Improvement of sinterability and mechanical properties of ZrB2 ceramics by the modified borothermal reduction methods

Starting from ZrO₂ and boron (molar ratio: 1:4), four ZrB₂ powders were synthesized by borothermal reduction method, three of which were designed to introduce minor modifications by combining solid solution with Ti and/or water-washing. The sinterability, microstructures, mechanical properties and thermal conductivity were investigated. In comparison with the conventional borothermal reduction, the modified methods offered significant improvement in terms of densification of ZrB₂ ceramics, particularly the mixture that included water-washing. Owing to the refined particle size and boron residues, ZrB₂ ceramics from the modified borothermal reduction which included water-washing demonstrated nearly full densification, Vickers hardness of 14.0 GPa and thermal conductivity of 82.5 W/mK after spark plasma sintering at 2000 °C for 10 min. It was revealed that the properties of ZrB₂ ceramics could be enhanced utilizing the proposed minor modification, starting from the same raw materials and adopting the same sintering conditions.     

Synthesis of TaB2 powders by borothermal reduction

Borothermal reduction processes of Ta₂O₅ with boron under vacuum were investigated. Ta₂O₅ reacted with boron to form various borides (TaB₂, Ta₃B₄, and TaB), depending on the boron/Ta₂O₅ molar ratio and temperature. In order to prepare pure TaB₂ powders, two routes were developed. The first route was one‐step heat treatment at 1550°C. With boron/Ta₂O₅ molar ratio of 9.0, pure TaB₂ powders with strong agglomeration were synthesized by the first route, and the particle size and oxygen content were 0.7 μm and 0.9 wt%, respectively. The second route consisted of two‐step heat treatment at 800°C and 1550°C plus intermediate water washing. With lower boron/Ta₂O₅ molar ratio of 8.2, pure TaB₂ powders with less agglomeration and more uniform distribution were synthesized by the second route, and the particle size and oxygen content were 0.8 μm and 0.8 wt%, respectively. Moreover, the particle size similarity of TaB₂ powders by the two routes suggested that byproduct boron oxides, which were previously reported as the most important factor in promoting the coarsening of ZrB₂ powders by borothermal reduction, did not lead to the significant coarsening of TaB₂ powders.     

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 of TiC-TiB2 composite powders from carbon coated TiO2 precursors

Submicron TiC-TiB₂ composite powders were synthesized by carbo/borothermal reductions from a novel carbon coated TiO₂ precursors method. Reactants were also mechanically mixed for comparison. Precursors and conventionally mixed powders were reacted from 1100 to 1500 °C for 2 h in flowing argon at 1 L/min. Phase evolution, thermodynamic analysis, microstructures, and surface areas were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and Brunauer-Emmett-Teller (BET) surface area analyzer, respectively. Products synthesized from precursors had high purity of TiC-TiB₂ mixture, fine particle size, narrow size distribution, regular shape, loose agglomeration, and high surface area. Powders with different compositions were also prepared by controlling the ratio between reactants. The sintering tests demonstrated that produced powders can be sintered to a 99.6% relative density using 5 wt% nickel additives at 1550 °C for 2 h in flowing argon atmosphere.     

Self‐propagating mechanosynthesis of HfB2 nanoparticles by a magnesiothermic reaction

A mechanically induced self‐sustaining reaction (MSR) was used to synthesize hafnium diboride nanoparticles. Along this route, magnesium was selected as a robust reducing agent for co‐reduction in boron and hafnium oxides in a combustive manner. Combustion occurred after a short milling period of 12 minutes. The hafnium diboride nanoparticles had a polygonal faceted morphology and were 50‐250 nm in diameter. The assessment of the processing mechanism revealed that the initial combustive reduction in B₂O₃ to elemental B by Mg was the major step for progressing the overall reaction. After that, HfO₂ can be reduced to elemental Hf, followed by the synthesis of HfB₂ phase.     

