Hafnium Diboride Research Articles

Densification and oxidation behavior of spark plasma sintered Hafnium Diboride-Hafnium Carbide composite

This study reports on the sintering and oxidation behaviors of Hafnium Diboride (HfB2) and Hafnium Carbide (HfC) based ultra-high temperature ceramic (UHTC) composites. Pure Hafnium Diboride, Hafnium Carbide, and HfB2-50 vol% HfC composite were consolidated using spark plasma sintering (SPS) without the use of sintering aids. HfB2-HfC composite displayed a high sintered density of 98% as compared to 87% density of pure HfB2. The increased density of the HfB2-HfC composite is attributed to the bimodal powder distribution, which allowed for the smaller HfC particle to occupy the voids between the larger HfB2 particles resulting in improved packing efficiency. Moreover, the higher planar surface energy, for the (111) and the (200) planes for HfC, contributed to the smaller HfC particle being a significant driving force in the sintering process. Oxidation studies of each material were carried out by exposing them to a high-speed plasma jet in a temperature exceeding 2700 °C. The results of these studies show an improved oxidation resistance of HfB2-HfC composite by 54% and 70% over pure HfC and HfB2, respectively. The enhanced oxidation behavior is attributed to B2O3 filling in the porosity between the HfO2 scale and protecting the underlying material. This study provides a new alternative to improve the sinterability of UHTC diborides by introducing another UHTC (i.e., HfC) to form a fully dense composite without sintering aid and superior oxidation resistance.

Interaction of HfB2 with Nickel and Ni–20% Cr Alloy (Nichrome)

Interaction in the Ni–HfB2 and NiCr–HfB2 systems is studied. No new reaction products are found to form in the system with pure nickel in solid-phase interaction up to the contact melting point. Solidphase interaction in the NiCr–HfB2 system resulting from the interdiffusion of boron from hafnium diboride and chromium from nichrome leads to a Cr2B layer in the contact area. Contact melting begins at 1370 ± 10°C in the Ni–HfB2 system and at 1450 ± 10°C in the NiCr–HfB2 system. The starting metals and Hf3Ni20B6 and Ni3Hf phases, including the Cr2Ni3B6 phase in the NiCr–HfB2 system, are basic products of the liquid-phase interaction in the hypoeutectic region of the phase diagrams of both systems. In the hypereutectic region of the phase diagrams, the metal component disappears and unreacted HfB2 appears additionally in the NiCr–HfB2 system. Both systems are eutectic and have eutectic melting points of 1370 ± 10°C for Ni–HfB2 and 1450 ± 10°C for NiCr–HfB2 at 20 ± 2% HfB2 in their quasibinary phase diagrams. Therefore, nickel and nichrome can be used to activate sintering since they act as a vanishing liquid phase. On the other hand, hightemperature oxidation-resistant hafnium diboride products can successfully perform in contact with nickel and nichrome products in the temperature range up to 1200°C.

Investigating the Performance and safety features of Pressurized water reactors using the burnable poisons

Investigating the performance and safety features of Pressurized water reactors (PWRs) using the burnable poisons (BPs) are evaluated in this paper. Modeling and simulating of PWR assembly from Vera core physics benchmark are carried out using the MCNPX code. The effect of using different burnable poisons (eight) on the neutronic parameters are studied. Moreover, the types of BPs either as integral burnable absorbers (IBAs) or as burnable poison rods (BPRs) are investigated. The effect of these poisons on the effective multiplication factor is the first parameter that simulated. Gadolinium oxide (Gd2O3) and Erbium oxide (Er2O3) are used as IBAs that mixed homogenously with uranium fuel. The results indicated that Erbium is more effective than Gadolinium due to the fast depletion of gadolinium that shortens the core life. Zirconium diboride (ZrB2), Lutetium Oxide (Lu2O3) and Protactinium Oxide (PaO2) are also investigated as IBAs that are used as coating the outer surface of fuel rod. The results demonstrated that Protactinium is more effective than other poisons. Moreover, the impact of Pyrex (SiO2-B2O3), WABA (Al2O3-B4C) and Hafnium di-boride (HfB2) are included and analyzed. Effective multiplication factor and beta effective are evaluated at the beginning of life of reactor operation. Finally, via the Pin by pin power distribution at BOC and EOC on the assembly level, maximum and minimum power values for the different cases are compared.

