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).     

In-situ synthesis and densification of HfB2 ceramics by the spark plasma sintering technique

Hafnium diboride (HfB₂) ceramics were in-situ synthesized and densified by the spark plasma sintering (SPS) method using HfO₂ and amorphous boron (B) as starting powders. Both synthesis and densification processes were succesfully accomplished in a single SPS cycle with one/two step heating schedules, which were designed by considering thermodynamic calculations made by Factsage software. In two step heating schedule, soaking at 1000 °C, which was supposed to be the synthesis temperature of HfB₂ particles, caused a creep like behaviour in final ceramic microstructures. A single step synthesis/densification schedule at 2050 °C with a 30 min hold time under 60 MPa uniaxial pressure leads to obtain monolithic HfB₂ ceramics up to 94% of it's theoretical density. Considering the literature, low hardness values (max. 12 GPa) were achieved, which were directly attributed to the low bonding between HfB₂ grains in terms of the residual stresses occurred during the synthesis and cooling steps. Samples produced by applying one step heating schedule showed transgranural fracture behaviour with a, fracture toughness of 3.12 MPa m^1/2. The fracture toughness of the samples produced by applying two step heating schedule was higher (5,06 MPa m^1/2) and the fracture mode changed from transgranular to mixed mode.     

Synthesis,consolidation and characterization of monolithic and SiC whiskers reinforced HfB2 ceramics

Spark Plasma Sintering is used for the fabrication of highly dense HfB₂ monolithic and HfB₂–26 vol.% SiC_w composite. Reactive SPS from elemental reactants is preferred for the preparation of bulk HfB₂ instead of classical sintering. The desired phase is rapidly formed through a solid–solid combustion synthesis mechanism, while full densification is achieved in 30 min at 1350 A when the applied pressure is switched from 20 to 50 MPa after the synthesis reaction. A 99.4% dense whiskers-reinforced HfB₂ ceramic matrix composite is also obtained in 30 min by SPS (I = 1350 A, P = 20 MPa) using SHSed HfB₂ powders and SiC_w. Nevertheless, whiskers degradation into SiC_p resulted under such conditions (temperature up to 1830 °C). On the other hand, the presence of whiskers is clearly evidenced in 96% dense products obtained when the applied current was decreased down to 1200 A (1700 °C) while P was increased to 60 MPa.     

Microstructure refinement and mechanical properties improvement of HfB2–SiC composites with the incorporation of HfC

HfB₂ and HfB₂–10 vol% HfC fine powders were synthesized by carbo/boronthermal reduction of HfO₂, which showed high sinterability. Using the as-synthesized powders and commercially available SiC as starting powders, nearly full dense HfB₂–20 vol% SiC (HS) and HfB₂–8 vol% HfC–20 vol% SiC (HHS) ceramics were obtained by hot pressing at 2000 °C/30 MPa. With the incorporation of HfC, the grain size of HHS was much finer than HS. As well, the fracture toughness and bending strength of HHS (5.09 MPa m^1/2, 863 MPa) increased significantly compared with HS (3.95 MPa m^1/2, 654 MPa). Therefore, it could be concluded that the incorporation of HfC refined the microstructure and improved the mechanical properties of HfB₂–SiC ceramics.     

Production of TiB2 by SHS and HCl leaching at different temperatures: Characterization and investigation of sintering behavior by SPS

Sub-micron sized TiB₂ ceramic powders were prepared via self-propagating high-temperature synthesis (SHS) followed by HCl leaching at different temperatures. Purified powders obtained using optimum process parameters were consolidated by field assisted sintering technology/spark plasma sintering (FAST/SPS) technique. Phase and microstructural analyses of both the powder and sintered samples were carried out by X-ray diffractometer (XRD) and scanning electron microscope (SEM). The chemical analyses and particle size measurements of the specimen were conducted by inductively coupled plasma-mass spectrometry (ICP-MS) and dynamic light scattering (DLS) techniques. The final properties of the sintered sample were determined in terms of density and microhardness. The effects of different HCl leaching temperatures on the formation, microstructure, particle size, purity and sintering behavior of the SHS-produced TiB₂ powders were investigated. The SHS reaction of TiO₂-B₂O₃-Mg powders as a starting mixture yielded MgO, Mg₃(BO₃)₂ and Mg beside the desired phase TiB₂. All three magnesium containing by-products were completely removed by performing hot HCl leaching. TiB₂ powders after SHS reaction and leaching with 9.3 M HCl for 30 min at 80 °C revealed a minimum purity of 98.4% and a homogenous particle size distribution with an average particle size of 536 nm. In the ultimate SPS experiment which was conducted at 1500 °C for 5 min under a pressure of 50 MPa, a relative density of 94.9% and a micro-hardness value of 24.56 GPa were achieved.     

