TaB2 Research Articles

Insight into the elastic anisotropy and thermodynamics properties of Tantalum borides

The first-principle method was used to investigate the elastic anisotropy and thermodynamic properties of Tantalum borides (B–Ta) compounds. The anisotropic indexes, the 3D surface constructions and 2D projections of Young's modulus indicate that B–Ta compounds are anisotropic. The order of elastic anisotropy is B2Ta > BTa2> BTa > B2Ta3>B4Ta3>B6Ta5. Dynamical and thermodynamic properties indicate that B–Ta compounds can keep the dynamical stability at ground state and high temperature. The calculated Debye temperatures of the various B–Ta compounds predict the good mechanical strength at elevated temperatures.

Effects of Boron Addition on the Microstructure and Mechanical Properties of (Ti,Ta)(C,N)-Co Based Cermets

In this work, a titanium–tantalum carbonitride based cermet, with cobalt as the binder phase and boron as a sintering additive, was developed by a mechanically induced self-sustaining reaction process using two different methodologies. The boron additive was added to prevent the formation of brittle intermetallic compounds generally formed during the liquid phase sintering step due to the excessive ceramic dissolution into the molten binder phase. A systematic study was carried out to understand the effects of boron addition on the nature of the phases, microstructure, and mechanical properties of cermets. With the boron addition, the formation of two different boride solid solutions, i.e., (Ti,Ta)B2 and (Ti,Ta)3B4, was observed. Moreover, the nature of the binder was also modified, from the (Ti,Ta)Co2 brittle intermetallic compound (for cermets without boron addition) to ductile and tough (Ti,Ta)Co3 and α-Co phases (for cermets with boron addition). These modifications caused, as a general trend, the increase of hardness and toughness in cermets.

A Tale of Two Tantalum Borides as Potential Saturable Absorbers for Q-Switched Fiber Lasers

In this paper, we analyze the performance of two tantalum-based boride (TaB and TaB 2 ) microparticles as potential saturable absorbers for high-power fiber lasers. Both materials are ultrahigh temperature ceramics with melting points above 3000 °C, but with different crystalline structures: TaB has an orthorhombic structure (nearly isotropic), whereas TaB 2 has a hexagonal structure (uniaxial, anisotropic). Despite their different crystalline structures, the microparticles have a similar low fluence attenuation (between 2.3 and 2.60 dB/μm) and modulation depths (around 2.0 dB/μm), but remarkable different saturation fluences: TaB has a saturation fluence of 160 μJ/cm 2 , whereas TaB 2 has a saturation fluence of 110 μJ/cm 2 . The measured damage thresholds are 112 and 10 6 mJ/cm 2 /pulse for TaB and TaB 2 , respectively. When incorporated to a fiber laser, the materials produce pulses with durations of 345 ns, lower than those reported by our group in previous papers. The results show that the materials can find potential applications in high-power Q-switched lasers.

Comparative study on microstructure and oxidation behaviour of ZrB2-20 vol% SiC ceramics reinforced with Si3N4/Ta additives

Multi-stage Spark Plasma Sintering (MS-SPS) of ZrB2-20 vol% SiC-10 vol% Si3N4 (ZSS) and ZrB2-20 vol% SiC-10 wt% Ta (ZST) composites were carried out at 1900 °C for 3 min to densify the ZrB2-based composites. The microstructure of ZSS composite consisted of new secondary phases (ZrO2, BN, ZrN) along with SiC and ZrB2. Interestingly, ZST sample exhibited core-shell/rim structure and it comprised of ZrB2 core, (Zr,Ta)B2 rim, SiC, ZrO2 and (Zr,Ta)C phases. The presence of new phases and annihilation of Si3N4 or Ta additive in both the sintered samples was clear indication of involvement of sintering reactions. The SEM of cross sectional samples revealed presence of three distinctive layers for the ZrB2 samples after the isothermal oxidation at 1600 °C for 10 h. In particular, no SiC depleted layer was observed for ZSS and its presence was evident in ZST composite. The weight gain (varied between 15.25 and 16.66 mg/cm2) of the ZrB2 composites was comparable and significant difference in the oxide layer thickness (changed from 255 to 476 μm) was noticed. Overall the ZST sample offers better oxidation resistance in view of protective nature of its oxide layer.

