A comparative study on combustion synthesis of Ta-B compounds

A comparative study on the preparation of various tantalum borides (including Ta₂B, Ta₃B₂, TaB, Ta₅B₆, Ta₃B₄, and TaB₂) in the Ta-B system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of their corresponding stoichiometries. Both combustion temperature and reaction front velocity increased and then decreased with increasing boron content in the powder mixture. The fastest flame front with a reaction temperature of 1732 °C and a propagation rate of 11.2 mm/s was observed in the sample of Ta:B = 1:1. The combustion temperature (1205 °C) and flame-front velocity (3.82 mm/s) for the powder compact of Ta:B = 2:1 were the lowest. According to the XRD analysis, single-phase TaB and TaB₂ were produced from the samples of Ta:B = 1:1 and 1:2, respectively. However, multiphase products were synthesized from the samples of other stoichiometries. In the final products from Ta-rich samples of Ta:B = 2:1 and 3:2, two boride phases, Ta₂B and TaB, along with a large amount of residual Ta were detected. The products yielded from boron-rich reactants of Ta:B = 5:6 and 3:4 were composed of TaB, Ta₃B₄, and TaB₂. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with the formation of TaB and TaB₂ by solid state combustion were determined to be 131.1 and 181.4 kJ/mol, respectively.     

Effects of excess boron on combustion synthesis of alumina–tantalum boride composites

TaB- and TaB₂–Al₂O₃ in situ composites were fabricated by thermite-incorporated combustion synthesis from the powder mixtures of different combinations, including Ta₂O₅–Al–B, Ta₂O₅–Al–B₂O₃–B, and Ta₂O₅–B₂O₃–Al. Effects of excess boron were studied on the combustion dynamics and phase constituents of final products. For the B₂O₃-containing samples, the reaction was less exothermic and aluminothermic reduction of Ta₂O₅ and B₂O₃ was less complete, resulting in the deficiency of boron and the presence of TaO₂ and Ta. For the samples containing elemental boron, the occurrence of borothermic reduction of Ta₂O₅ also caused the loss of boron. Experimental evidence showed that boron in excess of the stoichiometric amount substantially enhanced the formation of tantalum borides, which in turn facilitated the reduction of Ta₂O₅ by Al. Consequently, the samples rich with boron in the molar proportions of Ta₂O₅:Al:B=3:10:9 and 3:10:16 (i.e., B/Ta=1.5 and 2.67) were found to be the optimum stoichiometries of producing TaB- and TaB₂–Al₂O₃ composites through a self-sustaining combustion process.     

Mechanism and microstructural evolution of combustion synthesis of ZrB2–Al2O3 composite powders

Mechanism of combustion synthesis (CS) of ZrB₂–Al₂O₃ composite powders was systematically analyzed by a combustion front quenching method (CFQM). The microstructural evolution during the CS process was investigated by field-emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectrometer (EDS). The combustion temperature and wave velocity were measured by the data acquisition system. Moreover, the phase constituents of the final product were examined by X-ray diffraction (XRD). The thermal behaviors of the stoichiometic powders under the thermal exposure were characterized using differential scanning calorimetry (DSC) and thermogravimetric (TG). The results showed that the combustion reaction started from the melting of the B₂O₃ and Al particles, which was followed by the formation of ZrO₂–B₂O₃–Al solution. The ignition temperature of this system was determined to be around 800 °C. B and Al₂O₃ were then precipitated from the solution. As the CS reaction proceeded, Zr and Al₂O₃ were produced by the reaction between ZrO₂ particles and Al and precipitated from the solution. ZrB₂ could then be formed by the direct reaction between Zr and B. Finally, the ZrB₂–Al₂O₃ composite powders were obtained. Furthermore, a model corresponding to the dissolution–precipitation mechanism was proposed.     

Using the Electromotive Force of Condensed System Combustion to Estimate the Parameters of Heat Transfer through an Obstacle

The paper describes a method for measuring the delay of combustion wave propagation through an obstacle and other heat-transfer parameters by continuous recording of electrical signals arising in a burning condensed system. This method is suitable for systems and obstacles that have marked electric conductivity. Results of investigation of combustion wave propagation through a tantalum obstacle in the 3Zr + 2WO₃ system using the proposed method are presented. Key words: gas-free system, combustion, mechanical strains, obstacle, combustion e.m.f., heat transfer, thermal parameters.     

