Phase relations in the system CaO-B2O3-SiO2

Phase relations in the lime-rich portion of the system CaO-B₂O₃-SiO₂ have been studied by microscopy, infrared spectroscopy and X-ray powder diffraction of heated mixtures and quenched charges. Extensive solid solution of B₂O₃ in Ca₂SiO₄ occurs along the Ca₂SiO₄-Ca₃B₂O₆ boundary, which has been studied in detail. It contains a ternary compound, Ca₁₁B₂Si₄O₂₂, which is stable to liquidus temperatures, melting incongruently to Ca₂SiO₄ and liquid at 1420 °C. Ca₁₁B₂Si₄O₂₂forms a eutectic with Ca₃B₂O₆ at 1400 °C and, in the ternary system, with CaO and Ca₃B₂O₆ at 1390 °C.     

Densification behaviour and mechanical properties of pressureless-sintered B4C–CrB2 ceramics

B₄C based ceramics composites with 0–25 mol% CrB₂ were fabricated by pressureless sintering in the temperature range 1850°C to 2030°C. The CrB₂ addition enhanced the densification of B₄C due to the CrB₂–B₄C eutectic liquid phase formation. Both a high strength of 525 MPa and a modest fracture toughness of 3.7 MPa m^1/2 were obtained for the B₄C–20 mol% CrB₂ composite with a high-relative density of 98.1% after sintering at 2030°C. The improvement in fracture toughness is thought to result from the formation of microcracks and the deflection of propagating cracks resulting from the thermal expansion mismatch of CrB₂ and B₄C.     

Sintering and characterization of Al2O3-B4C composites

The effect of B₄C on the densification, microstructure and mechanical properties of pressureless sintered Al₂O₃-B₄C composites have been studied. Sintering was performed without sintering additives with varying B₄C content from 0–40 vol %. Up to 20 vol % B₄C, more than 97% theoretical density was always obtained when sintered at 1850 °C for 60 min. On increasing the sintering time from 30–120 min, there was no change in density. The result of X-ray diffraction analysis showed that no reaction occurred between Al₂O₃ and B₄C. The grain growth of Al₂O₃ was inhibited by B₄C particles pinned at the grain boundary and the grain-boundary drag effect. The critical amount of B₄C to drag the grain boundary migration effectively was believed to occur at 10 vol % B₄C sintered at 1850 °C for 60 min. The maximum three-point flexural strength was found to be 550 MPa for the specimen containing 20 vol % B₄C, and the maximum microhardness was 2100 kg mm^–2 for 30 vol % B₄C specimen.     

New approach to MoSi2/SiC intermetallic-ceramic composite with B4C

The effects of SiC and B₄C additives in the MoSi₂ matrix on the microstructures and mechanical properties at room temperature were investigated. Their coefficients of thermal expansion (CTE) were also evaluated up to 1200°C by a thermal mechanical analysis (TMA). The experimental results show that the Mo₂B₅ reinforced phase was formed in situ in the hot-pressed MoSi₂/SiC/B₄C composites. Both the Mo₂B₅ phase and the SiC phase significantly improved the mechanical behavior of MoSi₂. Besides, the SiC with a high content up to 40 vol% could be added into the MoSi₂ composite with the B₄C additive. As a result, a dense and homogenous MoSi₂/SiC/B₄C composite was obtained, which possessed a relatively high bending strength and fracture toughness. Meanwhile, the CTE of the MoSi₂/SiC/B₄C composites linearly decreased with the increasing SiC content, which dropped to 21% at 1200°C in comparison with the pure MoSi₂ when adding 40 vol% SiC. This MoSi₂/SiC/B₄C composite system is very important for developing new applications at elevated temperature, particularly for high-temperature coating applications.     