Effect of tantalum addition on microstructure and oxidation of spark plasma sintered ZrB2-20vol% SiC composites

Abstract Almost full density (>99% theoretical density (ρth)) was achieved for ZrB2-20vol% SiC-Xwt.% Ta (X = 2,5, 5 and 10) composites after Spark Plasma Sintering (SPS) (Temperature: 1900 °C, Pressure: 50 MPa; Time: 3 min). The microstructure of ZrB2-based composites exhibited core-rim structure and it consists of major crystalline phases (ZrB2 core, (Zr, Ta)B2 rim, SiC), minor amounts of ZrO2 and (Zr, Ta)C solid solution phases. Both the specific weight (from 22.91 to 18.77 mg/cm2) and oxide layer thickness (401–195 μm) of ZrB2-20vol% SiC composites decreased with increasing addition of Ta after the isothermal oxidation at 1500 °C for 10 h in air. The cross-sectional microstructure of oxidized samples displayed presence of a stack of three distinctive layers, which includes thick dense SiO2 top layer, SiC depleted intermediate layer and unreacted bulk. The present work clearly demonstrated the advantage of tantalum addition in improving the oxidation resistance of ZrB2-20vol% SiC.

Oxidation protection of (ZrTa)B2–SiC–Si coating for graphite materials

ABSTRACTTo protect graphite materials from oxidation, a dense (ZrTa)B2–SiC–Si coating was for the first time prepared on graphite substrate by slurry dipping and vapour silicon infiltration process. (ZrTa)B2 phase was synthesised by in-situ reaction using ZrC, TaC, B4C, phenolic resin and Si as raw materials. The phase compositions and microstructure of the obtained coating were analysed by XRD and SEM. The isothermal oxidation tests demonstrated that the in-situ coating had excellent oxidation resistance ability, after oxidation for 468 and 347 h at 1273 and 1773 K, respectively, the weight gains were 0.07% and 0.33%, respectively. No signs of oxidation of graphite substrate were found after oxidation, and two layers could be observed in the coating, namely, oxide layer containing Zr and Ta oxides and (ZrTa)B2–SiC–Si layer. A model for the evolution of the oxide layer was proposed to interpret the oxidation failure mechanism of the coating.

Mechanical Synthesis of Tantalum Borides and Modeling of Solid Solutions of Boron in Tantalum

Two mechanisms of mechanochemical solid-phase reactions are observed when tantalum and boron powder mixtures are ground in the same conditions but at different contents of the starting components. The substitutional solid solution decomposes at 50 at.% B in the starting mixture and stimulates the self-propagating mechanochemical synthesis reaction to form the TaB phase. The supersaturated interstitial solid solution, whose formation is controlled by the diffusion of boron atoms into tantalum, shows up at a boron content of 66 at.% and gradually decomposes to form the TaB2 phase. Formalism is proposed for estimating the free Gibbs energy of the interstitial and substitutional solid solutions with the regular solution model. The use of this formalism shows that grinding of the tantalum and boron powders in a planetary-ball mill produces a combined interstitial and substitutional solid solution (two boron atoms substitute one tantalum atom) at the initial stage. Both the substitutional and interstitial mechanisms lead to decrease in the solution’s Gibbs energy. When the substitutional solid solution prevails over the interstitial solution, the Gibbs energy of the substitutional solution becomes minimal at 50 at.% B in tantalum and causes its disintegration.

Part 2. Structure, mechanical and thermophysical properties of consolidated ceramics based on (Hf,Ta)B2

(Hf,Ta)B2-based ceramics were produced by hot pressing (HP) and spark plasma sintering of powders obtained by combustion synthesis in Hf-Ta-B system. The dense ceramics possess exceptionally high hardness (60–70 GPa) and Young's modulus (584 GPa). The thermal conductivity is 30.3–52.9 W/m K depending on the phase composition. Plasma torch testing of the single- and two-phase (Hf,Ta)B2-based ceramics at 1500–2500 °C demonstrated the formation of a protective multi-layer oxide film with good adhesion to the ceramic, which increases the enthalpy of decomposition of ceramics up to 390 kJ/g.

