The effect of sintering temperature on some properties of Cu-SiC composite

Copper matrix composites reinforced with 1 wt.%, 2 wt.%, 3 wt.% and 5 wt.% SiC particles were fabricated by powder metallurgy method. Cu and Cu-SiC powder mixtures were compacted with a compressive force of 280 MPa and sintered in an open atmospheric furnace at 900-950 °C for 2 h. Within the furnace compacted samples were embedding into the graphite powder. The presence of Cu and SiC components in composites was verified by XRD analysis. Optical and SEM studies showed that Cu-SiC composites have a uniform microstructure in which silicon carbide particles are distributed uniformly in the copper matrix. The results of the study on mechanical and electrical conductivity properties of Cu-SiC composites indicated that with increasing SiC content (wt.%), hardness increased, but relative density and electrical conductivity decreased. The highest electrical conductivity of 98.8% IACS and relative density of 98.2% were obtained for the Cu-1 wt.%SiC composite sintered at 900 °C and this temperature was defined as the optimum sintering temperature.     

Microstructures and mechanical properties of the in situ TiB-Ti metal-matrix composites synthesized by spark plasma sintering process

In situ synthesized TiB reinforced titanium matrix composites have been synthesized by spark plasma sintering (SPS) process at 950-1250 °C, using mixtures of 15 wt% TiB₂ and 85 wt% Ti powders. The effects of the sintering temperature on densification behavior and mechanical properties of the TiB-Ti composites were investigated. The results indicated that with rising sintering temperatures, relative densities of the composites increase obviously, while the in situ TiB whiskers grow rapidly. As a result, bending strength of the TiB-Ti composites increases slowly at the combined actions of the factors referred above. Fracture toughness of the composites is improved remarkably due to the large volume fraction of Ti matrix, the crack deflection, pull-out and the micro-fracture of the needle-shaped TiB grains. The results also suggested that TiB-Ti composite sintered at 1250 °C by SPS process exhibits the highest relative density of 99.6% along with bending strength of 1161 MPa and fracture toughness of 13.5 MPa m^1/2.     

Synthesis of ultrafine/nanocrystalline W-(30-50)Cu composite powders and microstructure characteristics of the sintered alloys

Ultrafine/Nanocrystalline W-Cu composite powders with various copper contents (30, 40 and 50 wt.%) have been synthesized by sol-spray drying and a subsequent hydrogen reduction process. The powders were consolidated by direct sintering at temperatures between 1150 and 1260 °C for 90 min. The powder characteristics and sintering behavior, as well as thermal conductivity of the sintered alloys were investigated. The results show that the synthesized powders exist in ultrafine composite particles containing numerous nanosized particles, and the composition distributed very homogeneously. As the copper contents increase, the grain size of the powders decreases. The subsequent sintered parts show nearly full density with the relative density more than 99% at the temperature of 1250 °C. The sintered parts have very fine tungsten grains embedded in a bulk matrix. With increased copper contents, the tungsten grain size decreases and the microstructural homogeneity of the sintered alloys improves further. The thermal conductivity properties, while a little lower than that of the theoretical value, depend on the copper contents.     

Effect of sintering temperature and time on densification, microstructure and properties of the PZT/ZnO nanowhisker piezoelectric composites

Lead zirconate titanate (PZT) based piezoelectric composites embedded with ZnO nanowhiskers (ZnO_w) were investigated to clarify the optimal sintering condition for densification, microstructure, and electrical properties. The samples are characterized by X-ray diffraction analysis and scanning electron microscopy. The results show that the increase of the sintering temperature and time is quite effective in improving the densification and piezoelectric properties of the PZT/ZnO_w composites. However, the relative density and piezoelectric properties deteriorate as the composites are sintered over the optimal sintering condition. Particularly, the PZT/ZnO_w composites sintered at 1150 °C for 2 h show excellent electrical properties of piezoelectric constant d₃₃ ∼ 471 pC/N, relative dielectric constant ? ∼ 3838, planar electromechanical coupling factor k_p ∼ 0.543, remnant polarization P_r ∼ 23.2 μC/cm² and coercive field E_c ∼ 9.2 kV/cm.     

Preparation and properties of the barium borate glassy matrix composite materials containing fused silica and monoclinic zirconia

In this work, the LTCC composite materials containing fused silica and monoclinic zirconia ceramic particles, respectively, which based on the composite matrix composed of the barium borate glassy matrix and α-alumina ceramic particles, were prepared by traditional solid-state preparation process at a sintering temperature of 900 °C. Sintering mechanism and physical properties, e.g. dielectric properties and coefficient of thermal expansion (CTE), of the LTCC composite ceramics are investigated and discussed in detail in terms of their mineral phase composition. The results indicate that a barium borate glassy phase can be easily formed from a barium borate compound, which is obtained by the chemical combination of barium hydroxide octahydrate and an aqueous solution of boric acid, at a sintering temperature of 900 °C. In turn, the barium borate glassy melts can supply a liquid phase sintering aid for the fabrication of the LTCC composite ceramics with the sintering temperature of 900 °C during sintering. The introduction of the α-alumina ceramic particles to the barium borate glassy matrix can improve the sintering behavior whereas the presence of fused silica or monoclinic zirconia particles in the composite ceramics is important to adjust the dielectric, thermal and mechanical properties of the LTCC composite materials. The work may be referenced for the fabrication of multi-layer LTCC structures with tailored physical properties.     