Phase control during synthesis of nanocrystalline ultrahigh temperature tantalum‐hafnium diboride powders

Ta_1?xHf_xB₂ material is attractive for various aerospace applications. In this study, 2 low‐cost approaches were adopted to synthesize nanocrystalline Ta_0.5Hf_0.5B₂ solid solution and related composite powders. The first was based on carbothermal reduction reaction (CTR) of intimately mixed tantalum‐hafnium‐boron oxide(s) and carbon obtained from aqueous solution processing of TaCl₅, HfCl₄, B₂O₃, and sucrose as precursors. It was found that when using this method, due to the low solubility of each other for Ta₂O₅ and HfO₂ and the difference in reactivity of those 2 oxides with carbon (as well as B₂O₃), individual TaB₂ (‐rich) and HfB₂ phases always form separately. Those borides tend to remain phase separated due to the slow inter‐diffusion between them. However, it was observed that addition of copper “catalyst” noticeably accelerates the inter‐diffusion and the solid solution formation. The second approach was based on alkali metal reduction reaction, in which TaCl₅ and HfCl₄ are directly reacted with sodium borohydride (NaBH₄). This method yields a single phase Ta_0.5Hf_0.5B₂ solid solution nanopowders in one step at much lower temperatures (e.g., 700°C) by avoiding the oxides formation and the associated phase separation of individual borides as observed in the CTR‐based process.     

Transparent glass and glass-ceramic in the binary system NaPO3-Ta2O5

Transparent and homogeneous tantalum phosphate glasses were prepared in the binary system (100-x)NaPO₃-xTa₂O₅ with x varying from 10 to 50 mol%. Thermal, structural, and optical properties, as well as crystallization mechanisms, were investigated by thermal analysis, X-ray diffraction, Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopies, optical absorption, transmission electron microscopy in terms of Ta₂O₅ content. FTIR and Raman results support the tantalum insertion in the phosphate chains with [TaO₆] polyhedra cross-linking the phosphate units. At higher Ta₂O₅ content, [TaO₆] clusters are formed and connected to the phosphate network by P-O-Ta bonds. This structural evolution is in good agreement with the thermal features measured by differential scanning calorimetry (DSC) with a strong increase of the T_g temperatures up to 920°C, high thermal stability against crystallization for low Ta₂O₅ content and increasing of crystallization tendency for the most Ta-concentrated samples. Besides, due to the progressive insertion of [TaO₆] units, the precipitation of Na₂Ta₈O₂₁ perovskite-like phase was identified in the sample with 50 mol% of Ta₂O₅. The optimal heat treatment conditions were identified using DSC measurements and a transparent glass-ceramic from 50NaPO₃ to 50Ta₂O₅ composition was prepared. The obtaines glass-ceramic has great potential for optical applications, such as host for rare-earth ions, nonlinear optical materials, and ferroelectric domain.     

Improving the adhesive,mechanical, tribological properties and corrosion resistance of reactive sputtered tantalum oxide coating on Ti6Al4V alloy via introducing multiple interlayers

Tantalum oxide film has become an investigation focus for surface modification materials in the biomedical field owing to its outstanding biocompatibility, anti-corrosion, and anti-wear performances. However, tantalum oxide films exhibit poor adhesion because of the mismatch between the properties of the film and the substrate. In this study, a novel multilayer tantalum oxide coating of Ta_mO_n/Ta_mO_n-TiO₂/TiO₂/Ti (code M-Ta_mO_n) was deposited on Ti6Al4V by magnetron sputtering with Ta_mO_n single-layer coating as control. The purpose of this work is to evaluate the influence of the introduced Ta_mO_n-TiO₂/TiO₂/Ti multi-interlayer on the microstructure, adhesive, mechanical, and anti-corrosion properties of reactive sputtered tantalum oxide coatings. The outcomes show that the Ta_mO_n-TiO₂/TiO₂/Ti intermediate layer improves the bonding strength between the Ta_mO_n layer and Ti6Al4V matrix from 17.83 N to over 50 N and enables the Ta_mO_n coating to have an increased H/E and H³/E² ratio, decreased friction coefficient and wear rate, raised potential, and reduced corrosion current density. The improved properties of the multilayer system are attributed to the positive effects of the inserted multiple interlayers in reducing the residual stress in the coating, coupling the mechanical performance between the layer and the substrate, blocking the continuous growth of penetrating defects in a film with columnar structure. These experimental results provide a workable route for improving the properties of the tantalum oxide coating on Ti6Al4V alloy for medical applications.     