Synthesis of sub-micro sized high purity zirconium diboride powder through carbothermal and borothermal reduction method

Sub-micro sized zirconium diboride (ZrB₂) powders were successfully prepared via the boro/carbothermal reduction method using zirconium oxide and boron carbide as the primary raw materials. The prepared mixtures were thermally reacted at 1250 °C for 1 h. The optimized composition range containing the lowest oxide and carbide impurity, which was 0.14% of oxygen and 0.3% of carbon contents, was determined using crystallographic and elemental analysis. The particle size was reduced from 5 µm to 245 nm by the addition of B₄C as a reductant within a composition range that maintained the highest purity. The morphology changed from faceted to angular hexagonal bar-like with a simultaneous growth in particle size. Changes in the particle structure were a result of the existing liquid B₂O₃ phase during the reaction. The 245-nm particles contained 12.1% oxygen content and 16.2% oxygen content for the 5-μm particle in the circumstance in which limited oxides could be produced.     

Microstructure and properties of NiAl/TiC composite synthesized by spark plasma sintering of mechanically activated elemental powders

In this study, NiAl/TiC_0.95 composite was synthesized by reactive spark plasma sintering of mechanically activated elemental powders. The microstructure and properties of activated powders and sintered samples were evaluated. The elemental powders were milled after different milling times and as-mixed and 10 h milled powder mixtures were sintered by the reactive spark plasma sintering method. The phase and the microstructure changes were evaluated by x-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy, respectively. The XRD pattern of 0 h milled powder after sintering showed that Ni₃Al, Ni₂Al₃ beside NiAl and TiC_0.75 formed. While after the sintering of 10 h mechanically activated powder, the Ni₃Al and Ni₂Al₃ were eliminated and NiAl remained with TiC_0.95. The nanoindentation result of the SPSed sample showed a hardness of 12.2 ± 0.1 GPa with an elastic modulus of 25.0 ± 0.5 GPa.     

Effect of ZrB2 powders on densification,microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB₂-SiC ceramics, three synthesis routes were investigated for the production of ZrB₂, prior to the fabrication of ZrB₂-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB₂, as a precursor material for the production of ZrB₂-SiC. It was determined that reduction in the primary ZrB₂ particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB₂-SiC ceramics, with ZrB₂ derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m^-1K^-1).     

Structural investigation of mechanically activated nanocrystalline BaTiO3 powders

In this article, in order to obtain tetragonal nanocrystalline BaTiO₃, structural investigations of mechanically activated BaTiO₃ powder have been performed. A mercury porosimetry analysis and scanning electron microscopy method have been applied for determination of the specific pore volume, porosity and microstructure morphology of the samples. The lattice vibration spectra of nonactivated and activated powders, their phase composition, lattice microstrains and the mean size of coherently diffracting domains were examined by Raman spectroscopy and the X-ray powder diffraction method. The average crystal structure of obtained nanocrystalline powders, estimated from X-ray diffraction data, gave evidence of retained, but slightly sustained tetragonality of powders, even for particles as small as ∼30 nm. Raman spectroscopy also gave clear evidence for local tetragonal symmetries, in particular through the presence of a band at ∼307 cm⁻¹.     

Synthesis of ZrB2-SiC powders in one-step reduction process

ZrB₂-SiC composite powders were synthesized through one-step reduction process of ZrO₂, B₄C, carbon black, silicon or silica under flowing argon. Effects of B₄C contents, calcination temperatures and different silicon sources on the phase composition and morphology were investigated. Combining the X-ray diffraction (XRD) results and scanning electron microscope (SEM) images, the spherical ZrB₂-SiC powders ranging from 100?nm to 300?nm would be prepared with silicon at 1500?°C for 60?min when n(B)/n(Zr) was at 2.4. As using silica as the raw material, the obtained ZrB₂ and SiC particles in the powders exhibited different shapes and sizes. The SiC grains were uniformly formed among the ZrB₂ grains.     