Combustion synthesis of ultra-high-temperature materials based on (Hf,Ta)B2. Part 1: The mechanisms of combustion and structure formation

Macrokinetic features of the combustion in Hf-Ta-B system were studied. Thermodynamic analysis and measured effective activation energies (Еа = 200 kJ/mol and 230 kJ/mol) suggest that chemical reactions which occur through gas and liquid phase play a crucial role in the combustion process. The sequence of the structure formation in the combustion wave of investigated mixtures was studied. The feature of this system is a narrow post-combustion zone, caused by high heat losses. The composition of synthesized phases does not change from the combustion zone to the zone of final products, leading to the chemical inhomogeneity and existence of intermediate phases. SHS without applied pressure leads to the formation of solid solutions on the base of (Hf,Ta)B2. Porous combustion products were used to prepare powders with particles size < 30 µm (median size 5 µm) for further sintering of bulk ceramics.

Combustion synthesis of ZrB2-TaB2-TaSi2 ceramics with microgradient grain structure and improved mechanical properties

Two schemes for fabricating the composite powder ZrB2–TaB2–TaSi2 by SHS are discussed in this study: (1) elemental synthesis in the Zr–Ta–B and Ta–Si mixtures followed by mixing of the combustion products, and (2) elemental synthesis in the Zr–Ta–Si–B mixture. The macrokinetic features of combustion of the Zr–Ta–Si–B mixtures, the mechanism of structure and phase formation in the combustion wave, and the structure and properties of the combustion products were studied. Primary crystals of tantalum and zirconium diborides were formed in the preheating zone as a result of gas-phase mass transfer of boron onto the surface of metal particles. In the combustion zone, melting of Si–B eutectic and Zr particles took place, followed by the formation of borides and silicides of tantalum and zirconium. In the zones of post-combustion and secondary structure formation, zirconium diboride partially interacted with tantalum diboride and formed solid solution (Ta,Zr)B2. Hot pressing of SHS powders produced bulk ceramics with the relative density of 95–98%. Structure and mechanical properties of the hot pressed ceramics depend significantly on the selected scheme of powder synthesis. The ceramics fabricated using scheme 2 have a specific microgradient grain structure and significantly higher hardness and crack resistance (HV10 =19.2 GPa; K1c=3.5 MPa*m1/2) as compared to the similar composite prepared using scheme 1 (13.9 GPa and 2.4 MPa m1/2).

Effects of TaSi2 addition on room temperature mechanical properties of ZrB2-20SiC composites

The effects of TaSi2 addition on the room temperature mechanical properties of ZrB2–20SiC (volume fraction, %) composite were investigated. Dense ZrB2–20SiC-xTaSi2 (x = 3, 6, 10) composites were prepared by hot pressing the mixture of ZrB2, SiC and TaSi2 powders at 1850–1900 °C under 30 MPa in flowing Ar. In the as-prepared composites, apart from ZrB2, SiC and TaSi2, (Zr,Ta)B2 solid solution also existed around ZrB2 grains. It became a continuous layer and the thickness increased gradually with the increase of TaSi2 content. When the addition of TaSi2 was within the range of 3–10%, Vickers hardness of ZrB2–20SiC reduced slightly by about 1–2 GPa, but the flexural strength and fracture toughness increased simultaneously by more than 35%. These phenomena could be attributed to the relatively low hardness of TaSi2, and the decrease of ZrB2 grain size as well as the presence of a continuous (Zr,Ta)B2 solid solution layer around ZrB2 grains.

Self-propagating high-temperature synthesis of refractory boride ceramics (Zr,Ta)B2 with superior properties

The macrokinetic features of combustion in the Ta-Zr-B system were studied. Combustion is characterized by spin mode, suggesting the limiting role of gas-phase mass transfer of reagents. The mechanism of chemical reactions and phase formation in combustion wave was discussed. Primary layers of tantalum and zirconium borides were detected in the preheating zone at temperatures below the melting point of the reagents. After zirconium and boron melt, the temperature in the combustion zone reaches its maximum and zirconium diboride precipitates out of the oversaturated solution. Powders with a grain size of 1–3 μm were fabricated and hot-pressed into dense ultra-high-temperature ceramics (UHTCs). Boride ceramics with the record-setting hardness of 70 GPa, Young's modulus of 594 GPa, and elastic recovery of 96% were obtained. The measured heat conductivity of the solid solution (Zr,Ta)B2 was equal to 35–42 W/m K. Plasma torch tests demonstrated high oxidation resistance of the obtained ceramics at 2900–3000 °C.

