Synthesis of ultra‐fine TaB2 nano powders by liquid phase method

Ultra‐fine TaB₂ powders were synthesized by a liquid phase method using tantalum ethoxide, boric acid and sucrose as the sources of tantalum, boron, and carbon. The TaB₂ precursor powders is a Ta–B–C–O network system, which were heat‐treated at lower temperature (1500°C) in normal argon atmosphere to obtain the TaB₂ powders. XRD confirms the presence of only hexagonal TaB₂, while EDS and XPS spectrums confirm the composition and element chemical states of TaB₂. The TEM images show a platelet shape of the TaB₂ powder and the monocrystal SAED pattern confirms the presence of hexagonal TaB₂. Particle size distribution curves show that particle size of the TaB₂ powders distributes in the range of 30‐160 nm, whose mean particle size is 106 nm.     

TiB2 Powders Synthesis by Borothermal Reduction in TiO2 Under Vacuum

TiB₂ powders were synthesized by borothermal reduction in nanoscale TiO₂ with boron under vacuum. Reaction processes were investigated, and the effect of by‐product B₂O₃ was evaluated. Results showed that TiO₂ was firstly reduced by boron to form TiBO₃ and Ti₂O₃, and then to produce TiB₂ and B₂O₃ with increasing temperature. The reaction processes of TiB₂ powders synthesis included two‐step reduction in TiO₂ by boron and the removal of B₂O₃. The presence of B₂O₃, which was previously reported as the most important factor in promoting the coarsening of ZrB₂ and HfB₂ powders by borothermal reduction, did not lead to significant coarsening of TiB₂ powders. Due to the minor effect of B₂O₃, TiB₂ powders with small particle size and low oxygen content could be prepared by direct heat treatment of TiO₂ and boron at 1550°C under vacuum for 1 h. The particle size and oxygen content of synthesized TiB₂ powders were ~0.9 μm and ~1.7 wt%, 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.     

Improvement of densification and microstructure of HfB2 ceramics by Ta/Ti substitution for Hf

Ultrafine HfB₂ powders were synthesized by the combination of borothermal reduction of HfO₂ and solid solution of 5 mol% TiB₂ or 5 mol% TaB₂, prototypically, (Hf_0.95Ti_0.05)B₂ and (Hf_0.95Ta_0.05)B₂. The influence of substitution on the particle growth, high-temperature stability, densification, microstructure, and mechanical properties of HfB₂ was investigated. Results showed that the particle sizes of HfB₂, (Hf_0.95Ti_0.05)B₂ and (Hf_0.95Ta_0.05)B₂ powders prepared by borothermal reduction at 1500°C were 1.73, 0.87, and 0.21 µm, respectively. The substitution of TaB₂ led to a greater decrease in particles size than TiB₂. After heat treatment at 1800°C, the particle sizes of HfB₂, (Hf_0.95Ti_0.05)B₂ and (Hf_0.95Ta_0.05)B₂ powders increased to 2.60, 1.59, and 0.32 µm, respectively, indicative of the good high-temperature stability of TaB₂-substituted HfB₂. The relative densities of HfB₂, (Hf_0.95Ti_0.05)B₂ and (Hf_0.95Ta_0.05)B₂ ceramics after spark plasma sintering at 2000°C were 76.1%, 85.2% and 99.8%, respectively. The fully dense (Hf_0.95Ta_0.05)B₂ ceramics with fine microstructure showed comparably high Vickers hardness of 21.1 GPa combined with flexural strength of 521.2 MPa. It was proved that the solid solution of TaB₂ could effectively inhibit the grain growth of HfB₂ powders, and improve the densification, microstructure, and mechanical properties of HfB₂ ceramics.     

Synthesis,densification, and microstructure of TaC‐TaB2‐SiC ceramics

The usual way to prepare TaC‐TaB₂ ceramics by adding B₄C to TaC leads to formation of residual C, which degrades samples’ mechanical properties. To eliminate the residual C, we suggest incorporating Si together with B₄C into TaC ceramics, resulting in new ultrahigh‐temperature ceramics (TaC‐TaB₂‐SiC). Dense ceramics (>99%) with SiC volume fraction ranging from 15.86% to 41.04% were fabricated by reactive spark plasma sintering at 1900°C for 5 minutes. The formation of SiO₂‐based transient liquid phase was evidenced by the “film” in intermediate products, which can promote densification. The fine‐grained microstructure in final products was found to be associated with the in situ formed SiC, which impeded TaC and TaB₂ grains from coarsening by the pinning effect. Besides, ultrafine TaB₂ grains (~100 nm) produced during the reaction and then rearranged in liquid also contributed to grain refinement. Compared to TaC‐TaB₂(‐C) ceramics prepared from TaC and B₄C, the acquired composites exhibit better mechanical properties, due to their fine‐grained microstructures and the elimination of residual C.     