Quantitative prediction of the structure and properties of Li2O–Ta2O5–SiO2 glasses via phase diagram approach

Tantalum silicate glasses serve as laser host materials to take advantage of their high refractive index and the ability to tailor their physical properties in the design of high-performance photonic and photoelectric components. However, successful attainment of feature control in tantalum-doped materials remains a longstanding problem due to the limited understanding of local structure around the tantalum ions, a problem that lies at the heart of predicting the micro- and macroscopic properties of these glasses. Herein, we present a novel approach for predicting the local structural environments in tantalum silicate glass based on a phase diagram approach. The phase relations and glass formation region of Li₂O–Ta₂O₅–SiO₂ ternary systems are explored to calculate the structure and additive physical properties of lithium tantalum silicate glasses. These measured and calculated results are in good quantitative agreement, indicating that the phase diagram approach can be applied broadly to Li₂O–Ta₂O₅–SiO₂ ternary glass systems. Using the phase diagram approach, the local structure of tantalum can be directly obtained. Each Ta atom is surrounded by six atoms, and its polyhedron, the TaO₆ octahedron, bonds through oxygen to Li and Ta. As a network modifier, Ta⁵⁺ depolymerizes the silicate glass structure by modulating the local structure of lithium atoms in Li₂O–Ta₂O₅–SiO₂ ternary glass system. The compositional dependence of structure in lithium tantalum silicate glasses is quantitatively determined based on the structure of the nearest neighbor congruent compound through the lever rule. These findings offer a precise prediction of tantalum silicate glass properties with quantitative control over local structural environment of the disordered materials.     

The effect of P/Ta ratio on sorbitol dehydration over modified tantalum oxide by phosphoric acid

A series of modified hydrated tantalum oxides (Ta₂O₅·nH₂O) with various contents of phosphoric acid were prepared and firstly used as the catalysts for sorbitol dehydration to isosorbide. The catalytic performance of Ta₂O₅ was improved significantly after modification, and the isosorbide selectivity over 0.8P/TaO was improved from 3.8 to 48.3% with nearly 100% sorbitol conversion. Characterization with NH₃-TPD, FT-IR, Raman and XRD demonstrated that the introduction of phosphate has transformed Ta₂O₅ into two kinds of new crystalline phases exhibiting various surface acidities. The P-OH species in TaH(PO₄)₂ were mainly responsible for the promotion of catalytic performance over modified oxides.     

Preparation and Characterization of UltraHigh‐Temperature Ternary Ceramics Ta4HfC5

Tantalum hafnium carbide (Ta₄HfC₅) powders were synthesized by solvothermal treatment and carbothermal reduction reactions from an inorganic hybrid. Tantalum pentachloride, hafnium chloride, and phenolic resin were used as the sources of tantalum, hafnium, and carbon, respectively. Pyrolysis of the complexes at 1000°C/1 h initiated the carbothermal reduction to result in multiplex phases including tantalum carbide and hafnium oxide which after heat treatment at 1400°C–1600°C transformed to single‐phase solid solution Ta₄HfC₅ by solid solution reaction. The mean crystallite size of Ta₄HfC₅ particles was less than 80 nm, and the composition of Ta, Hf, and C elements was near stoichiometric and homogeneously distributed in the powder samples. XRD pattern for Ta₄HfC₅ powders was analyzed.     