A new combustion route for synthesis of TaB2 nanoparticles

Tantalum diboride (TaB2) nanoparticles were synthesized through a mechanically induced self-sustaining reaction (MSR). In this method, the ternary system of Mg/Ta2O5/B was employed in which, magnesium was used as a reducing agent for reduction of tantalum oxides in a combustive regime. The processing route of TaB2 by the solid-state combustion was very short-term and the product purification was extremely easy and rapid. The synthesis mechanism was studied and revealed that magnesiothermic reduction of tantalum oxide is the initiator of the total reaction, while borothermic reduction of the oxide may occur in parallel.

Spark plasma sintering and high-temperature strength of B6O–TaB2 ceramics

Tantalum diboride – boron suboxide ceramic composites were densified by spark plasma sintering at 1900°C. Strength and fracture toughness of these bulk composites at room temperature were 490MPa and 4MPam1/2, respectively. Flexural strength of B6O–TaB2 ceramics increased up to 800°C and remained unchanged up to 1600°C. At 1800°C a rapid decrease in strength down to 300MPa was observed and was accompanied by change in fracture mechanisms suggestive of decomposition of boron suboxide grains. Fracture toughness of B6O–TaB2 composites showed a minimum at 800°C, suggestive a relaxation of thermal stresses generated from the mismatch in coefficients of thermal expansion.Flexural strength at elevated temperatures for bulk TaB2 reference sample was also investigated.Results suggest that formation of composite provides additional strengthening/toughening as in all cases flexural strength and fracture toughness of the B6O–TaB2 ceramic composite was higher than that reported for B6O monoliths.

Flexural strength behavior of a ZrB2–TaB2 composite consolidated by non-reactive spark plasma sintering at 2300 °C

A bulk zirconium–tantalum diboride ceramic composite was consolidated by non-reactive spark plasma sintering (SPS) at 2300°C. In order to consolidate the ZrB2–44wt% TaB2 composite and restrict grain-growth, a special loading procedure was used. Pressure was applied and released at 2150°C and 1250°C, respectively. These SPS conditions allowed us to obtain a crack-free bulk composite with a grain size of 4–8μm. Flexural strength at temperatures up to 1800°C was measured for the ZrB2–TaB2 composite. Importantly, at 1600°C, the strength was 336±23MPa, which is superior to that of monolithic ZrB2. Moreover, the ZrB2–TaB2 composite only showed plastic behavior at 1800°C, a finding that is atypical for ZrB2-based ceramics.

Critical oxidation behavior of Ta-containing ZrB2 composites in the 1500–1650 °C temperature range

The microstructure evolution of ZrB2 hot pressed with 15 vol% TaSi2 was studied in the as-sintered state and after oxidation for 15min at 1500 and 1650°C in stagnant air. In the pristine material, the original ZrB2 nuclei are surrounded by a mixed (Zr,Ta)B2 solid solution. Refractory Ta-compounds are located at triple junctions and wetted grain boundaries are distinctive of this ceramic. After oxidation, the solid solution evolves into ZrO2 grains encasing intragranular nano-structured TaB2 particles. Here we show that the operating limit temperature of this composite is related to the critical oxidation of TaB2 to Ta2O5 above 1600°C, accompanied by large volume expansion and local liquid formation, which ruptures the ZrO2 grains and structure.

Tantalum Diboride Obtained by Reactive Sintering–Properties and Wear Resistance

The paper present mechanical properties and wear behavior of tantalum diboride at temperatures in the range from room temperature up to 800°C. The materials for studies were obtained using Spark Plasma Reactive Sintering method from tantalum and boron powders. The mixture was prepared according to the reaction Ta+2B→TaB2. Reactive sintering processes were carried out at temperature of 2100°C during 30 minutes and pressure of 48MPa. During the heating, exothermic synthesis reaction, with visible change of sample height and temperature increase of about 160°C, takes place. The obtained materials have an apparent density of 11.22g/cm3, the Young's modulus of 500 GPa and Vickers hardness of 19.5 GPa. The tribological investigations were carried out using ball-on-disc method in dry friction conditions, at temperatures in the range from room temperature up to 800°C. The influence of temperature on the friction coefficient, specific wear rate and Vickers hardness of sintered materials was determined. In order to study the mechanisms of the surface oxide layer formation, the microstructural analyzes of cross-section were carried out. The values of friction coefficient up to 600°C were stable. The increase of temperature up to 700°C resulted in significant decrease of the friction coefficient to the value of 0.22. The microstructure analysis of the materials indicates that the oxidation of the surface takes place above the temperature of 500°C. The negative effect of the oxides surface layer formation is a gradual decrease of the material surface hardness.