Component-Interaction Mechanism in the Quasibinary HfB2–Cr System

The interaction of hafnium boride and chromium in the HfB2–Cr system is studied. Solid-phase interaction occurs at temperatures up to 1650 ± 30°C to form chromium boride (Cr2B) on the chromium side of the HfB2–Cr system. The diffusion of boron from hafnium diboride to chromium is the control link of this interaction process. At 1650 ± 30°C, a liquid phase of eutectic composition with a contact angle on hafnium diboride of 20° shows up in the reaction area. The interaction proceeds with the dissolution of each contacting element in the eutectic, equalizing their concentrations through liquid-phase diffusion. The eutectic nature of the quasibinary HfB2–Cr phase diagram, a small contact angle formed by the eutectic on hafnium diboride, decreasing with higher temperature, and high chromium vapor pressure, promoting uniform chromium distribution over the entire charge through the vapor phase allow chromium to be considered a promising addition that activates the sintering of hafnium diboride.

Способы повышения механических характеристик керамики на основе диборидов циркония и гафния (Обзор)

This paper describes the main methods for improving the mechanical properties of ceramic materials based on zirconium and hafnium diborides, such as strength and crack resistance. Particular attention is paid to the influence of the sintering method, as well as additives introduced into the powder composition, in particular hard fibers, second-phase powder particles and “whiskers”, on the mechanical characteristics of the ceramics. On the basis of the analysis of literature data, it is shown that the spark plasma sintering (SPS) method allows to obtain ceramic samples with increased mechanical characteristics due to their high density and low defects. The addition of silicon carbide filamentous crystals or fibers to the ceramics can increase the crack resistance of the sintered material up to 6.0–8.5 MPa×m1/2. These conclusions can be useful in the development of ultra-high-temperature ceramic materials.

Abnormal physical behaviors of hafnium diboride under high pressure

Hafnium diboride (HfB2) is one of the most promising hard-brittle ceramic materials with unique physical properties. Here, we have synthesized the well-crystallized HfB2 by a high-pressure solid-state reaction and employ in situ high-pressure synchrotron radiation angle-dispersive X-ray diffraction to investigate the size-effect of HfB2. An abnormal physical behavior of HfB2 under high pressure is observed. The microsized HfB2 shows slight anisotropy along a and c axes; however, the nanosized HfB2 reveals a different compression behavior with pressure-induced shell structural transition from a disordered symmetric amorphous shell state to an asymmetric amorphous shell state. In addition, the results indicate that the pressure calibrations are sensitive to the grain size. The present findings offer insights into the physical behaviors of different sized HfB2, which may also provide valuable information for other transition metal borides under high pressure.

A numerical approach to the heat transfer and thermal stress in a gas turbine stator blade made of HfB2

Conventional gas turbine blades often face corrosion and oxidation at high temperatures. For resolving these problems, another way in addition to the cooling ducts is selecting appropriate materials for the fragments manufacturing. The present work offers a solution to reduce the metallurgical issues at higher temperatures by using ultra-high temperature ceramics. Despite the rotor blades, stator ones are exposed to hot fluid without any rotation and consequence centrifugal forces. Because brittle materials such as ordinary ceramics are not applicable in the case of high tension forces, hafnium diboride is selected to examine its feasibility for the fabrication of gas turbine stator blades. Thermal stress and deformation of gas turbine stator blades are investigated numerically. Comsol Multiphysics software, utilizing Finite Element Method, was used to analyze the heat transfer and possibility of failure in the stator blades. HfB2 can be a resistant alternative for manufacturing the turbine blades, which does not fail against the applied compressive stresses according to both Von Mises and Coulomb-Mohr theories. In addition, a comparison study between HfB2 and two other diboride ceramics, ZrB2 and TiB2, is conducted.

