Diboride Ceramics Research Articles

Nanostructures from Zirconium Diboride and Alumina Ceramics

In this paper, we describe the observation of nanostructure formation under microwave processing of zirconium diboride powder with aluminum oxide. These nanostructures appear to be formed from arcing resulting from a microwave-induced electric field across conductive ZrB2 particles. Microwave heating allows process times <1 min. The arcing rapidly heated the ZrB2 and nearby alumina creating nanotubes and nanorods. The morphology of these nanostructures was characterized using scanning and transmission electron microscopy (TEM), showing ∼100 nm tube wall thicknesses with widths up to a few micrometer and lengths to 40 μm. Energy-dispersive X-ray spectroscopy showed the composition of the nanotubes included aluminum, oxygen, zirconium, and boron. Fast Fourier transform's of the TEM images provided characterization of the lattice parameters. Morphology, composition and crystallography resemble both single-crystal aluminum borate and boron-mullite nanotubes. The produced nanostructures could be used to reinforce high-temperature materials, improve the durability of metal surfaces or as needles for intracellular drug delivery.

Effect of thermal mismatch induced residual stresses on grain boundary microcracking of titanium diboride ceramics

The effect of thermal mismatch induced residual stresses on grain boundary microcracking in titanium diboride (TiB2) ceramics has been studied by finite element method. A cohesive zone model was used to simulate the microcracking initiation in four-point bending specimens. In particular, the microcracking was assumed to occur at a grain boundary which is located in the center of the specimen, surrounded by a thermally anisotropic area. The predicted failure strength appears to be significantly reduced by the presence of residual stresses when the cohesive energy of the microstructure is small. The failure load from experiments has been used to determine the critical damage parameters for microcracking initiation in both pristine and aluminum-infiltrated TiB2. A viscous regularization technique is employed in the simulations to improve the rate of convergence of the solution and the effect of the value of the viscosity parameter on the simulation results, has been investigated. The effect of grain size, grain orientation, and number of employed thermally anisotropic grains, on the microcracking is also discussed.

Pressureless sintering mechanisms and mechanical properties of hafnium diboride ceramics with pre-sintering heat treatment

A pressureless sintering process with two pre-sintering heat treatments was developed for the densification of HfB2. The use of two isothermal holdings, at 1650 and 2000 °C, prior to the final sintering temperature (2200 °C) proved to be more effective at removing oxygen impurities and rearranging the initial powder compact microstructure. The mechanical properties of HfB2 with 2 wt.%B4C, including hardness (19.5 GPa), E modulus (529 GPa) and strength (469 MPa), were comparable to hot-pressed samples or higher than the values using other sintering aids.

Reactive hot pressing of zirconium diboride

The reaction of zirconium and boron was investigated as a potential route to form dense monolithic zirconium diboride (ZrB 2) ceramics. Attrition milling of the precursors produced nanosized (less than 100 nm) zirconium metal particles that reacted with boron to form ZrB 2 with an average particle size of less than 100 nm at temperatures as low as 600 °C. Scanning electron microscopy of ZrB 2 compacts heated to 1450 °C and 1650 °C showed average particle sizes of 0.6 μm and 1.0 μm, respectively, suggesting that the fine particle size was maintained during densification. Ceramics with a relative density of ∼99% were produced by hot pressing at 2100 °C. Dense ZrB 2 produced by the reactive hot pressing process had mechanical properties that were comparable to ceramics produced by conventional processes. The four-point flexure strength of ZrB 2 produced in this study was 434 MPa.

Temperature dependent hardness and strength properties of TiB 2 with TiSi 2 sinter-aid

From the perspective of high temperature structural applications, it is important to evaluate temperature dependent mechanical properties of titanium diboride (TiB 2) ceramics. The present study reports the effect of TiSi 2 content (up to 10 wt.%) and temperature on hardness and strength of TiB 2. The hardness properties were measured from room temperature (RT)—900 °C in vacuum; the four-point flexural strength properties were evaluated at selected temperatures in air up to 1000 °C. An attempt has been made to discuss the difference in hardness and strength properties with sinter-aid amount and microstructure. Our experimental results clearly indicated that the addition of 2.5 wt.% TiSi 2 to TiB 2 resulted almost full densification at a lower hot pressing temperature of 1650 °C without compromising on the high temperature strength and hardness properties. The hot pressed TiB 2–2.5 wt.% TiSi 2 ceramic could retain moderate strength of more than 400 MPa and hardness of 9 GPa at 1000 °C and 900 °C, respectively.

