torch at 160–200 A. Resulting welds were cross-sectioned and analyzed to determine which compositions were weldable and to characterize microstructural evolution in welded samples. As compositions approached the ternary eutectic, the welds had smaller SiC grains and exhibited better weldability. Penetration depth of welds was controlled by a combination of current input and welding speed. The ternary eutectic in the system was found at 36.9 ± 1.3 vol% SiC, 42.7 ± 1.5 vol% ZrB₂, and 20.4 ± 1.9 vol% ZrC and its melting temperature was 2330 ± 23 °C. A ternary phase diagram for the SiC-ZrB₂-ZrC was constructed and proposed via microstructural analysis of arc melted pellets on binary joins between each binary eutectic and the ternary eutectic in the system.     

Enthalpies of formation of rare-earth borides from first principles. Comparison with experimental values

Calculations based on the density functional theory have achieved considerable reliability for the prediction of the properties of materials. In the present work, we have obtained enthalpies of formation of binary intermetallic compounds of boron with the rare-earth elements. These values have been calculated using the Vienna ab-initio simulation package in the generalized gradient approximation. In the Y-B system, we predict that Y₂B₅ in the mP28-Gd₂B₅ type structure is thermodynamically stable. We have compared the calculated enthalpies of formation of compounds to the values obtained with calorimetric methods. An excellent agreement is observed for some compounds: YB₄, LaB₆, LaB₄, PrB₄, and NdB₄. However, the experimental values of the formation enthalpies of the rare earth diborides are systematically less exothermic than the calculated values except for the ScB₂ compound where the experimental value is considerably more negative than the calculated one. The evolution of the enthalpies of formation along the series Sc, Y, La and La to Gd to Lu is discussed. The behaviour of the Y and Gd with respect to boron is found to be very similar. In a lesser extent, it is the same for the Sc and Lu with respect to boron.     

Design of wear resistant boron-modified supermartensitic stainless steel by spray forming process

In this paper the chemical composition of the supermartensitic stainless (SM) was modified with the addition of small boron contents (0.3, 0.5 and 0.7 wt.%) and processed by spray forming aiming the development of functionalized stainless steel with higher wear resistance. The addition of boron to the SM leads to the formation of continuous network of M₂B type borides uniformly distributed in the refined microstructure promoted by the spray forming process. The wear resistance was evaluated by two different methodologies: (1) the standardized dry sand/rubber wheel test (ASTM G65); and (2) a plate-on-cylinder (POC) wear test which was designed to simulate in laboratorial scale the tribosystems found in wear of risers and casings. It was shown that the wear mechanisms that take place in both tests are quite different, but in all cases increasing the boron content is always accompanied by an increase in the wear resistance. Electrochemical analyses were performed to evaluate the corrosion resistance of the designed alloys. It could be seen that corrosion properties similar to the commercial SM can be achieved in the SM modified with 0.7 wt.% of boron if an over content of chromium is added to the chemical composition.     

Destabilization of LiBH4 by MH2 (M = Ce, La) for hydrogen storage: Nanostructural effects on the hydrogen sorption kinetics

Destabilization of LiBH₄ by addition of metal hydrides or borohydrides is a powerful strategy to develop new promising hydrogen storage systems. In this study, we compare the destabilization behavior of the LiBH₄ by addition of MH₂ (M = La, Ce). A notable improvement in the hydrogen desorption temperature, the rate and the weight percentage of hydrogen released is observed for LiBH₄-MH₂ with respect to LiBH₄. Formation of LaB₆ and CeB₆ after dehydriding of the composites is proved by PXRD. Remarkable hydrogen storage reversibility of LiBH₄-MH₂ composites is confirmed under moderate conditions: 400 °C and 6.0 MPa of hydrogen pressure for 4 h without catalyst. The LiBH₄-LaH₂ composite exhibits improved hydrogen desorption performance compared with LiBH₄-CeH₂ composite, but the hydrogen storage reversibility is inferior. Notably, the LiBH₄-CeH₂ nanocomposite produced by in situ formation of CeH₂ from Ce(BH₄)₃-LiH displays excellent hydrogen storage properties. The addition of ZrCl₄ as a catalyst improves dehydriding kinetics. The mechanism underlying the enhancement in the LiBH₄-MH₂ composites is also discussed. Our study is the first work about reversible hydrogen storage in LiBH₄-LaH₂.