Low temperature synthesis of TaB2 nanorods by molten‐salt assisted borothermal reduction

TaB₂ powders were synthesized by a molten‐salt assisted borothermal reduction method at 900°C‐1000°C in flowing argon using Ta₂O₅ and amorphous B as starting materials. The results indicated that the presence of liquid phase, such as B₂O₃ and NaCl/KCl, accelerated the mass transfer of reactant species and resulted in the complete finish of the reaction at low temperatures. The obtained TaB₂ powders exhibited a flow‐like shape assembled from nanorods grow along [001] direction or c‐axis. The morphology of the synthesized TaB₂ powders could be tailored by the amount of B₂O₃ or NaCl/KCl.     

A two-step heating strategy for low-temperature fabrication of high sinterability and fine MgAlON powder with MgAl2O4 as Mg source

Based on the reaction sequence during synthesis of MgAlON powder by solid-state reaction, a two-step heating strategy is proposed to low-temperature fabricate fine MgAlON powder of high sinterability by using MgAl₂O₄ as Mg source, respectively together with AlON and Al₂O₃+AlN. By introduction of an additional dwelling at 1550 °C to the first heating step, more α-Al₂O₃ dissolve into the solid solution at this temperature. By this way, overlarge particles of Al₂O₃ by agglomeration could be avoided in the next heating step to enable fast full reaction at a lower temperature. By dwelling 30 min at 1550 °C followed by 60 min at 1700 °C, single phase MgAlON powders were successfully prepared by solid-state reaction of all the two batches. The fine MgAlON powder synthesized by MAS+Al₂O₃+AlN batch exhibited high sinterability as the MgAlON ceramics pressureless sintered by this powder at 1880 °C without dwelling showed a transmittance up to 68.3%. The phase assemblage and morphology evolution of the mixture during solid-state reaction were monitored, which verified the effectiveness of the proposed two-step heating strategy. The low synthesis temperature of the two-step heating scheme benefits to prepare pure MgAlON powder with small particle size.     

Preparation of TaB2-SiC oxidation protective coating for carbon materials by liquid phase sintering

To control the microstructure and amounts of TaB₂ phase in the TaB₂-SiC coating, a novel liquid phase sintering method was developed on the basis of in-situ reaction method to prepare the TaB₂-SiC coating, which includes synthesis of TaB₂ powders and further preparation of TaB₂-SiC coating. With Ta₂O₅, B₂O₃ and C employed as raw materials, hexagonal TaB₂ powders were prepared by carbothermal reduction method at 1500?°C, whose mean particle size is 491?nm. The TaB₂, SiC, C powders, and the low melting point phases Si and silica sol were used to prepare the TaB₂-SiC coating by liquid phase sintering at 2373?K. The thickness of the coating is about 350?µm. Compared with the SiC coating, the weight loss of the samples modified by TaB₂ decreased from 17.7% to 11.8%, and the average weight loss rate of the fastest weightloss zone reduced from ?6?×?10^?3 mg?cm^?2 s^?1 to ?5?×?10^?3 mg?cm^?2 s^?1. During oxidation, the Ta-oxides would gradually dissolve in the silicate glass to form Ta-Si-O glass ceramics with dendritic structure, which significantly improved the toughness and stability of the glass layer. The Ta-Si-O glass ceramics possesses the ability of sealing and arresting the microcracks, which can enhance the oxidation protective ability of the coating.     

Oxidation of TaB2-SiC coatings prepared by spark plasma sintering and effect of pre-oxidation treatments

To suppress the oxidation of TaB₂-SiC coatings, the effects of pre-oxidation temperature on the oxygen hindering properties of TaB₂-SiC coatings were investigated to prepare TaB₂-SiC coatings with enhanced oxidation behavior. The addition of 40 wt% TaB₂ made the oxygen permeability of the coating decrease by 62.16%. However, excessive TaB₂ weakened the oxygen hindering ability of the coating due to the large ion complex ability of Ta⁵⁺. The pre-oxidation temperature at 1500 °C led to a homogeneous dispersion of Ta-oxide nanocrystal particles in the Ta-B-Si-O complex-phase glass layer. In contrast with the untreated samples, the active factor and inert factor values of the TaB₂-SiC coating after pre-oxidation treatment at 1500 °C decreased by 43.12% and 17.33%, respectively, which improved the dynamic stability of the coating during oxidation.     