(Ti,W,Cr)B2 coatings produced by dc magnetron sputtering

Transition metal diboride coatings of composition (Ti_0.44W_0.29Cr_0.27)B_1.90 were deposited on silicon substrates by dc magnetron sputtering of compound targets. The chemical composition of the targets is transferred to the sputtered films. The as-deposited films are amorphous as indicated by grazing incidence X-ray diffraction. Investigations with electron microscopy revealed that the films show a columnar nano-structure. Annealing at temperatures between 1000 °C and 1300 °C leads to the formation of nano-crystalline precipitations, which can be attributed to (Ti,W,Cr)B₂, β-(W, Ti, Cr)B and W₂B₄ phases. Annealing can be used to tailor the average grain size of the precipitates, making these films a good candidate for hard coatings re-enforced by nano-structuring.     

Advances in pressureless densification of boron carbide

The coarsening processes which occur concurrently with the sintering of B₄C were deduced using differential dilatometry, weight loss and grain size measurements. B₂O₃ coatings on B₄C particles stalled the onset of densification until these coatings decomposed away at 1840°C. Oxide gaseous species formed in this process were postulated to be one conduit of particle coarsening. At higher temperatures, the volatilization of B₄C itself were interpreted to be the second vapor phase coarsening mechanism, starting at temperatures above 2000°C. Carbon additions or soaking the compact in a hydrogen-containing atmosphere at 1350°C removed the oxide coatings at temperatures below those in which sintering or coarsening occurred. Accelerating densification above 2130°C was attributed to nonstoichiometric volatilization, leaving carbon behind to facilitate activated sintering.     

Formation process of tungsten borides by solid state reaction between tungsten and amorphous boron

Various kinds of tungsten borides were synthesized by solid state reaction between tungsten and amorphous boron powders. The mixed powders with various compositions (B/W = 0.4 to 13.0) were treated at 800 to 1550° C for 0 to 120 min in a stream of argon. Four kinds of boride phases such as W₂B, WB, W₂B₅ and WB₄ were formed, although the boride phase having the composition of the highest boride, WB₁₂, did not appear. The formation of W₂B was initiated approximately at 1000° C in excess of tungsten content. On the other hand, in the excess boron content, the formation of WB, W₂B₅ and WB₄ was initiated approximately at 800, 950 and 1200° C, respectively. The maximum formation amount and crystallinity of WB and W₂B₅ was found in nearly 10 at % excess boron content in their own stoichiometric compositions. The only crystalline phase of WB₄ was prepared with a large excess boron content. However, the formation behaviour of WB₄ showed that WB₄ is metastable above 1400° C. The stability of WB₄ phase could be increased by the presence of excess boron.     

Studies on the formation of whiskers and platelets of B4C and BN

The formation of whiskers and platelets of B₄C and BN has been studied through carbothermal reduction of B₂O₃. In the absence of any additive, neither whiskers nor platelets have formed from B₂O₃ and carbon black. K₂CO₃ which forms a low melting liquid and NiCl₂ which act as catalyst in gasification of carbon were used to facilitate the growth of whiskers and platelets. NiCl₂, K₂CO₃, carbon black and B₂O₃ were reacted in a weight ratio (NiCl₂:K₂CO₃:C:B₂O₃= 5:5:12:17.4) and studied the formation of B₄C and BN in the temperature range of 940°C to 1500°C in 1-atm. argon and 1-atm. nitrogen respectively. Whiskers and platelets of different sizes have formed at 1100–1500°C. The whiskers have been observed to form by vapor-liquid-solid growth mechanism. The effect of NiCl₂ and K₂CO₃ on the morphology of B₄C and carbon has been studied. NiCl₂ and K₂CO₃ have been found to accelerate the growth of whiskers and platelets.     

