Influence of TiO2 on the physical properties of low-temperature ceramic vitrified bond and mechanical properties of CBN composites

The microstructures and properties of vitrified bond abrasive tools made of CBN grains and advanced vitrified bond systems with different TiO₂ doping amounts were investigated. Based on the experimental observations and analysis, the incorporation of TiO₂ in appropriate amount (4 wt.%) was beneficial to the improvement on flowing ability and thermal expansion property of the vitrified bond systems, and mechanical properties of the CBN composites including bending strength and Rockwell hardness were obviously improved. On the basis of discussion for microstructure, the CBN grains were better covered by vitrified bond and acquired less pores when the content of TiO₂ reached 4 wt.%. These results were related to the role of TiO₂ in the glass network structure which was analyzed by Fourier transform infrared spectroscopy (FTIR).     

Thermal conductivity of diamond composites sintered under high pressures

Thermal conductivity at room temperature of diamond composites of two types: with a diamond skeleton and with diamond grains imbedded in a non-diamond matrix was evaluated in dependence of the diamond grain size (d) varied from a ten of microns to 500 μm. The thermal conductivity of the compacts with diamond skeleton obtained in the Cu–diamond system at high pressure of 8 GPa strongly increases with diamond particles size approaching the maximum value of 9 W/cm K at d ≈ 200 μm. The compacts sintered in the Cu–Ti–diamond, Al–Si–diamond and Si–diamond systems at lower pressure (2 GPa) are formed predominantly owing to the presence of the binder. It was found for these conditions that the thermal conductivity is less sensitive to the diamond grain size, reaching the value of 6 W/cm K for the composites with SiC–Si matrix.     

Influence of diamond particles content on the critical load for crack initiation and fracture toughness of SiOC glass–diamond composites

The crack initiation load and fracture toughness were characterized as a function of diamond particle content, up to 25 vol%, in silicon oxycarbide glass matrix by means of Vickers indentation and single edge notch beam (SENB) technique, respectively. The larger fracture toughness value of 3.21 ± 0.3 MPa m^1/2 was reached for 20 vol% diamond content composites and the value was 4 times higher than that of the unreinforced glass. The addition of diamond particles greatly influenced the crack initiation load, which increased from 2.9 to 49.0 N. The enhancement in the fracture toughness and crack initiation load can be explained by both the intrinsic mechanical properties of diamond (especially the elastic properties; E ～ 1100 GPa) and the diamond/SiOC glass interfacial bonding. A clear correlation was found between the fracture energy, the reinforced interparticle spacing and the residual stress arising upon cooling due to thermal expansion mismatch between the matrix and the diamond particles.     

The effect of BaO and Al2O3 addition on the crystallization behaviour of cordierite glass ceramics in the presence of V2O5 nucleant

The effect of V₂O₅ nucleant on crystallization of stoichiometric cordierite glass ceramics in the presence of various amounts of BaO and Al₂O₃ additives were investigated by DTA, XRD and SEM. It was shown that 3 wt.% V₂O₅ and 1.5 wt.% BaO were the optimum amounts of the additives effective in inducing both surface and bulk crystallization in the above glass ceramics.This resulted in ～90 wt.% cordierite after a 3 h heat treatment at 1020 °C. The specimens possessing 4–5 wt.% Al₂O₃ in excess of the stoichiometric cordierite composition, developed mullite along with cordierite in the temperature range of 1045–1055 °C, whereas in the specimen containing 6 wt.% excess Al₂O₃, mullite was detected as the sole crystallization product.     

Microstructure and properties of diamond/SiC composites prepared by tape-casting and chemical vapor infiltration process

This article reported a novel method for preparing diamond/SiC composites by tape-casting and chemical vapor infiltration (CVI) process, and the advantages of this method were discussed. The diamond particle was proved to be thermally stable under CVI conditions and the CVI diamond/SiC composites only contained diamond and CVI-SiC phases. The SEM and TEM results showed a strong interfacial bonding existed between diamond and CVI-SiC matrix. Due to the strong bonding, the surface HRA hardness could reach up to 98.4 (HV 50 ± 5 GPa) and the thermal conductivity (TC) of composites was five times higher than that of pure CVI-SiC matrix. Additionally, the effects of diamond particle size on microstructure and properties of composites were also investigated. With the increasing of particle size, the density and TC of composites with the size 27 μm reached 2.940 g/cm³ and 82 W/(m K), respectively.     