Improved Oxidation Resistance of Zirconium Diboride by Tungsten Carbide Additions

The effect of tungsten carbide (WC) additions on the oxidation resistance of zirconium diboride (ZrB2) was studied. Zirconium diboride ceramics, nominally pure and with the addition of 4 mol% WC, were densified by pressureless sintering. During densification, the WC that was added to the ceramic dissolved into the ZrB2 matrix, forming a solid solution. Oxidation behavior was evaluated using thermal gravimetric analysis up to 1500°C and isothermal furnace oxidation at 1500° and 1600°C. The WC additions reduced both the weight gain and the oxide scale thickness. After oxidation at 1600°C for 3 h, the mass gain decreased from 22 mg/cm2 for nominally pure ZrB2 to 8 mg/cm2 for the WC‐containing ceramic. Analysis showed that the WC additions improved the oxidation resistance of ZrB2 by promoting densification of the zirconia scale formed during oxidation through liquid‐phase sintering.

High-strength porous titanium diboride ceramics: Microstructure and mechanical properties

Porous TiB2 ceramics have been fabricated by compacting TiB2 powders with cold isostatical pressing, followed by pressureless sintering. The microstructural feature, relative density, flexural strength, and fracture toughness were investigated. The experimental results revealed that the porosity of the fabricated porous TiB2 was controllable between ∼ 50 and ∼ 84 per cent depending on the sintering temperature and initial compacted pressures. The flexural strength and fracture toughness were improved with the increased relative density. The improved mechanical properties of the sintered porous TiB2 were attributed to the enhanced neck bonding and growth.

Cohesive zone modeling of grain boundary microcracking induced by thermal anisotropy in titanium diboride ceramics

This paper addresses the residual stresses and their effect on microcracking in polycrystalline ceramic materials. Residual stresses at microstructural level in titanium diboride ceramics, as a result of thermal expansion anisotropy, were analyzed by finite element method using Clarke’s model. Damage mechanics based cohesive zone model was applied to study grain boundary microcracking, propagation and arrest. Quantitative relations between temperature variation, grain boundary energy, grain size, final microcrack length as well as microcracking temperature are established.

Pressureless Sintering of ZrB2–SiC Ceramics

A pressureless sintering process was developed for the densification of zirconium diboride ceramics containing 10–30 vol% silicon carbide particles. Initially, boron carbide was evaluated as a sintering aid. However, the formation of a borosilicate glass led to significant coarsening, which inhibited densification. Based on thermodynamic calculations, a combination of carbon and boron carbide was added, which enabled densification (relative density &gt;98%) by solid‐state sintering at temperatures as low as 1950°C. Varying the size of the starting silicon carbide particles allowed the final silicon carbide particle morphology to be controlled from equiaxed to whisker‐like. The mechanical properties of sintered ceramics were comparable with hot‐pressed materials with Vickers hardness of 22 GPa, elastic modulus of 460 GPa, and fracture toughness of ∼4 MPa·m1/2. Flexure strength was ∼460 MPa, which is at the low end of the range reported for similar materials, due to the relatively large size (∼13 μm long) of the silicon carbide inclusions.

Homogeneous TiB2 Ceramics Achieved by Electric Current‐Assisted Self‐Propagating Reaction Sintering

Using spark plasma sintering techniques, homogeneous microstructures of titanium diboride (TiB2) ceramics were obtained by sintering of boron and titanium powder mixtures. The results show that an additional electric current is essential for achieving a large number of evenly distributed ignition points that ensure that the self‐propagating reaction simultaneously takes place within the entire volume. The effects of the electric current, the use of Mg additions, and the heating rates on the resulting TiB2 ceramic densities and microstructures are discussed.