Fabrication of TaB2/mullite composites by combustion synthesis with excess silicon and B2O3 additions

Preparation of TaB₂/mullite composites from a cost- and energy-effective mixture of reactants was conducted by self-propagating high-temperature synthesis (SHS). The sample stoichiometry of 1.18Ta₂O₅+(2.36x)B₂O₃+6Al+(2y)Si was formulated with x=1.0–1.3 and y=1.0–2.0 for the study of the effects of excess B₂O₃ (x>1.0) and Si (y>1.0) on combustion characteristics and product compositions. The synthesis reaction involved coreduction of Ta₂O₅ and B₂O₃ by Al and Si. The reaction exothermicity and combustion velocity increased slightly with increasing B₂O₃ but decreased considerably with Si. Formation of TaB₂ and mullite was improved by adopting excess B₂O₃ and Si to compensate for their evaporation loss at high combustion temperatures up to about 1600 °C. The sample with x=1.3 and y=1.5 was shown to yield the optimum formation of TaB₂ and mullite. Mullite grains with a tubing-like shape were produced from in situ formed SiO₂ and Al₂O₃ and had an atomic composition close to that of 3:2 mullite (3Al₂O₃·2SiO₂).     

Effect of the scale factor on the combustion of chromium oxide mixtures and on gravity separation of combustion products

The effect of the diameter and height of the reaction volume on combustion of mixtures of CrO₃/Cr₂O₃/C/Al and CrO₃/Cr₂O₃/B₂O₃/Al was studied. The limits of gravity separation with regard to the height and diameter of the metal and oxide phases in the molten combustion products were determined. To explain the results, we propose a qualitative model of the process in which the liquid-phase state of the medium in the combustion front and behind the combustion front determines the characteristics of the processes taking place in the systems studied.     

Production of ultra-high temperature carbide (Ta,Zr)C by self-propagating high-temperature synthesis of mechanically activated mixtures

The combustion temperatures and rates of mechanically activated (MA) Ta–Zr–C mixtures depending on the initial temperature T₀ are determined. The self-heating phenomenon is observed in argon atmosphere at T₀>380 K due to oxidation of the surface of zirconium particles by adsorbed oxygen. Zirconium oxide is formed in the combustion zone at the initial stage of chemical interaction; it is subsequently transformed into zirconium carbide. In addition, tantalum carbide is formed in the combustion zone, while the binary tantalum–zirconium carbide (Ta,Zr)C is formed closer to the post-combustion zone. In order to maintain the layer by layer stationary combustion mode of SHS, the initial temperature T₀ needs to be 298 K, while the duration of mechanical activation needs to be less than 5 min. After longer mechanical activation, the mixtures are prone to bulk combustion even at low initial temperatures. Single-phase (Ta,Zr)C carbide with the lattice parameter of 0.4479 nm was synthesized by forced SHS compaction in a sand mold.     

Deformations inside burning specimens

The dynamics of particle motion inside burning specimens producing solid combustion products was studied by flash radiography. In the experiments, we used specimens of aTi+C+20% TiC mixture, inside which marks in the form of strips of a ofZr+WO ₃ mixture or a tantalum powder were placed. The specimens were burnt in a semiclosed rigid casing with exhaust of impurity gases through slags. It is established that, just behind the combustion front, particles of combustion products begin to move in the direction opposite to the direction of motion of the combustion front. In the central zone of specimens, the particle velocity reaches values comparable with the combustion velocity of the specimens (∼20 mm/sec), whereas, on the periphery, the particle velocity is close to zero. Institute of Experimental Physics, Sarov 607190. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 4, pp. 78–83, July–August, 1997.     

Effect of preheating on synthesis of tantalum nitride by self-propagating combustion

An experimental investigation of self-propagating high-temperature synthesis (SHS) of tantalum nitride (TaN) was conducted with tantalum compacts in nitrogen of 0.27–1.82 MPa. Effects of sample density, nitrogen pressure, and preheating temperature on the flame-front propagation velocity, combustion temperature, degree of conversion, and product composition were studied. Results showed that the SHS process of the tantalum/nitrogen reaction was characterized by the steady propagation of a planar combustion front, followed by a prolonged afterburning reaction. The flame-front velocity increased with nitrogen pressure, but decreased with sample density. Preheating the sample prior to ignition contributed higher combustion temperatures, thus leading to an increase in the conversion percentage. For the unpreheated samples, the conversion increased significantly with nitrogen pressure and reached around 80% at 1.82 MPa of N₂. With preheating temperatures between 150 and 300 °C, the conversion was increased by about 15% when compared with that without preheating. The nitride phase TaN was identified by XRD as the dominant composition in the combustion product.     

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.     