Near net-shape fabrication of alumina glass composites

The purpose of the present study is to fabricate alumina glass composites by melt infiltration with better dimensional control through reducing both the presintering and infiltration temperature. Main efforts were put to develop glasses that are chemically compatible with alumina. After extensive investigations, a glass of 21SiO₂–24B₂O₃–35Al₂O₃–15Li₂O–5CaO wt.% was successfully developed. The glass shows good chemical compatibility with alumina at elevated temperatures and low viscosity above 900 °C. Dense alumina glass composites can be fabricated by the melt infiltration process at 950 °C, which is 150 °C lower than the current state-of-art. Investigations showed improved net-shape capability for the newly developed composites, where the total linear shrinkage for the sintering and infiltration at 950 °C is less than 0.1%, as compared with the shrinkage of 0.5% induced by the presintering and infiltration at 1100 °C. Preliminary mechanical tests showed that the fracture strength and toughness of the composites are 303 MPa and 3.4 MPa m^1/2, respectively. The lower processing temperature and the better dimensional control are the major advantages for the newly developed alumina glass composites.     

Effect of H2/Ar plasma on growth behavior of ultra-nanocrystalline diamond films: The TEM study

Incorporation of H₂ species into Ar plasma was observed to markedly alter the microstructure of diamond films. TEM examinations indicate that, while the Ar/CH₄ plasma produced the ultrananocrystalline diamond films with equi-axed grains (~ 5 nm), the addition of 20% H₂ in Ar resulted in grains with dendrite geometry and the incorporation of 80% H₂ in Ar led to micro-crystalline diamond with faceted grains (~ 800 nm). Optical emission spectroscopy suggests that small percentage of H₂-species (< 20%) in the plasma leads to partially etching of hydrocarbons adhered onto the diamond clusters, such that the C₂-species attach to diamond surface anisotropically, forming diamond flakes, which evolve into dendrite geometry. In contrast, high percentage of H₂-species in the plasma (80%) can efficiently etch away the hydrocarbons adhered onto the diamond clusters, such that the C₂-species can attach to diamond surface isotropically, resulting in large diamond grains with faceted geometry. The field needed to turn on the electron field emission for diamond films increases from E₀ = 22.1 V/μm (J_e = 0.48 mA/cm² at 50 V/μm applied field) for 0% H₂ samples to E₀ = 78.2 V/μm (J_e < 0.01 mA/cm² at 210 V/μm applied field) for 80% H₂ samples, as the grains grow, decreasing the proportion of grain boundaries.     

Interface structure and mechanical properties of the brazed joint of TiC cermet and steel

A commercially available Ag–Cu braze alloy foil with Zn was used to join TiC cermet and steel. According to the experimental observations, the interface structure is (Cu, Ni)/Ag (s.s.) + Cu (s.s.)/(Cu, Ni)/(Cu, Ni) + (Fe, Ni) from TiC cermet to steel side. With increased brazing temperature or time, the amounts of (Cu, Ni) near the base metals/Ag–31Cu–23Zn interface and (Cu, Ni) + (Fe, Ni) near the Ag–31Cu–23Zn/steel interface increase, while the amount of Ag (s.s.) + Cu (s.s.) in the middle of the braze alloy decreases. The whole joining process consists of diffusion and solution among atoms of the braze alloy foil and base metals. The maximum shear strength is 120.7 MPa for the joint brazed at 850 °C for 10 min.     

Effect of processing technique on the transport and mechanical properties of vapour grown carbon nanofibre/rubbery epoxy composites for electronic packaging applications