Oxidation mechanisms under water vapour conditions of ZrB2-SiC and HfB2-SiC based materials up to 2400 °C

This study aims at observing and understanding the oxidation mechanisms of ZrB₂-20 vol%SiC (ZS), HfB₂-20 vol%SiC (HS) and HfB₂-20 vol%SiC- 3 vol%Y₂O₃ (HSY) materials up to 2400 °C under water vapour conditions. After SPS sintering, fully densified samples were oxidized at several temperatures with 30 vol% H₂O/70 vol% Ar during 20 s. Weight variations as well as post-test microstructural and XRD analyses allowed understanding the influence of the composition on the oxidation behavior and the evolution of each oxide sublayer. Below 1550 °C, oxidation is limited, and thin oxide layers are observed. At 1900 and 2200 °C, ZS and HS show mechanical damage (cracks, spallation), while HSY keeps its structural integrity and interlayer adherence. The addition of Y₂O₃ reduces the damages due to thermal stresses in the material due to the stabilization of the cubic phase of HfO₂, and the formation of a Y₂Si₂O₇ interphase that mitigates thermal expansion mismatch between the SiC-depleted layer and the HfO₂ layer.     

Effects of SiC or MoSi2 second phase on the oxide layers structure of HfB2-based composites

Monolithic HfB₂, HfB₂-30 vol% SiC and HfB₂-10 vol% MoSi₂ composites were prepared by SPS and oxidized in stagnant air at 1500 °C for 70 min. The microstructure of the oxide layer cross-sections showed that the oxidation extents were as follow: monolithic HfB₂ > HfB₂-30 vol%SiC > HfB₂-10 vol% MoSi₂.According to the EDS Line-scan, only one porous oxide layer containing a minor amount of B₂O₃was found on the HfB₂ oxidized surface whereas a thick silicate glass layer and a porous oxide layer below that existed on the surface of HfB₂-30 vol% SiC. After oxidation, the surface of HfB₂-10 vol% MoSi₂ had a narrow silicate-oxide compact layer covered by a very thin glass layer. X-ray diffraction patterns of the oxidized surfaces showed the monolithic HfB₂,the HfB₂-30 vol% SiC and HfB₂-10 vol% MoSi₂composites contain, upon oxidation, only m-HfO₂ phase, mainly m-HfO₂ with a minor amount of HfSiO₄ and mainly HfSiO₄ with a minor amount of m-HfO₂ phases, respectively. Based on the observations in this study, it is suggested that the elimination of the porous layer and subsequent increase of the HfSiO₄ phase are the main reasons for the better oxidation resistance of HfB₂-10 vol% MoSi₂.     

Fabrication of high-performance Y2O3 stabilized hafnium dioxide refractories

Y₂O₃ stabilized HfO₂ ceramics with 5 mol% and 8 mol% Y₂O₃, respectively, were successfully fabricated by pressureless sintering at 1600 °C for 1 h. The phase distribution and transformation, microstructure, and relative density were investigated for Y₂O₃ stabilized HfO₂ ceramics. Cubic phase was observed in the Y₂O₃ stabilized hafnium dioxide while pure HfO₂ showed monoclinical phase only. The SEM images of fractured surface indicated two kinds of structures existed in modified HfO₂, the solid solution region and uniform polygon grains, and some holes caused by Kirkendall effect. Refractoriness test showed that high temperature volumetric stability of the material can be effectively enhanced by adding Y₂O₃ into HfO₂.     

Synthesis and spark plasma sintering of sub-micron HfB2: Effect of various carbon sources

The present work describes a simple process to synthesise HfB₂ powder with sub-micron sized particles. Hafnium chloride and boric acid were used as the elemental sources whilst several carbon sources including sucrose, graphite, carbon black, carbon nanotubes and liquid and powder phenolic resin were used. The carbon sources were characterised using thermogravimetric analysis and transmission electron microscope. The mechanism by which the structure of the carbon source used, affects the size and morphology of the resultant HfB₂ powder was studied; the HfB₂ powders were characterised using X-ray diffraction and scanning and transmission electron microscopy. The powder synthesised using powder phenolic resin had a surface area of 21 m² g⁻¹ and a particle size distribution between 30 and 150 nm. This was sintered using SPS to a relative density of 94% of theoretical density (TD) at 2100 °C and 50 MPa pressure without the help of any sintering aids.     