Analysis of the precipitation process of secondary phase MgNi2.5B2 and superconducting properties in Ni-doped MgB2 bulk

A polycrystalline MgB₂ sample with 4 at.% Ni powder addition has been synthesized by a solid-state reaction at 750 °C. Two different Ni powders, raw (40 μm) and ball-milled (10 μm) particles were selected to clarify the influence of the Ni particle size on the secondary phase MgNi_2.5B₂ formation and the critical current density J_c of MgB₂ bulks. According to the investigation of the sintering, it was found that the smaller Ni particles may significantly decrease the formation temperature of the MgNi_2.5B₂ ternary compound to 550 °C. Combined with scanning electron microscopy (SEM) analysis and magnetic measurements, it was suggested that the addition of fine Ni particles led to MgNi_2.5B₂ inclusion precipitating along the spiral terraces of MgB₂ grains and with small particles sizes. The unique microstructure is responsible for the smaller reduction in J_c, which partly eliminates the effect of weak links compared with the coarse Ni particles. Besides, the formation mechanism of the microstructures and the relative location of MgNi_2.5B₂ varying from cluster at the MgB₂ grain boundaries to a screw arrangement around the MgB₂ grains with the decrease of the Ni grain sizes were also discussed in detail.     

Microstructure and mechanical properties of pulsed electric current sintered B4C-TiB2 composites

Monolithic B₄C, TiB₂ and B₄C-TiB₂ particulate composites were consolidated without sintering additives by means of pulsed electric current sintering in vacuum. Sintering studies on B₄C-TiB₂ composites were carried out to reveal the influence of the pressure loading cycle during pulsed electrical current sintering (PECS) on the removal of oxide impurities, i.e. boron oxide and titanium oxide, hereby influencing the densification behavior as well as microstructure evolvement. The critical temperature to evaporate the boron oxide impurities was determined to be 2000 °C. Fully dense B₄C-TiB₂ composites were achieved by PECS for 4 min at 2000 °C when applying the maximum external pressure of 60 MPa after volatilization of the oxide impurities, whereas a relative density of 95-97% was obtained when applying the external pressure below 2000 °C. Microstructural analysis showed that B₄C and TiB₂ grain growth was substantially suppressed due to the pinning effect of the secondary phase and the rapid sintering cycle, resulting in micrometer sized and homogeneous microstructures. Excellent properties were obtained for the 60 vol% TiB₂ composite, combining a Vickers hardness of 29 GPa, a fracture toughness of 4.5 MPa m^1/2 and a flexural strength of 867 MPa, as well as electrical conductivity of 3.39E+6 S/m.     

Precipitation mechanism of BN in the ternary system of B-Mg-N

The phase relations in the ternary system B-Mg-N resulting from the reaction between Mg and BN at 2.5 GPa were investigated by means ofin situ differential thermal analysis (DTA) supported by a conventional quenching technique. One compound, Mg₃B₂N₄, was newly formed when melted Mg reacted with hexagonal BN (hBN) solid above 1150° C and formed the eutectic with Mg and hBNat 737 and 1295° C, respectively. It was also confirmed that hBN crystals were precipitated from the liquid above 1300° C. The catalytic mechanism controlling cBN formation in the system Mg-BN was discussed from the experimental results of the present work. The lower temperature limit of cubic BN (cBN) growth region appears to be closely related to the initial formation of liquid phase due to a eutectic of Mg₃B₂N₄-BN.     

Effect of forms of B2O3-SiO2 as an additive on magnetic properties of SrFe12O19 ferrites

Strontium ferrites (SrO · 5.5Fe₂O₃) were prepared using hot-rolled mill scale as a source of iron oxides. Calcination was performed at 1200°C for 2 h. The fused B₂O₃-SiO₂, with various mole ratios of B₂O₃ to SiO₂, was added to the calcined ferrites during the milling stage. The ferrites were formed anisotropically. The fused additives are quite effective to enhance the magnetic properties; (BH)_max can reach 3.0 MGOe for the ferrite sintered at 1220°C for 2 h. In general, (BH)_max is independent of the mole ratio of B₂O₃ to SiO₂ and dominantly influenced by the sintering temperature. The addition of 0.3 or 0.5 wt% fused additives showed no significant difference in the magnet quality. The quality of the magnet was decreased with increasing mole ratio of B₂O₃ to SiO₂ when unfused B₂O₃-SiO₂ was used under the same processing conditions.     