High-temperature oxidation and plasma torch testing of MoSi2–HfB2–MoB ceramics with single-level and two-level structure

Influence of the composition and structure of the heterophase ceramics in system MoSi2–HfB2–MoB on the thermal conductivity, kinetics and mechanisms of oxidation at 1200 °C and 1650 °C, including the plasma torch testing at 2000 °C, was investigated. Dense ceramics with heterogeneous single-level structure (SLS) and two-level structure (TLS) were obtained by the hot pressing of powders produced by self-propagating high-temperature synthesis (SHS) in combustion mode. TLS ceramics have a lower thermal conductivity as compared to the SLS ceramics with similar elemental composition. Addition of hafnium diboride leads to the increase of mass change during the oxidation due to the formation of HfO2 and HfSiO4, which are denser than SiO2. In the case of the SLS ceramics, at 1200 °C a two-layered oxide film is formed. The upper layer is comprised of amorphous Hf-doped oxide layer, and the lower layer is crystalline crystoballite α-SiO2. TEM investigation of samples oxidized at 1650 °C revealed the formation of HfSiO4 precipitates in the α-SiO2 matrix. In the case of the TLS ceramics, regardless of oxidation temperature two-layered oxide film consists of upper SiO2 and lower HfSiO4. TLS ceramics demonstrated the highest oxidation resistance under the plasma torch and kept its structural integrity during 180 s at 2000 °C.

Optimized control rod designs for Generation-IV fast reactors using alternative absorbers and moderators

The characteristics of Generation-IV fast reactors strongly impact the design of its control rods. The traditional control rod is a cluster of vented pins with boron carbide (B4C) as the absorber. Due to the gas release, the neutron-induced swelling, the melting risk, and the high loss of the reactivity worth, B4C impact the safety performance of control rods and thus limit their operating lifetime.Various alternative absorbers to B4C are assessed in detailed designs according to their neutronic performance, safety performance, and waste management. The hafnium diboride (HfB2) process a high thermal conductivity, a high melting point, and a small neutron-induced swelling, which contributes to an enhanced safety performance. The europium oxide (Eu2O3) process a very small loss of the absorption ability for a long-time utilization. Hafnium hydride (HfH1.62) has a good neutronic performance, but its hydrogen desorption issue should be further investigated.The absorption cross-section decreases generally with the neutron incident energy. Moreover, an important spatial self-shielding effect is found in the control rod, which leads to a peak temperature and peak burnup in the outer zone while a non-optimized utilization of absorber in the inner zone. In this work, the hydrogen-containing moderator is used in independent pins to replace some absorber pins in control rods. The moderator is able to increase significantly the absorption ability of Eu2O3 and of gadolinium oxide (Gd2O3). The moderator optimizes the utilization of absorbers, such as HfB2 and B4C, and save their investment. By homogenizing the absorption distribution, the application of moderators has only a limited influence on safety performance or waste management.

Heretophase Ceramics in the Hf–Si–Mo–B System Fabricated by the Combination of SHS and Hot Pressing Methods

This work is devoted to the fabrication of heterophase powder and consolidated ceramics based on hafnium and molybdenum borides and silicides by combining self-propagating high-temperature synthesis (SHS) and hot pressing (HP). Composite ceramic HfB2–HfSi2–MoSi2 SHS powders are fabricated according to the magnesium-thermal reduction flowsheet from oxide raw materials, in which the combustion wave has temperatures of 1750–2119 K and rather high mss combustion rates of 8.4–9.3 g/s. The structure of synthesized SHS powders consists of relative coarse MoSi2 grains up to 10 μm in size, submicron elongated HfB2 grains mainly located inside the MoSi2 grains, and rounded Si precipitates. The composition with a lower boron concentration contains numerous polyhedral HfSi2 grains smaller than 10 μm in size. The resulting powders have an average particle size of ~6 μm with a maximal size up to 26 μm. The phase compositions of the ceramics consolidated by the HP method and synthesized SHS powders are identical. The microstructure of compact samples consists of faceted elongated HfB2 grains 0.5–10.0 μm in size, polyhedral HfSi2 and MoSi2 grains up to 8–10 μm in size, and silicon interlayers. The consolidated ceramics have a high structural and chemical homogeneity, low residual porosity of 1.1–1.7%, high hardness of 11.7–12.6 GPa, and thermal conductivity of 62–87 W/(m K).