Densification and dielectric properties of strontium bismuth tantalate ceramics

Abstract

The effects of B₂O₃ doping on the sintering processes and dielectric properties of SrBi₂Ta₂O₉ were investigated in this study. The sinterability of SrBi₂Ta₂O₉ was significantly enhanced by doping B₂O₃ due to the formation of liquid phase at elevated temperatures. The addition of B₂O₃ also led to inhibited thermal decomposition and enhanced grain growth of SrBi₂Ta₂O₉. The amount of doped B₂O₃ was found to substantially influence the dielectric constants of SrBi₂Ta₂O₉ ceramics. When a proper amount of B₂O₃ was added, the dielectric constants greatly increased. For facilitating the densification process and improving the dielectric properties of SrBi₂Ta₂O₉, it is important to control the doping amount of B₂O₃.     

Synthesis of single-phase high-entropy diboride powders and their spheroidization process

Spherical (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders with a uniform particle size distribution are successfully prepared using a novel industrial approach, which combines spray-drying process and thermal plasma sintering technology together. In this, single-phase (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders are first synthesized via a borothermal reduction process using a mixture of individual metallic oxides and boron powders as starting materials. The influence of boron powder content on the structure of prepared powders is researched. Then, (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ granules are prepared after wet-grinding and spray-drying process, which exhibit a spherical shape and homogeneous element distribution. RF induction thermal plasma is finally used to sinter the granulated particle, and the apparent density of sintered spherical powders is increased to 2.57 g/cm³ from 1.43 g/cm³. Such powders are in potential demand for additive manufacturing techniques, and the successful synthesis of spherical (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders may guide the way toward the preparation of many other spherical high-entropy diboride powders.     

Molten-salt-assisted combustion synthesis of B4C powders: Synthesis mechanism and dielectric and electromagnetic wave absorbing properties

A novel electromagnetic wave (EMW) absorber was prepared by combustion synthesis. Boron carbide (B₄C) powders with different grain sizes using a molten-salt-assisted combustion technique with B₂O₃, CB (carbon black), and Mg powders as starting materials, and NaCl as an additives. The effects of the NaCl content on the phase compositions and the microstructure of the products were characterized. A combustion front quenching method was used to elucidate the mechanism for the B₄C powders synthesis. The dielectric, and EMW absorbing properties in the X-band were also investigated. The results showed that the addition of NaCl significantly reduced the grain size of B₄C powders. Nanoscale B₄C powders with cubic polyhedral structures were synthesized using 6 wt% NaCl (labeled as N-6). According to the quenching test results can be obtained that the first step in the combustion synthesis was melting B₂O₃ into a glassy substance. At the same time, Mg melted and formed a liquid pool into which the NaCl dissolved, followed reduction of the B₂O₃ to B. The formed B eventually reacted with CB to form B₄C, and the B₄C particles precipitated from the matrices. The N-6 sample exhibits optimal dielectric and EMW absorbing properties, because of a high specific surface area that enhances interfacial and space charge polarization.     

Combustion synthesis of ultra-high-temperature solid solutions (ZrxNb1-x)B2. Part 1: The mechanisms of combustion and structure formation

The self-propagating high-temperature synthesis (SHS) in Zr–Nb–B system has been thoroughly investigated with a focus on its macrokinetics and phase formation mechanisms. The activation energies were determined by analyzing the relationship between the combustion rate and temperature. For compositions NbB₂–40%ZrB₂ and NbB₂–50%ZrB₂, the increase in the initial temperature has resulted in a significant change in combustion rate's dependence on temperature, signifying a possible shift in the reaction mechanisms. The quenched combustion front technique (QCF) method in conjunction with thermodynamic and ab initio calculations were used to analyze the sequence of phase formation events, including the formation of niobium and zirconium borides through gas transport reactions involving volatile boron suboxide BO in the heating zone, followed by the emergence of a zirconium-boron melt in the combustion zone and formation of main fraction of niobium and zirconium borides. Interactions between the primary niobium and zirconium borides and the melt result in the formation of solid solutions; however, at sub-optimal combustion conditions multiple non-equilibrium solid solutions are retained in the products. To address this issue, a machine learning model was developed, attaining coefficients of determination (R²) of 89% for combustion temperature and rate predictions, thus enabling the fine-tuning of macrokinetic parameters of the SHS process in the system.