Synthesis of ultra-fine hafnium carbide powders combining the methods of liquid precursor conversion and plasma activated sintering

Ultra-fine hafnium carbide (HfC) powders were synthesized using a novel method combining liquid precursor conversion and plasma activated sintering (PAS). Solution-based processing was used to achieve a fine-scale mixing of the reactants, and further treatment by PAS allowed fast formation of HfC. We investigated the effect of the type of acid used during the liquid precursor conversion on the synthesized powders, where mixtures were prepared using salicylic acid, citric acid, or a combination of these. The results show that pure HfC powders (with an average particle sizes of 350 nm) were obtained at a relatively low temperature (1550 °C) using a HfOCl₂·8H₂O precursor with the mixed acids. The oxygen content of the synthesized powders was only 0.97 wt%. The type of acid had a significant effect on the synthesis product. When using only citric acid, the temperature required to produce pure hafnium carbide increased to 1700 °C. In the case of a salicylic acid precursor, pure HfC was not obtained, even at a synthesis temperature of 1700 °C.     

机械活化勃姆石-二氧化硅混合物合成莫来石

以勃姆石和无定形二氧化硅为原料,按n(A l2O3):n(SiO2)=3:2配制成勃姆石-二氧化硅起始混合物,先采用高能球磨进行0~40 h的机械活化,然后在1 100~1 600℃的空气气氛中热处理3 h。通过对高能球磨前后物料以及热处理前后物料的XRD和SEM分析,研究了高能球磨对物料的晶体形态及其莫来石开始生成温度和莫来石完全生成温度的影响。结果表明:(1)机械活化使得起始物料组分由晶态向无定形态转化,减小了物料的颗粒尺寸和晶粒尺寸,提高了起始物料组分的混合均匀性。(2)未经机械活化的物料在1 300℃煅烧后开始有莫来石生成,而经40 h机械活化的物料在1 100℃煅烧便有莫来石相出现,即机械活化使莫来石的开始生成温度至少降低了200℃。(3)未经机械活化的物料在1 600℃煅烧没有完全生成莫来石,而经40 h机械活化的物料在1 300℃煅烧便生成了纯莫来石相,即机械活化使得莫来石的完全生成温度至少降低了300℃。     

Synthesis and characterization of BaAl2Si2O8 using mechanically activated precursor mixtures containing coal fly ash

BaAl₂Si₂O₈ (BAS) was synthesized by solid state reaction, using coal fly ash [CFA, containing (in wt.%) 75–78% silico-aluminous glassy phase, 14.6% mullite, 5.2% quartz, 1.9% magnetite, plus other minor phases] as main raw material. A BaCO₃–CFA–Al₂O₃ powder mixture of stoichiometric BAS composition was mechanically activated in an attrition mill for up to 12 h and then sintered at 900–1300 °C. The monoclinic BAS phase (Celsian) was desired because it has better mechanical and thermal properties than the hexagonal BAS polymorph (Hexacelsian), but this tends to form first, remaining frequently metastably at low temperatures; besides, the Hexacelsian to Celsian conversion is sluggish and difficult to occur. The reaction rate, the apparent density and the mechanical properties of the synthesized materials increased with increasing milling time and sintering temperature. The mineralizing effect caused by some of the CFA impurities produced Hexacelsian to Celsian conversions higher than those previously reported for mechanically activated BAS materials.     

Influence of SiC on phase and microstructure of ZrB2 powders synthesized via carbothermal reduction at different temperatures

ZrB₂ powders were successfully prepared via carbothermal reduction of ZrO₂ with H₃BO₃ and carbon black under flowing argon. By introducing SiC species into reaction mixtures, the effects of SiC addition on phase composition and morphology of ZrB₂ powders thermally treated at different temperatures were investigated. The resultant samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). The highly pure ZrB₂ with the mean size of 5?µm could be obtained at 1600?°C for 90?min and the grains presented columnar shapes. After addition of SiC, ZrB₂ revealed relatively better crystallinity and finer particle size. Regular columnar ZrB₂ grains ranging from 1 to 2?µm were seen existing after reaction at 1500?°C for 90?min.     