Vapour grown carbon nanofibre (VGCNF)/rubbery epoxy (RE) composites were produced, by either mechanical mixing, three-roll milling (RM) or combined ultrasonication/mechanical mixing. Incorporation of VGCNFs resulted in significant enhancements in the thermal and electrical conductivities of the material. Appropriate selection of processing technique and parameters can help to maximise the potential of VGCNF additions by improving their dispersion in the matrix. The composites produced by RM have superior transport properties compared with those produced by other techniques. The thermal conductivity of such composites at 40 wt.% VGCNFs reached 1.845 W/m K, a 10-fold increase compared to RE alone. The thermal conductivity data of VGCNF/RE composites best fits to the Hatta–Taya model. The lowest electrical percolation threshold is at 2 wt.%, obtained for composites produced by RM. The thermal conductivity of VGCNF/glassy epoxy (GE) composites at 12 wt.% is 10% lower than the corresponding RE composite but its electrical conductivity is 2 orders of magnitude higher than the corresponding RE composite. VGCNFs at 40 wt.% increase the compressive strength of rubbery epoxy by ～5× but the compressive modulus of 40 wt.% VGCNF/RE composite is 12 times lower than that of 12 wt.% VGCNF/GE composite, demonstrating highly compliant nature of RE composites.     

Microstructure and electrical properties of BaTiO3/Cu ceramic composite sintered in nitrogen atmosphere

BaTiO₃/xCu composite ceramics with x = 0–30 wt.% were fabricated by the traditional mixing oxide method and their microstructure, relative density, electric conductivity, permittivity and dielectric loss were measured as a function of the Cu mass fraction. The X-ray diffraction (XRD) patterns indicated that the dense composite has no chemical reaction between BaTiO₃ and Cu during sintering, and the relative diffraction intensity of Cu increased with the increase of Cu. The electric properties showed that the percolation threshold of BaTiO₃/Cu composites was x = 0.25 and its conductivity increased as the Cu content increased after that. With increasing Cu content up to 30 wt.%, the permittivity (?_r) markedly increased from ～3000 for monolithic BaTiO₃ to ～8000 at 1 kHz. Additionally, the temperature coefficient of this system was less than 5% in the temperature range of 25–115°C.     

Oxidation behaviour of Si3N4–TiN ceramics under dry and humid air at high temperature

The oxidation in air of Si₃N₄-based ceramics containing 35 vol.% of TiN secondary phase and different amounts of sintering additives has been studied at different temperatures up to 1400 °C in dry or humid environment. The oxidation starts by crystal growth of TiO₂ at the surface, then a multilayered scale develops under the rutile layer from 1000 °C. This subscale is composed of silicon nitride in which TiN particles are oxidized to agglomerates of rutile, glass and pores. The oxidation process is controlled by the matter transports, which take place in the intergranular phase. These transport phenomena are affected by the changes in distribution and composition of the glassy phase and by humidity which modifies the glass network structure and thus the in-diffusion rate. From 1200 °C, Si₃N₄ grains are also oxidized, the additional glass formed closes the residual porosities yielding scales more compact and developing an autoprotective behavior. At 1400 °C, glass phase crystallizes into cristobalite and the rutile top layer becomes discontinuous. Only composites with low amounts of sinter additives keep an autoprotective oxidation mode.     

In situ doping of diamond coatings with silicon,aluminum and titanium through a modified laser-based CVD process

The aim of this study was to demonstrate the feasibility of in situ doping of chemical vapor deposition (CVD) fabricated diamond coatings through simultaneous evaporation of solids in a CVD plasma-based process. In order to achieve maximum flexibility and energy density, a laser-based plasma-jet CVD process was chosen, and expanded with the introduction of dopant rods. The rods, with diameters varying from 0.5 mm to 3.0 mm, were fed at rates from 0.25 mm/min to > 100 mm/min, and positioned 3 mm below the optical breakthrough which generates the plasma. Gas flows of 20.0 slm (standard liters per min) argon, 2.0 slm hydrogen and 0.02 slm methane were used for diamond coating deposition. At a surface temperature of about 1100 °C, an average linear diamond growth rate of 20 μm/h was achieved. The materials selected as solid precursors for the rods were SiO₂, Al₂O₃, and Ti due to their differing electrical characteristics, as they are an insulator, semiconductor, and conductor, respectively. The evaporation rate of these rods varied by more than six orders of magnitude, from < 1 × 10^− 8 g/min (Ti) to > 7 × 10^− 2 g/min (SiO₂). The doped diamond coatings were produced by simultaneous evaporation and CVD. To prove that the precursors were vaporized and the atomic bonds were broken by the plasma, the optical emission spectra are compared with published and calculated spectral lines. Analyses of the layers were performed using EDX (energy-dispersive X-ray) spectroscopy and WDS (wavelength dispersive X-ray spectroscopy). As a result, the maximum doping densities in the diamond coating were determined, and were 3.460 wt.% for silicon, 0.957 wt.% for aluminum, and 0.03 wt.% for titanium. To prove the diamond-like characteristics of these coatings, Raman measurements were performed.     