Electrochemical Synthesis of Nanomaterials in Molten Salts

For the first time by electrochemical synthesis from NaCl-KCl-NaF-GdF3-KBF4 melt at temperature 1023 K and with utilization of molybdenum cathode the gadolinium hexaboride nanotubes were synthesized. Most likely GdB6 nanotubes formation is ruled by intercalation mechanism. Gadolinium hexaboride lattice is the framework of boron atom octahedrons, in gaps of which the gadolinium atoms are situated. These gaps are spacious enough to host the metal atoms without deformation and even with reserve of vacant space. Electrochemical synthesis of GdB6 occurs at potentials negative enough for alkali metal cations electroreduction. So alkali metal cations intercalate into the emptiness of hexaboride lattice where cations are reduce in situ. The metal atoms have larger radius than its cations so they provoke mechanical strain. At the certain value of the strain GdB6 splits with nanotubes formation. Nanoneedles of TaO with tetragonal crystal lattice were obtained by electrolysis of CsCl melt, containing monooxofluoride complexes of tantalum. It was determined the correlation between the redox potential of molten system, temperature and stability of tantalum monoxide. Multi-layered tantalum boride structures were produced via electrolysis in the NaCl-KCl-NaF-K2TaF7-KBF4 melt. The overall thickness of the layers, synthesized under direct current regime, was 30-50 μm, and they consisted of numerous fine layers (nanolayers). A new generation of highly active and stable catalytic nanostructured coatings for the water-gas shift reaction by high temperature electrochemical synthesis in molten salts has been developed. The steady-state reaction rates for the Mo2C/Mo coatings were higher than those for the bulk Mo2C and commercial Cu/ZnO/Al2O3 catalysts over in the temperature range explored. The catalytic activity is enhanced by at three orders of magnitude comparing to that of the pure Mo2C phase. The methanation reaction was completely suppressed in the whole temperature range studied on the Mo2C/Mo systems. The catalytic activity remained constant during 5000 hours on-stream. The coatings were also stable during the thermal cycling, while the activity of commercial catalysts tends to decrease with time. A novel microstructured reactor/heat-exchanger containing eight sections with a cross-section of 10 mm x 10 mm and a length of 100 mm has been designed and constructed. Each section of the reactor contains flat, perforated Mo plates and Mo wires with a diameter of 250 mm and a length of 100 mm coated with a porous Mo2C layer. Nanoneedles of silicon were obtained in chloride-fluoride melts containing K2SiF6 by potentiostatic electrolysis which accompanied by reaction of disproportionation.

Consolidation and grain growth of tantalum diboride during spark plasma sintering

Bulk tantalum diboride ceramics have been consolidated by spark plasma sintering (SPS) at 1700–2200°C using moderate pressure of 30MPa. The effect of temperature and dwell time on the densification, microstructure and mechanical properties were investigated. SPS resulted in dense, crack-free specimens with strength ranging from 280 to 550MPa. The room-temperature strength of TaB2 ceramics was analyzed as a function of the grain size. Activation energy for the grain growth process was estimated as 1078±31kJmol−1.

Process and composition dependence of optical properties of zirconium, hafnium and tantalum borides for solar receiver applications

Ultra-high temperature boride ceramics have proved to show promising properties for novel solar receivers. The present work shows a further step towards their actual application, investigating how sintering technique and starting powders composition affect the properties of final materials. Thus we report on the comparative characterization of ZrB2, HfB2 and TaB2 produced by high pressure and pressureless techniques and with different amounts of MoSi2 sintering aid. We investigate microstructural, mechanical and optical properties, in the perspective to assess the material potential for novel solar absorbers operating at higher temperatures than those currently available. Moreover, a systematic study has been carried out on ZrB2, producing with fixed high pressure sintering technique, a series of samples with MoSi2 compositions in the range 5–50vol%. We show that the content of silicide and silicide-related secondary phases in the final pellets affects either the mechanical performance and the optical behavior. Thus, as far as the optical properties are concerned, the MoSi2 amount should be the lowest as possible to ensure a proper material consolidation whilst enhancing the absorbance/spectral selectivity.