Stable and hard hafnium borides: A first-principles study

We investigate the stability of hafnium borides at zero pressure via the evolutionary crystal structure prediction and first-principles calculations. Our results indicate that the well-known P6/mmm-HfB2 is the only thermodynamically stable phase at zero temperature and pressure, and two more phases (Pnma-HfB and Fm3¯m-HfB12) become thermodynamically stable at higher temperatures. We compute the mechanical properties including bulk, shear and Young’s moduli, Vickers hardness, and fracture toughness for all stable and metastable hafnium borides (∼30 phases) and then study in detail the effect of boron concentration and topology of B-sublattice on their mechanical properties. We show that not only the concentration of boron, but also the topology of the boron sublattice is important for the mechanical properties of hafnium borides. Among the predicted stable and low-energy metastable hafnium borides, the highest possible hardness is exhibited by P6/mmm-HfB2 with graphenelike boron sheets and by phases with 3D boron networks and high B/Hf ratios (e.g., Pnnm-HfB5 and Fm3¯m-HfB12).

Ultrahigh-Temperature Ceramic Aerogels

We demonstrate the synthesis of high-surface-area, low-density refractory aerogels. The monolithic hafnium boride (HfB2) and zirconium boride (ZrB2) aerogels are prepared via borothermal reduction ...

High temperature strength of an ultra high temperature ceramic produced by additive manufacturing

Abstract In this study hafnium diboride was fabricated using the additive manufacturing technique robocasting. Parts have been successfully produced with complex shapes and internal structures not possible via conventional manufacturing techniques. Following pressureless sintering, the monolithic parts reach densities of 94–97% theoretical. These parts exhibit bending strength of 364 ± 31 MPa at room temperature, and maintain strengths of 196 ± 5 MPa up to 1950 °C, which is comparable to UHTC parts produced by traditional means. These are the highest temperature mechanical tests that a 3D printed part has ever undergone. The successful printing of the high density HfB2 demonstrates the versatile range materials that can be produced via robocasting using Pluronic pastes.

Hafnium Diboride as a saturable absorber for Q-switched lasers

ABSTRACTHafnium diboride (HfB2) is used in hypervelocity re-entry vehicles (such as intercontinental ballistic missile (ICBM) or ICBM heat shields), nuclear reactors and aerodynamic leading edges because of its strong tensile strength and high thermal resistance. HfB2 is an ultra-high-temperature ceramic (UHTC) with a melting point of 3250°C. In this paper, we study the performance of HfB2 as a saturable absorber in Q-switched lasers. HfB2 has a non-saturable absorptance of 2.5 dB/µm, a saturable absorptance of 0.8 dB/µm and a saturation fluence of 12 µJ/cm2. In addition, the saturation lifetime is estimated as 989.3 ps, while the recovery lifetimes are estimated as 254.9 and 22.1 ps. As a saturable absorber in our Q-switched laser, we observed pulses with durations between 880 and 2000 ns. With the addition of an acousto-optic modulator, we have observed Q-switching mode-locking, with pulses as low as 250 ns. HfB2 can potentially work at very high power, since its damage fluence is no less than 361 mJ/cm2.

Thermal Expansion of Micro- and Nanocrystalline HfB2

Hafnium diboride nano- and microcrystals are studied by high-temperature X-ray diffraction in the temperature range of 300–1500 K. HfB2 nanocrystals are found to have a greater thermal expansion coefficient than HfB2 microcrystals. The thermal expansion of HfB2 is found to be anisotropic with respect to the unit cell axes.

Materials characterisation and mechanical properties of Cf-UHTC powder composites

Ultra-high temperature ceramic composites based on carbon fibre, Cf, preforms impregnated with hafnium diboride, HfB2, powder and then densified with carbon by chemical vapour infiltration, CVI, have been mechanically tested to measure the room temperature flexural, interlaminar shear, compressive and tensile strengths. The latter was also measured at 1000 °C. All the composites suffered a degree of delamination during the different mechanical tests but the strength values obtained were at least equal to, or better than, those previously reported in the literature for ultra-high temperature ceramic (UHTC)-based composites. Importantly, in spite of the oxidation of the tensile samples tested at 1000 °C, similar tensile strength values were obtained at both temperatures, suggesting that the materials can resist elevated temperatures. The samples tested at higher temperature did show greater evidence of fibre pull out, possibly due to a weaker fibre-matrix interface as a result of oxidative degradation. The results also suggested that the 0° orientation plies in the Cf preform structure offered greater resistance to mechanical stresses; this suggests that composites can now be designed to offer even greater strength values.