Improvements in mechanical properties of TiB2 by the dispersion of B4C particles

Densities up to 99% of the theoretical value were achieved by hot-pressing of TiB₂-B₄C composites at 1700° C for 1 h using 1 vol % Fe as a sintering aid. The microstructure consists of dispersed B₄C particles in a fine-grained TiB₂ matrix. Addition of B₄C particles increases the fracture toughness of TiB₂ (to 7.6 MPa m^1/2 at 20 vol % B₄C) and yields high fracture strength (to 700 MPa at 10 vol % B₄C). Microstructural observations indicate that the improved strength is a result of a higher density, smaller grain size and intergranular fracture, and the toughness increase is a result of crack deflection around the B₄C particles.     

Synthesis, densification, and mechanical properties of TaB2

Tantalum diboride (TaB₂) was synthesized by reducing Ta₂O₅ using B₄C and graphite at 1600 °C under flowing Ar. The powder had an average particle size of 0.4 μm with both needle-like and rounded particles. The TaB₂ powder was hot pressed to relative densities of 97% at 2000 °C (3.6 μm grain size) and 98% at 2100 °C (5.3 μm grain size). Mechanical properties were measured for TaB₂ hot pressed at 2100 °C and were comparable to those of the commonly studied diborides, ZrB₂ and HfB₂. The Young's modulus was 551 GPa, Vickers' hardness was 25.6 GPa, flexure strength was 555 MPa, and fracture toughness was 4.5 MPa-m^1/2.     

The synthesis of cBN using Ca3B2N4

The phase relation of the system Ca-B-N was investigated at a pressure of 2.5 GPa by both differential thermal analysis (DTA) and a quenching method. When BN reacted with Ca at temperatures higher than 1150° C, Ca₃B₂N₄ was formed together with small amounts of Ca₃N₂ and CaB₆. The system of Ca₃B₂N₄-BN has a eutectic relationship at 1316° C and 2.5 GPa. The synthesis of cBN was established by using Ca₃B₂N₄ under the thermodynamic stable conditions of cBN. It could be seen that cBN crystals grew repeatedly in Ca₃B₂N₄ melt, which advanced into the hBN layer. The cBN crystals obtained in this way were of outstanding purity and of good quality. This indicated that hBN dissolved and precipitated as cBN in Ca₃B₂N₄ melt without any appreciable change of molar boron/nitrogen ratio. In the system of Ca-B-N, the low-temperature limit of cBN formation was closely related to the eutectic relationship between BN and Ca₃B₂N₄.     

Process and wear behavior of monolithic SiC and short carbon fiber-SiC matrix composite

The process and wear behavior of monolithic SiC and 10 vol. % short carbon fiber-SiC matrix (C-SiC) composite have been studied. The results indicate that, among ethyl alcohol, acetone, n-hexane and n-octyl alcohol, n-octyl alcohol was the most effective dispersing agent in dispersing both SiC powder and short carbon fiber. Among AlN, Al₂O₃, B₄C, graphite, AlN/B₄C, AlN/graphite, B₄C/graphite and Al₂O₃/B₄C, the most effective sintering aid for the fabrication of SiC and C-SiC composite was a mixture of 2 wt% AlN and 0.5 wt% graphite. The monolithic SiC hot-pressed at 2100°C exhibited higher density but lower flexural strength than those hot-pressed at 2000°C due to a grain growth effect. For the C-SiC composite, both density and strength of the composite hot-pressed at 2100°C were generally higher than those hot-pressed at 2000°C. The density and strength of C-SiC composite were lower than those of monolithic SiC under the same hot pressing conditions due to a higher porosity level in the composite. When monolithic SiC slid against C-SiC composite, the weight losses of SiC and the composite were each less than that of self-mated SiC or self-mated C-SiC. In the self-mated SiC tribosystem, a mechanically stable film could not be established, resulting in an essentially constant wear rate. When sliding against C-SiC, a thin, smooth and adherent debris film was quickly formed on the SiC surface, resulting in a lower wear.     