Synthesis and microstructure of Gd2O3-doped HfO2 ceramics

The effect of Gd₂O₃-doping on the crystal structure, surface morphology and chemical composition of the Gd₂O₃–HfO₂ system is reported. Gd₂O₃–HfO₂ ceramics with variable composition were prepared by varying the Gd₂O₃ composition in the range of 0–38 mol% balanced HfO₂. X-ray diffraction (XRD) analysis indicates that the Gd₂O₃ concentration influences the crystal structure of the Gd₂O₃–HfO₂ ceramics. Pure HfO₂ and Gd₂O₃ crystallize in monoclinic and body centered cubic structure, respectively. The Gd₂O₃–HfO₂ ceramics exhibit mixed monoclinic and fluorite structure when the Gd₂O₃ concentration is varied from 4 to 12 mol%. At 20 mol% of Gd₂O₃, existence of only the fluorite phase was found. Increasing the Gd₂O₃ concentration to 38 mol% results in the formation of single-phase pyrochlore Gd₂Hf₂O₇ (a = 5.258 Å).     

Microstructure and properties of Al2O3–TiC nanocomposites fabricated by spark plasma sintering from high-energy ball milled reactants

In situ synthesis of Al₂O₃–TiC nanocomposite powders from a mixture of titanium, graphite, and Al₂O₃ powders by high-energy ball milling (HEBM) and its consolidation through spark plasma sintering (SPS) were investigated. After being milled for 25 h at ambient temperature, the powder mixtures were mainly composed of homogeneous nanosized Al₂O₃ particle and amorphous TiC solid solution. The relative density of the samples consolidated by SPS technique in vacuum at 1480 °C for 4 min reached 99.2%. The final products exhibited very fine microstructure, and the grain sizes of Al₂O₃ and TiC were about 400 nm and 200 nm, respectively, with a flexure strength of 944 ± 21 MPa, Vickers hardness 21.0 ± 0.3 GPa, fracture toughness 3.87 ± 0.2 MPa m^1/2, and electrical conductivity 1.2787 × 10⁵ S m⁻¹.     

Spark plasma sintering of Sm2O3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm₂O₃) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm₂O₃ as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm₂O₃ as sintering additive, with a thermal conductivity value about 118 W m⁻¹ K⁻¹) for the 3 wt.% Sm₂O₃-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm₂O₃ greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm₂O₃ dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.     

Sintering behavior,microstructural evolution,and mechanical properties of ultra-fine grained alumina synthesized via in-situ spark plasma sintering

Ultra-fine grained Al₂O₃ was fabricated by in-situ spark plasma sintering (SPS) process directly from amorphous powders. During in-situ sintering, phase transformation from amorphous to stable α-phase was completed by 1100 °C. High relative density over 99% of in-situ sintered Al₂O₃ was obtained in the sintering condition of 1400 °C under 65 MPa pressure without holding time. The grain size of in-situ sintered Al₂O₃ body was much finer (~400 nm) than that of Al₂O₃ sintered from the crystalline α-Al₂O₃ powders. For in-situ sintered Al₂O₃ from amorphous powders, we observed a characteristic microstructural feature of highly elongated grains in the ultra-fine grained matrix due to abnormal grain growth. Moreover, the properties of abnormally grown grains were controllable. Fracture toughness of in-situ sintered Al₂O₃ with the elongated grains was significantly enhanced due to the self-reinforcing effect via the crack deflection and bridging phenomena.     

Ablation resistance of HfB2-SiC coating prepared by in-situ reaction method for SiC coated C/C composites

To improve the ablation resistance of SiC coating, HfB₂-SiC coating was prepared on SiC-coated carbon/carbon (C/C) composites by in-situ reaction method. Owing to the penetration of coating powders, there is no clear boundary between SiC coating and HfB₂-SiC coating. After oxyacetylene ablation for 60 s at heat flux of 2400 kW/m², the mass ablation rate and linear ablation rate of the coated C/C composites were only 0.147 mg/s and 0.267 µm/s, reduced by 21.8% and 60.0%, respectively, compared with SiC coated C/C composites. The good ablation resistance was attributed to the formation of multiple Hf-Si-O glassy layer including SiO₂, HfO₂ and HfSiO₄.     

Mechanical properties of nano-grain SiO2 glass prepared by spark plasma sintering

Silicon carbide (SiC) layers were deposited on silica (SiO₂) glass powder by rotary chemical vapor deposition (RCVD) to form SiO₂ glass (core)/SiC (shell) powder; this powder was consolidated by spark plasma sintering (SPS). SiO₂ glass powder with a particle size of 250 nm was coated with 5–10-nm-thick SiC layers. The resultant SiO₂ glass (core)/SiC (shell) powder was consolidated to form a nano-grain SiO₂ glass composite at a relative density above 90% by SPS in the sintering temperature range of 1573–1823 K. The Vickers hardness and fracture toughness of the SiO₂ glass composite at 1723 K were found to be 14.2 GPa and 5.4 MPa m^1/2, respectively.     