Refractory Diborides of Zirconium and Hafnium

This paper reviews the crystal chemistry, synthesis, densification, microstructure, mechanical properties, and oxidation behavior of zirconium diboride (ZrB2) and hafnium diboride (HfB2) ceramics. The refractory diborides exhibit partial or complete solid solution with other transition metal diborides, which allows compositional tailoring of properties such as thermal expansion coefficient and hardness. Carbothermal reduction is the typical synthesis route, but reactive processes, solution methods, and pre‐ceramic polymers can also be used. Typically, diborides are densified by hot pressing, but recently solid state and liquid phase sintering routes have been developed. Fine‐grained ZrB2 and HfB2 have strengths of a few hundred MPa, which can increase to over 1 GPa with the addition of SiC. Pure diborides exhibit parabolic oxidation kinetics at temperatures below 1100°C, but B2O3 volatility leads to rapid, linear oxidation kinetics above that temperature. The addition of silica scale formers such as SiC or MoSi2 improves the oxidation behavior above 1100°C. Based on their unique combination of properties, ZrB2 and HfB2 ceramics are candidates for use in the extreme environments associated with hypersonic flight, atmospheric re‐entry, and rocket propulsion.

Low‐Temperature Densification of Zirconium Diboride Ceramics by Reactive Hot Pressing

Zirconium diboride–silicon carbide ceramics with relative densities in excess of 95% were produced by reactive hot pressing (RHP) at temperatures as low as 1650°C. The ZrB2 matrix was formed by reacting elemental zirconium and boron. Attrition milling of the starting powders produced nanosized (&lt;100 nm) Zr particulates that reacted with B below 600°C. The reaction resulted in the formation of nanoscale ZrB2 crystallites that could be densified more than 250°C below the temperatures required for conventional ZrB2 powder. Because of the low‐temperature densification, the resulting ZrB2 grain sizes were as small as 0.5±0.30 μm for specimens densified at 1650°C and 1.5±1.2 μm for specimens densified at 1800°C. Vickers hardness, elastic modulus, and flexure strength of fully dense materials produced by RHP were 27, 510, and 800 MPa, respectively.

Pressureless Sintering of Zirconium Diboride

Zirconium diboride (ZrB2) ceramics were sintered to a relative density of ∼98% without applied external pressure. Densification studies were performed in the temperature range of 1900°–2150°C. Examination of bulk density as a function of temperature revealed that shrinkage started at ∼2100°C, with significant densification occurring at only 2150°C. At 2150°C, isothermal holds were used to determine the effect of time on relative density and microstructure. For a hold time of 540 min at 2150°C, ZrB2 pellets reached an average density of 6.02±0.04 g/cm3 (98% of theoretical) with an average grain size of 9.0±5.6 μm. Four‐point bend strength, elastic modulus, and Vickers' hardness were measured for sintered ZrB2 and compared with values reported for hot‐pressed materials. Vickers' hardness of sintered ZrB2 was 14.5±2.6 GPa, which was significantly lower when compared with 23 GPa for hot‐pressed ZrB2. Strength and elastic modulus of the ZrB2 were 444±30 MPa and 454 GPa, which were comparable with values reported for hot‐pressed ZrB2. The ability to densify ZrB2 ceramics without hot pressing should enable near‐net shape processing, which would significantly reduce the cost of fabricating ZrB2 components compared with conventional hot pressing and machining.

Wetting and spreading of liquid metals on ZrB2-based ceramics

The possibility to exploit commercially the peculiar characteristics of refractory metallic and ceramic materials and in particular of Zirconium diboride ceramics—a class of promising materials for high temperature applications—often depends to a great extent on the ability to join different ceramics one to the other or to special metallic alloys. As the behaviour of a metal-ceramic joint is ruled by the chemical and the physical properties of the interface, the knowledge of wettability, interfacial tensions and interfacial reactions is mandatory to understand what happens at the liquid metal-ceramic interface during joining processes.

Superconducting Characteristics of Polycrystalline Magnesium Diboride Ceramics Fabricated by a Spark Plasma Sintering Technique

Highly densified MgB2 superconductors were successfully fabricated using a spark plasma sintering (SPS) technique, and their superconductivity with respect to microstructural evolution was evaluated. Full densification with final density close to the theoretical density was achieved at a temperature of 1000°C within a total SPS processing time of 40 min. Both an MgB2 specimen sintered at 1000°C for 30 min and one sintered at 1050°C for 10 min exhibited a high critical transition temperature (Tc) similar to that of an MgB2 single crystal (39 K), and a very sharp superconducting transition width (ΔT) less than 0.5 K. In addition, high critical current densities (Jc) of 7.7 × 105 A/cm2 in a field of 0.6 T at 5 K and of 8.3 × 104 A/cm2 in a field of 0.09 T at 35 K were obtained. These excellent superconducting characteristics of the SPS‐processed MgB2 are attributed to uniformly distributed secondary MgO phase nanoparticles and well‐developed dislocations in the microstructure that may act effectively as extrinsic flux pinning sites, resulting in the strong pinning force showing a high Jc of 8.7 × 104 A/cm2 even in the condition of a field of 4 T at 5 K.