Preparation and characterizations of tantalum pentoxide (Ta2O5) nanoparticles and UV‐curable Ta2O5‐acrylic nanocomposites

Tantalum pentoxide (Ta₂O₅) nanoparticles with the sizes in the range of 20–50 nm were prepared via a chemical route in which the oleic acid (OLEA) was adopted as the surfactant for the synthesis process. X‐ray diffraction (XRD) revealed the as‐synthesized Ta₂O₅ transforms from amorphous to hexagonal and orthorhombic structures at the temperatures of 700°C and 750°C, respectively, illustrating the suppression of recrystallization temperature of Ta₂O₅ due to the particle size reduction. UV‐curable nanocomposites containing the Ta₂O₅ nanoparticles and acrylic matrix were also prepared. Thermogravimetry analysis (TGA) found an about 10–20°C improvement on the 5% weight‐loss thermal decomposition temperatures (T_ds). Dielectric measurement showed that the dielectric constant of nanocomposite increases with the increase in the filler loading without severe deterioration of dielectric loss. The increment of dielectric constants was ascribed to the addition of high‐dielectric inorganic fillers as well as the presence of interfacial polarization at the organic/inorganic interfaces. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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.     

Synthesis of HfB2 powders by mechanically activated borothermal reduction of HfCl4

HfB₂ powders were synthesized via a borothermal reduction route from mechanically activated HfCl₄ and B powder blends. Mechanical activation of the powder blends was carried out for 1 h in a high-energy ball mill using hardened steel vial and balls. Mechanically activated powders were subsequently annealed at 1100 °C for 1 h under Ar atmosphere. Then, purification processes such as washing with distilled water and leaching in HCl solution were applied for the elimination of the undesired boron oxide (B₂O₃) phase and the probable Fe impurity. The effect of boron amount on the microstructure of the resultant powders was investigated. The boron amount in the starting blends plays an important role in the formation of the HfO₂ phase. HfB₂ powders without any detectable HfO₂ were prepared by adding 20 wt% excess amount of boron. Microstructural analyses of the mechanically activated, annealed and purified powders were performed using X-ray diffractometer (XRD), particle size analyzer (PSA), stereomicroscope (SM), scanning electron microscope/energy dispersive spectrometer (SEM/EDS) and transmission electron microscope (TEM).     

Which phase of Ta2O5 being of the largest dielectric constant

As a promising dielectric material, tantalum pentoxide (Ta₂O₅) exhibits multi-phase co-existing under common fabrication conditions, so it is highly desired to predict theoretically which phase is of the largest dielectric constant. Considering that the specific operations and parameters employed in ab initio calculations may lead to considerably different lattice structure, we set the experimental lattice constants as the criteria to select the functional and parameters in density of functional theory and calculated the dielectric tensors of crystalline δ-, β-, L_SR-, B-, and Z-Ta₂O₅, showing that the dielectric constant is very sensitive to the lattice constants and δ-Ta₂O₅ possesses the largest dielectric constant, 375.1, which is about 10 times larger than those of the other four phases and the experimental measurement of common Ta₂O₅ samples with several phases co-existed, and furthermore, the dielectric constant of δ- and β-Ta₂O₅ could be enhanced to 1175.2 and 939.5, respectively, with the Ta atom replaced by Ti atom at a molar ratio of 7.14%, which is in qualitative agreement with the experimental observations of Ti-doped β-, L_SR-, and H-Ta₂O₅.     

Particle refinement of ZrB2 by the combination of borothermal reduction and solid solution

Flexible synthesis of ultra‐fine ZrB₂ powders was achieved by borothermal reduction in a mixture of ZrO₂, boron, and TiO₂. Without TiO₂ additive, coarse ZrB₂ powders with particle size of 0.81 μm were obtained, presumably due to good wettability and solubility of ZrB₂ in the byproduct B₂O₃. It was found that the particle growth of ZrB₂ was effectively inhibited by the solid solution of TiB₂ (≥1 mol%). The refinement mechanism was that the solid solution of in situ formed TiB₂ presumably lowered the wettability and solubility of ZrB₂ in the B₂O₃ liquid and significantly inhibited the coarsening of ZrB₂. The average particle size of resulting powders decreased to 0.37 μm with the addition of 10 mol% TiO₂.     