Oxidation resistance of carbon-ceramics composite materials sintered from ground powder mixtures of raw coke and ceramics

Carbon-SiC-B₄C composite materials were prepared from ground powder mixtures of petroleum raw coke, SiC and B₄C by powder sintering, without the use of any special binder and hot-pressing process. Dense composites with a fine microtexture were obtained. Oxidation tests were carried out on the composites at temperatures from 1000 to 1300° C under an air flow. The oxidation resistance depended strongly on the SiC/(SiC + B₄C) ratio and total contents of SiC and B₄C in the composites, which determined the compositions of B₂O₃ and SiO₂ in the protective film formed at the surface of the composite block during oxidation. In optimum ratios, from 63 to 87%, the composites showed such a high oxidation resistance that they were comparable with Si₃ N₄ at 1200° C.     

Densification process of TaC/TaB2 composite in spark plasma sintering

A TaC composite with ∼11 wt% in situ TaB₂ was fabricated by spark plasma sintering at 1600–1900 °C. The densification process was studied by analysis of the densifying shrinkage of the powder compacts. Three distinct stages for densification were determined. The starting powder mixture of TaC, B₄C and Ta completed reaction to form the desired TaC and TaB₂ at temperature <1583 °C. At around ∼1750 °C, the TaC/TaB₂ material significantly improved relative density to ∼95% with rapid grain growth. The final densification took place very rapidly at 1900 °C by releasing a high pressure vapor.     

Nickel-boron nanolayer evolution on boron carbide particle surfaces during thermal treatment

This study is focused on reduction of Ni₂O₃ and B₂O₃ in the Ni-B nanolayer on B₄C particle surfaces and understanding of the nanolayer composition and morphology changes. Initially, the nanolayer contains Ni₂O₃, B₂O₃, and amorphous boron. After 400 °C thermal treatment in a H₂-Ar atmosphere, Ni₂O₃ is reduced to nickel; the nanolayer morphology is maintained and the coated particles demonstrate magnetism. As the thermal treatment temperature is increased to 550 °C, B₂O₃ is reduced to boron, which reacts with nickel and forms Ni₂B. Simultaneously, the nanolayer evolves into nanoparticles. Thermal treatment temperature increase to 700-900 °C only causes Ni₂B particle growth but does not fundamentally change the composition or phase.     

High-Pressure Phase Transitions of M2O5(M = V, Nb, Ta) and Thermal Stability of New Polymorphs

The stability regions of various M₂O₅(M = V, Nb, Ta) polymorphs were studied by quenching samples from 600–1300°C at pressures in the range 5.0–8.0 GPa. Above 7.0 GPa, Nb₂O₅and Ta₂O₅were found to have a new polymorph (Zphase) in which the metal atoms are in sevenfold coordination. In addition, the samples contained the high-pressure polymorph B-M₂O₅ with the rutile-related structure. Differential thermal analysis at atmospheric pressure showed a weak, broad exotherm, indicating that the transformation of Z-M₂O₅into other phases begins at 100–150°C and reaches completion at 400°C in Nb₂O₅and 550°C in Ta₂O₅. At p 8.0 GPa and t> 750°C, a new high-pressure phase B-V₂O₅, isostructural with B-Nb₂O₅, was identified (a= 11.9640(6) Å, b= 4.6986(3) Å, c= 5.3249(3) Å, = 104.338(4)°, V= 290.01 ų, Z= 4, sp. gr. C2/c). At atmospheric pressure, B-V₂O₅transforms into -V₂O₅, with two strong exothermic peaks at 230 and 270°C. Experiments in the pressure range 5.0–8.0 GPa confirmed that the high-pressure phase -V₂O₅has a broad stability region.