Wettability of zirconium diboride ceramics by Ag, Cu and their alloys with Zr

Sintered ZrB 2 ceramics, pure and with 4 wt.% Ni as sintering aid, have been tested in contact with liquid Ag, Cu, Ag–Cu and Ag–Cu–Zr. “Pure” ZrB 2 ceramics are wetted by Ag–Zr alloys, and ZrB 2/Ni ceramics also by pure Cu. The wetting behaviour is briefly discussed in terms of existing wetting theories.

Interfacial Reactions between Titanium Nitride-Titanium Diboride Ceramics and Nickel

Interfacial Reactions between Titanium Nitride-Titanium Diboride Ceramics and Nickel

Pressureless Sintering and Properties of Titanium Diboride Ceramics Containing Chromium and Iron

The simultaneous addition of 0.5 wt% Cr and Fe was found to enhance the densification of TiB2. The densities of specimens that were sintered for 2 h at 1800° and 1900°C were 97.6% and 98.8% of the theoretical value, respectively. The mechanical properties of the specimen sintered at 1800°C, which had a strength of 506 MPa and a fracture toughness of 6.16 MPa·m1/2, were much better than those observed in the specimen sintered at 1900°C.

Contribution of recoil atoms to irradiation damage in absorber materials

Absorbing materials are used to control the reactivity of nuclear reactors by taking advantage of nuclear reactions (e.g., 10B(n,α) 7Li) where neutrons are absorbed. During such reactions, energetic recoils are produced. As a result, radiation damage in absorbing materials originates both from these nuclear reactions and from elastic collisions between neutrons and atoms. This damage eventually leads to a partial destruction of the materials, and this is the main limitation on their lifetime in nuclear reactors. Using a formalism developed to calculate displacements per atoms (dpa) in a multi atomic target, we have calculated damages in terms of displacements per atom in a (n,α) absorbing material taking into account geometrical effects of 10 boron self shielding and transmutation reactions induced by neutrons inside the absorber. Radiation damage is calculated for boron carbide and hafnium diboride ceramics in a Pressurized Water Reactor environment. It is shown that recoils produced by nuclear reactions account for the main part of the radiation damage created in these ceramics. Damages are calculated as a function of the distance from the center of an absorber pellet. Due to the self-shielding effect, these damage curves exhibit sharp maxima, the position of which changes in time.

Tonically active striate neurons acquire new activity through dopamine and gaba receptor-mediated mechanisms during behabioral learning

The high-temperature applications of zirconium diboride (ZrB2) ceramics are hindered due to their easy initial oxidation at temperature around 700 °C. Although mixing and coating methods have been introduced to enhance the oxidation resistance of ZrB2-based ceramics, their thorough comparative analysis still demands a more systematic exploration. Herein, spark plasma sintered ZrB2@20 vol% LaF3 and ZrB2+20 vol% LaF3 ceramics prepared by coating and mixing powders, respectively, were used to study the influence of dispersion method of LaF3 on their oxidation resistance. After oxidation at 1000 °C for 4 h, a thin oxide layer (~25 μm) and low weight gain (9%) confirmed the superior oxidation resistance of ZrB2+20 vol% LaF3 over ZrB2@20 vol% LaF3, which could be attributed to the formation of a dense surface layer due to the self-healing mechanism. In turn, the poor oxidation resistance of ZrB2@20 vol% LaF3 was caused by the “anchoring” of LaF3 cladding layer and locally-coated ZrB2 particles, which led to the in situ reactions and formation of a porous oxide scale (~400 μm). Finally, the discrepant oxidation resistance mechanisms of ZrB2–LaF3 ceramics obtained by different dispersion methods of LaF3 were proposed.