Bottom-Up Synthesis and Mechanical Behavior of Refractory Coatings Made of Carbon Nanotube-Hafnium Diboride Composites.

We use aligned carbon nanotube (CNT) forests as scaffolds to deposit hafnium diboride (HfB2) and fabricate millimeter-thick ultrahigh-temperature composite coating. HfB2 has a melting temperature of 3250 °C, which makes it an attractive candidate for applications requiring operation in extreme environments. Compared to typical refractory HfB2 processing, which requires temperatures exceeding 1500 °C, we use conformal HfB2 chemical vapor deposition (CVD) to coat CNT forests at a low temperature of 200 °C. During this process, nanometer-thin HfB2 films grow on the CNT surface and uniformly fill tall CNT forests, thus transforming nanometer film deposition to a scalable HfB2 coating technology. The conformal HfB2 coating process uses static (S-) CVD, where the precursor is fed into a closed system, enabling highly conformal coating and economically efficient utilization of the HfB2 precursor reaching 85%. The modulus and compressive strength of the composites are measured using flat-punch indentation of micropillars having various coating thickness. Filling the CNTs with HfB2 strengthens their node morphology and effectively enhances the mechanical properties. We study the nonlinear behavior of the material to extract a unique modulus value that describes the stress-strain response at any applied compression. At the highest HfB2 coating thickness of 45 nm, the solid fraction is increased from 2% for the bare CNTs to 36% for the composite; the modulus and strength reach 107 and 1.5 GPa, respectively. An analytical model is used to explain the mechanism of the measured structure-mechanical property scaling. Finally, the process is used to fabricate CNT-HfB2 films having 1.7 mm height, a centimeter square area, and only 5.8 × 10-6 nm/nm thickness gradient to demonstrate the potential for scalability.

Microstructure and flexural strength of hafnium diboride via flash and conventional spark plasma sintering

Microstructure evolution in bulk hafnium diboride ceramics prepared by spark plasma sintering in flash regime was compared with conventional spark plasma sintering. The conventional and flash spark plasma sintering resulted in ceramics with a high relative density exceeding 96% of their theoretical density. A remarkably fine grain size distribution was noticed for the specimen prepared in the flash regime. This atypical microstructure evolution provides a possible insight into the mechanism of flash sintering for conductive bulks. The room temperature flexural strength of the hafnium diboride processed by flash SPS was 650 MPa which is 140 MPa higher than the sample produced by conventional SPS.

Ablation properties of HfB2 coatings prepared by supersonic atmospheric plasma spraying for SiC-coated carbon/carbon composites

In order to improve the ablation resistance of carbon/carbon (C/C) composites, hafnium boride (HfB2) coatings were prepared on the surface of SiC-coated C/C composites by supersonic atmospheric plasma spraying (SAPS). The anti-ablation property was investigated in an oxyacetylene torch environment under different heat flux. The phase composition, surface and cross-section microstructure and ablation behavior of the prepared coated C/C composites were characterized by using X-ray diffraction and scanning electron microscopy equipped with energy dispersion spectroscopy. The HfB2 coating exhibited excellent ablation resistance under 2400 kW/m2 after ablation for 30 s and no obvious penetrated crack and critical defect could be observed in the coating. The increase of ablation rates with the formation of rugged surface structure and pores indicated that HfB2 coating suffered more serious mechanical denudation and thermochemical erosion. The formation of protective glassy Hf-O layer with low oxygen permeability, generated due to the oxidation of HfB2 phase, inhibited the inward diffusion of oxygen and improved remarkably the ablation resistance of C/C composites.

Special Features of Oxidation of Hafnium Diboride Nanoparticles of Different Dispersity

Products of oxidation of HfB2 particles with mean size 50–55 and 20–25 nm with air oxygen under polythermal and isothermal conditions have been studied by means of thermal analysis, scanning electron microscopy, X-ray energy-dispersive analysis, and elemental analysis. Rate constants of oxidation of the HfB2 nanoparticles have been determined.