Mechanical activation-assisted autoclave processing and sintering of HfB2-HfO2 ceramic powders

This study reports on the synthesis and consolidation of HfB₂-HfO₂ ceramic powders via mechanical activation-assisted autoclave processing followed by pressureless sintering (PS) or spark plasma sintering (SPS). HfCl₄, B₂O₃ and Mg starting powders were mechanically activated for 5 min to obtain homogeneously blended precursors with active particle surfaces. Autoclave synthesis was carried out at a relatively low temperature at 500 °C for 6 or 12 h. As-synthesized powders were purified from reaction by-products such as MgO and MgCl₂ by washing and acid leaching treatments. The characterization investigations of the as-synthesized and purified powders were performed by using an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM) and particle size analyzer (PSA). The purified powders with an average particle size of about 190 nm comprised the HfB₂ phase with an amount of 79.6 wt% in addition to the HfO₂ phase and a very small amount of Mg₂Hf₅O₁₂ phase after mechanical activation for 5 min and autoclave processing for 12 h. They were consolidated at 1700 °C both by PS for 6 h and SPS for 15 min. The Mg₂Hf₅O₁₂ phase decomposed during sintering and bulk samples only had the HfB₂ and HfO₂ phases. The bulk properties of the sintered samples were characterized in terms of microstructure, density, microhardness and wear characteristics. The HfB₂-HfO₂ ceramics consolidated by PS exhibited poor densification rates. A considerable improvement was obtained in the relative density (~91%), microhardness (~16 GPa) and relative wear resistance (2.5) values of the HfB₂-HfO₂ ceramics consolidated by SPS.     

Ablation resistance of HfC(Si,O)-HfB2(Si,O) composites fabricated by one-step reactive spark plasma sintering

A dense HfC(Si, O)-HfB₂(Si, O) composite was fabricated by reactive spark plasma sintering using HfC and SiB₆ as starting reactants. The best ablation resistance was obtained with the composite fabricated with the addition of 15 vol.% SiB₆. After ablation under an oxyacetylene flame for 60 s, the mass and linear ablation rates of this composite were ?0.007 mg cm^?2 s^?1 and ?0.233 μm s^?1, respectively. The negative ablation rates are the result of a slight mass gain/thickness increase, which indicate that the oxidation process was stable and mechanical scouring was limited during ablation. This enhanced ablation resistance was attributed to a unique double-layered oxide formation, which possessed lower oxygen permeability and better mechanical strength. The solid solution nature of the composite and its appropriate phase composition were responsible for the stable oxide structure formation.     

Properties and consolidation of nanocrystalline WSi2-SiC composite from mechanically activated powders by pulsed current activated combustion synthesis

Dense nanostructured WSi₂-SiC composite was synthesized by pulsed current activated combustion synthesis method within 2 min in one step from mechanically activated powders of WC and 3Si. Simultaneous combustion synthesis and consolidation were accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense WSi₂-SiC with relative density of up to 99.8% was produced under simultaneous application of a 80 MPa pressure and the pulsed current. The average grain size and mechanical properties of the composite were investigated.     

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.     

Synthesis of rod-like ZrB2 powders

Rod-like ZrB₂ powders were synthesised at 1500°C in vacuum by boro/carbothermal reduction using ZrO₂, B₄C and graphite as the starting materials. During the heating process, the ZrB₂ grains primarily grow along the c axis to form a rod-like morphology without any heterogeneous catalyst. The final products are pure rod-like ZrB₂ particles, which are thought to be promising starting powders to prepare high performance ultrahigh temperature ceramics with unique microstructures such as textured one through tape casting process.     

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.