Synthesis and characterization of nanometric yttrium-doped hafnia solid solutions

Nanometric-sized yttrium doped HfO₂ powders were obtained by applying metathesis and combustion reactions. The tailored composition of solid solutions was: Hf_1?xY_xO_2?δ with concentration “x” ranging from 0 to 0.2. HfCl₄ was used as a source of hafnium whereas Y(NO₃)₃·6H₂O was used as a source of yttrium. The obtained powders were annealed at different temperatures in order to induce crystallization of HfO₂. The influence of dopant concentration, annealing temperature and annealing time on powder properties was examined. The XRD analysis revealed that the crystal structure of HfO₂ depends on the dopant concentration. The samples doped with 20 mol% of yttrium and annealed at 1500 °C had high-temperature, cubic structure even after cooling to room temperature. The presence of relatively large amount of dopant was beneficial in stabilizing highly desirable cubic phase of HfO₂. It was found that the crystallite size lies in the nanometric range (<10 nm).     

Spark plasma sintering and characterization of ZrC-TiB2 composites

ZrC-based composites were consolidated from ZrC and TiB₂ powders by the Spark Plasma Sintering (SPS) technique at 1685 °C and 1700 °C for 300 s under 40-50-60 MPa. Densification, crystalline phases, microstructure, mechanical properties and oxidation behavior of the composites were investigated. The sintered bodies were composed of a (Zr,Ti)C solid solution and a ZrB phase. The densification behaviors of the composites were improved by increasing the TiB₂ content and applied pressure. The highest value of hardness, 21.64 GPa, was attained with the addition of 30 vol% TiB₂. Oxidation tests were performed at 900 °C for 2 h and the formation of ZrO₂, TiO₂ and B₂O₃ phases were identified by using XRD.     

Combustion synthesis of single-phase β-sialons (z = 2–4)

Single-phase β-sialon powders (z = 2–4) have been prepared with homogeneous compositions by the combustion synthesis. The raw materials (Si, Al and SiO₂) were combusted under N₂ pressure of 1 MPa. Without using a diluent, the reaction temperatures were very high (>2000 °C) and the combustion products contained Si and Al residues. With addition of commercial β-sialon (z = 1) as a diluent (up to 50 wt%), both the reaction temperatures and amount of residual Si and Al significantly decreased. The combustion reactions completed at 50 wt% dilution, and pure β-sialon phases were synthesized. When the combustion product itself is used instead of the commercial diluent, the phase content of desired z value increased with the repetition times until a single-phase powder is produced. The sinterability of the synthesized powders was then tested using 5 wt% Y₂O₃ as a sintering aid by the spark plasma sintering (SPS).     

CuAlO2 thermoelectric compacts by SPS and thermoelectric performance improvement by orientation control

We fabricated CuO/Al₂O₃ green compacts from plate-like Al₂O₃ and granular CuO powders by multi-press forming and investigated the alumina orientation using Lotgering's method. The results showed that Al₂O₃ particles preferentially aligned perpendicular to the pressure direction and the orientation degree increased as the forming pressure was increased. We proposed a model describing the movement of the alumina particles to explain the pressure effect on their orientation. The orientation calculation was in good agreement with those by Lotgering's method. Furthermore, we prepared the CuAlO₂ compacts by regular or spark plasma sintering (SPS). However, the compacts sintered by SPS exhibited higher orientation degree and density than those produced by regular sintering. The electrical conductivity values of the orientation-controlled compacts sintered by SPS reached 770 S m⁻¹ at 928 K, which was close to that of CuAlO₂ single crystal. The power factor of the CuAlO₂ compacts with highest orientation degree is as high as 5.95 × 10⁻⁵ W m⁻¹ K⁻¹ at 928 K. Therefore, we can conclude that orientation control is an effective method to enhance the thermoelectric performance of compact polycrystalline CuAlO₂ bulks.     

Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) is a powerful technique to produce fine grain dense ferrite at low temperature. This work was undertaken to study the effect of sintering temperature on the densification, microstructures and magnetic properties of magnesium ferrite (MgFe₂O₄). MgFe₂O₄ nanoparticles were synthesized via sol–gel self-combustion method. The powders were pressed into pellets which were sintered by spark plasma sintering at 700–900 °C for 5 min under 40 MPa. A densification of 95% of the theoretical density of Mg ferrite was achieved in the spark plasma sintered (SPSed) ceramics. The density, grain size and saturation magnetization of SPSed ceramics were found to increase with an increase in sintering temperature. Infrared (IR) spectra exhibit two important vibration bands of tetrahedral and octahedral metal-oxygen sites. The investigations of microstructures and magnetic properties reveal that the unique sintering mechanism in the SPS process is responsible for the enhancement of magnetic properties of SPSed compacts.