Fabrication and properties of lead-free machinable brass with Ti additive by powder metallurgy

The aim of this paper was to investigate the properties of Cu40Zn2.2Bi + Ti for the development of a new lead-free, high-strength and machinable brass by powder metallurgy. The effect of Ti addition on the mechanical properties and machinability of BS40-2.2Bi (Cu40Zn2.2Bi) brass was studied with respect to different contents of Ti addition. BS40 (Cu40Zn) and BS40-2.2Bi brass powders were prepared by water atomization process, and the β phase was retained in the raw powders predominately. The BS40-2.2Bi powder and Ti powder were elementally mixed to prepare BS40-2.2Bi + xTi (x = 0.3, 0.5 and 1.0 wt.%) premixed powders. The alloy powders and premixed powders were solidified at 1053 K for 600 s by spark plasma sintering (SPS) and extruded subsequently. It was observed that intermetallic compounds (IMCs) such as Ti₂Bi were formed via the reaction between additive Ti and Bi alloying elements, and improved the ductility of BS40-2.2Bi significantly. The yield strength (YS) and ultimate tensile strength (UTS) were increased by increasing the contents of Ti addition, however, the elongation showed a decrease trend and the machinability became worse. The optimal content of Ti addition was 0.3 wt.%, which served excellent mechanical properties and machinability comparing with BS40-2.2Bi. For example, it had a YS of 235 MPa, a UTS of 459 MPa and an elongation of 39%, which showed 4.9%, 4.1% and 18% higher than that of extruded BS40-2.2Bi brass, respectively.     

Au and Cu recovery from printed boards by decomposition of epoxy resin in supercritical water

The recovery of Au and Cu from a printed board using supercritical water was studied, and the effects of the treatment with and without an oxidant on the components of the printed board were compared.Over 99 wt.% of metallic Au and metallic Cu was recovered in the solid form by oxidant-free supercritical water treatment at 400 °C and 25 MPa. However, the maximal removal of solid organics was 70 wt.% due to char-generation. Promotion of hydrolysis and inhibition of char-generation enhanced the removal of solid organics to nearly 90 wt.% at 380 °C.Under oxidative conditions, complete separation of solid organics from the inorganics occurred within 60 min. Metallic Au was unaltered and successfully recovered. Over 90 wt.% of metallic Cu was oxidized within 5 min, and only Cu oxides were recovered after 30 min. Oxidation of organics was promoted by coexisting Cu compounds contained in the printed board.     

Hot pressing of bimodal alumina powders with magnesium aluminosilicate (MAS) addition

High quality alumina ceramics were fabricated by hot-pressed sintering using bimodal alumina with superfine component as raw material and magnesium aluminosilicate (MAS) glass as sintering aid. Densification behavior, microstructure evolution and mechanical properties of alumina were investigated from 1300 °C to 1450 °C. The bimodal alumina powders were sintered to 99.8% of the theoretical value at 1400 °C and a comparative dense microstructure with a few plate-like abnormal grains was observed. With increase of sintering temperature up to 1450 °C, many fine matrix grains were consumed and quite a few abnormal grains impinged upon each other. For the alumina ceramics hot-pressed from bimodal alumina with 30 wt.% superfine component, optimal mechanical properties were obtained at 1400 °C. The bending strength and fracture toughness were 522 MPa and 5.0 MPa m^1/2, respectively.     

The effect of RO oxides on microstructure and chemical durability of borosilicate glasses opacified by P2O5

The microstructure, devitrification and chemical durability of borosilicate glass specimens opacified by P₂O₅, with the general composition SiO₂ 70, B₂O₃ 12, Al₂O₃ 2, P₂O₅ 2, Na₂O (13 − X), RO X (wt.%) (R = Ca, Mg, Ba, Zn) were investigated after being subjected to various heat treatment conditions, using DTA, XRD and SEM. It was shown that while heat treatments at 1073 K and >1123 K were generally detrimental for the hydrolytic resistance of glasses, due to the enhanced phase separation or formation of excessive amounts of cristobalite, heating at 1123 K for 1 h usually improved the resistance due to the partial crystallization or microstructural changes of specimens. It was also found that a progressive decrease in hydrolytic and alkaline resistance occurred during prolonged heat treatment at 1123 K due to the formation of exessive amounts of cristobalite. It was also revealed that ZnO and MgO had the worst effect on chemical durabilities of specimens containing 7 and 5.5 wt.% Na₂O, respectively.     