Fast one-step carbothermal reduction and nitridation method using nano-sized Al2O3 and carbon black to prepare aluminium oxynitride powder for highly transparent ceramics

By fast heating the nano-sized Al₂O₃ and carbon black mixtures at 50°C/min to 1750°C for 30–120 min, single-phase AlON powders were successfully obtained by a fast one-step carbothermal reduction and nitridation (CRN) method. The AlON ceramics pressureless sintered at 1880°C for 150 min by these powders show high transmittances up to 83%–84%, which indicates that the proposed fast one-step CRN method is an effective and efficient way with strong robustness to synthesize single-phase AlON powder for highly transparent AlON ceramics. It was found that α-Al₂O₃ particles do not have enough time to aggregate and coalesce during heating due to the tremendously shortened heating span, which significantly inhibited particle coarsening until the formation of AlON starts. The fast-formed AlON further inhibits the coarsening of α-Al₂O₃ during dwelling. Consequently, single-phase AlON powder of small primary particles can be obtained after 30 min dwelling at 1750°C.     

Fully dense ZrB2 ceramics by borothermal reduction with ultra-fine ZrO2 and solid solution

The effects of ZrO₂ particle size (55 nm and 113 nm) and borothermal reduction routes (borothermal reduction with water-washing (BRW) and in situ 5 mol% TaB₂ solid solution, BRS) on synthesis and densification of ZrB₂ were investigated. Irrespective of reduction routes, the use of finer ZrO₂ powders as raw materials resulted in finer ZrB₂ powders. Compared to the powders derived from BRS, the powders derived from BRW had smaller particle size with higher oxygen content, especially the powders synthesized with finer ZrO₂. Irrespective of ZrO₂ particle size, the oxygen contents of ZrB₂ powders prepared by the BRS route were similar. Because of the high oxygen content, the ZrB₂ ceramics synthesized by BRW with finer ZrO₂ demonstrated the lowest relative density (90.5%), which resulted in the lowest Vickers’ hardness (14.2 ± 0.9 GPa). Due to the low oxygen content and small particle size of ZrB₂ powders, fully dense ZrB₂ ceramics (relative density: 99.6%) with highest Vickers’ hardness (16.0 ± 0.2 GPa) were achieved by BRS with finer ZrO₂ powders.     

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.     

Low temperature synthesis of pure phase TaB2 powders and its oxidation protection modification behaviors for Si-based ceramic coating in dynamic oxidation environments

To reveal the generation mechanisms of the Ta-Si-O glass ceramics layer in dynamic oxidation environments, a 40?wt% TaB₂-SiC coating was prepared by liquid phase sintering method. To obtain pure phase TaB₂ powders at lower temperature (1500?°C), excessive B₂O₃ powders were added in raw materials to eliminate the TaC byproduct phase. The hexagonal pure phase TaB₂ powders own average particle size of about 386?nm. During the TGA dynamic oxidation tests, after the modification of 40?wt% TaB₂, the initial weight loss temperature of the sample delayed by about 48%, while the weight loss percentage and rate in fastest weight loss zone decreased by about 61% and 53%, respectively. During oxidation, the generated Ta-oxides were peeled and carried away by the formed fluid SiO₂ glass layer to form “Ta-oxides halation” at first, which results the dissolution of Ta-oxides in the SiO₂ glass, thus forming the Ta-Si-O glass ceramics with dendritic structure. With the spread of the SiO₂ glass layer and growth of the Ta-Si-O dendrite, the Ta-Si-O glass ceramics gradually cover on the surface of the SiO₂ glass layer, forming the structure of Ta-Si-O/SiO₂ double glass layer that is capable of sealing and arresting of microcracks.     

Interactions of Ta2O5 with water vapor at elevated temperatures

Tantalum pentoxide (Ta₂O₅) and its solid solution phases are candidate coatings for components to be used in combustion environments. Thus, it is important to understand the response of Ta₂O₅ to high‐temperature water vapor, a product of combustion. Thermogravimetric methods are used to examine the oxide in reactant streams of controlled water vapor contents at 1250°C‐1450°C. The observed weight loss indicates a reaction of the general form ½ Ta₂O₅(s) + x H₂O(g)=TaO_y(OH)_x(g). Methodical variation in the water vapor pressure suggests the products are a mix of TaO(OH)₃(g) and Ta(OH)₅(g). Evidence of TaO(OH)₃(g) was observed with a sampling mass spectrometer. The measured hydroxide and oxyhyroxide vapor fluxes from Ta₂O₅ are compared with calculated vapor fluxes from SiO₂ and Al₂O₃. Ta₂O₅ exhibits fluxes similar to those from SiO₂ due to gaseous metal hydroxide formation.     

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.