Unsupported CuFe bimetallic nanoparticles for higher alcohol synthesis via syngas

CuFe bimetallic nanoparticles were synthesized by co-reduction method as model catalysts for HAS. Cu contacted with Fe component in the form of Cu–Fe alloy, CuFe₂O₄ and Cu(Fe)–CuFe₂O₄ interface in the fresh CuFe sample. However, Cu/Fe₃O₄ and Cu/FeC_x composites formed after activation. Cu–FeC_x center benefited alcohol formation which led to higher selectivity to total alcohol for CuFe than that for Fe and physical mixture of Fe and Cu nanoparticles. In addition, CuFe showed very high C_6 +OH selectivity in alcohol distribution and 33 wt.%–74 wt.% was achieved, demonstrating the potential for direct synthesis of C_6 +OH from syngas.     

Ultra-high elevated temperature strength of TiB2-based ceramics consolidated by spark plasma sintering

Spark plasma sintering of TiB₂–boron ceramics using commercially available raw powders is reported. The B₄C phase developed during reaction-driven consolidation at 1900 °C. The newly formed grains were located at the grain junctions and the triple point of TiB₂ grains, forming a covalent and stiff skeleton of B₄C. The flexural strength of the TiB₂–10 wt.% boron ceramic composites reached 910 MPa at room temperature and 1105 MPa at 1600 °С. Which is the highest strength reported for non-oxide ceramics at 1600 °C. This was followed by a rapid decrease at 1800 °C to 480–620 MPa, which was confirmed by increased number of cavitated titanium diboride grains observed after flexural strength tests.     

Infiltration mechanism of diamond/SiC composites fabricated by Si-vapor vacuum reactive infiltration process

Dense diamond/SiC composites were fabricated by Si vapor vacuum reactive infiltration of carbon-containing diamond porous preform at 1600 °C for 1 h. The microstructural evolution of the composites was investigated. The infiltration mechanisms during reactive infiltration were discussed. The composite consists of diamond, β-SiC and a small amount of Si. Epitaxial growth of nano-sized SiC on diamond and graphite surfaces occurred due to the diffusion-reaction mechanism in the initial stage of infiltration. Growth of micron-sized SiC with no preferential orientation was controlled by solution-precipitation mechanism in the final stage. The infiltration process was determined both by molecular diffusion and capillary effects. Explosive evaporation of molten Si, volume expansion of the solids and heat release during the reaction were the key factors contributing to the rapid densification of diamond/SiC composites. High thermal conductivity (580 W m^?1 K^?1) and low density (3.33 g cm^?3) of the composites were beneficial to thermal management applications.     

Hydrolysis-induced aqueous gelcasting for near-net shape forming of ZTA ceramic composites

A new near-net shape forming process called “hydrolysis-induced aqueous gelcasting” (GCHAS) is reported in this paper for the consolidation of ZTA composites, ZTA-30 (70 wt.% Al₂O₃ + 30 wt.% ZrO₂) and ZTA-60 (40 wt.% Al₂O₃ + 60 wt.% ZrO₂). For comparison purposes, ceramics having the same chemical compositions were also consolidated by hydrolysis-assisted solidification (HAS). All the starting suspensions contained a solids loading of 50 vol.%. In the precursor powder mixtures, 1–5 wt.% of Al₂O₃ was replaced by equivalent amounts of AlN to enhance or promote or co-promote the consolidation of suspensions by HAS or by GCHAS, respectively. The suspensions for GCHAS were prepared by dispersing the ZTA powder precursor mixtures in a premix solution of 20 wt.% MAM (methacrylamide), MBAM (methylenebisacrylamide) and NVP (n-vinylpyrrolidinone) in 3:1:3 ratio in de-ionized water. Ceramics consolidated via GCHAS exhibited superior mechanical properties after consolidation and after sintering for 1 h at 1600 °C in comparison to those consolidated by HAS.