Sintering behavior of W–30Cu composite powder prepared by electroless plating

Powder metallurgy technique was employed to prepare W–30 wt.% Cu composite through a chemical procedure. This includes powder pre-treatment followed by deposition of electroless Cu plating on the surface of the pre-treated W powder. The composite powder and W–30Cu composite were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Cold compaction was carried out under pressures ranging from 200 MPa to 600 MPa while sintering at 850 °C, 1000 °C and 1200 °C. The relative density, hardness, compressive strength, and electrical conductivity of the sintered samples were investigated. The results show that the relative sintered density of the titled composites increased with the sintering temperature. However, in solid sintering, the relative density increased with pressure. At 1200 °C and 400 MPa, the liquid-sintered specimen exhibited optimum performance, with the relative density reaching as high as 95.04% and superior electrical conductivity of IACS 53.24%, which doubles the national average of 26.77%. The FE-SEM microstructure evaluation of the sintered compacts showed homogenous dispersion of Cu and W and a Cu network all over the structure.     

A feasibility study of W-Cu composites production by high pressure compression of tungsten powder

For manufacturing a heavy duty W-Cu composite, a porous tungsten skeleton is required; which later can be filled by molten copper via infiltration technique. The compression force usually up to 200 MPa can be provided by cold isostatic press (CIP) and the temperatures used for sintering the green compacts are more 2000 °C. However, in this research, high pressure within the range of 200 to 663 MPa was used to produce high density green specimens (60-80%) by CIP while sintering was carried out at a moderate temperature of 1550 °C. The tungsten skeletons were infiltrated with molten copper at 1300 °C.The reduction of sintering temperature from over 2000 °C to 1550 °C for a highly densified W-skeleton not only resulted into a successful production of W-Cu composites but also the obtained physical and mechanical properties of these composites are comparable to those obtained for lower compaction pressures and sintering temperature higher than 2000 °C.     

Fabrication of Silver-Graphite Contact Materials Using Silver Nanopowders

Silver-graphite (AgC) composites are used in electrical switchgears as arcing as well as sliding contacts. AgC composite powders for electrical contact applications are conventionally prepared using micron-size silver powder. Present investigation is aimed at exploring the effect of nanosize silver powder, made by colloidal synthesis route, on the processing and properties of AgC contact materials. The AgC composite powders synthesized from micron-size and nanosize silver powders, respectively, are characterized for particle size distribution by dynamic light scattering technique, x-ray diffraction, and scanning electron microscopy. The bulk solid compacts produced by conventional powder metallurgy process of pressing, sintering, and repressing of AgC composite powders are evaluated for their density, microhardness, electrical conductivity, and microstructure. The study reveals that the use of nanosize silver powder not only leads to reduction in sintering temperature but also contributes in improving the properties of the AgC contact materials.     

The comparison of W/Cu and W/ZrC composites fabricated through hot-press

In this study the W/Cu and W/ZrC composites have been fabricated by hot-press and then their mechanical properties were compared in addition to their ablation resistance. To produce W-20vol.%Cu composite at first stage the elemental W and Cu powders were ball milled for 3 h in rotation speed of 200 rpm, in which 2% nickel was added in order to reduce the density. The mixed powders were hot-pressed for 1 h at 1400 °C and compact pressure of 30 MPa. Additionally W/40vol.%ZrC composite has been fabricated by hot-pressing of mixed W and ZrC powders in 30 MPa and 2200 °C for 1 h. Since these materials are used at elevated temperature applications, where ablation is the main source of material failure, after producing the composites their ablation resistance was evaluated in a real condition. The results show that not only W–ZrC composite is better than W–Cu composite in mechanical properties, but also in ablation resistance.     

Low temperature sintering of ZrC-SiC composite

High-energy ball milling and spark plasma sintering were adopted to prepare ZrC-SiC composite. Zirconium carbide, silicon, and graphite powders were used as raw materials. ZrC-30 vol.%SiC was sintered to a relative density of >96.1% at 1800 °C. The composite showed a fine microstructure. The fracture strength reached up to 523.4 MPa, Vickers’ hardness 18.8 GPa, fracture toughness 4.0 MPa m^1/2, and elastic modulus 390.5 GPa.     

放电等离子烧结温度对超细晶W-40Cu复合材料的影响

采用高能球磨法制备了W-40Cu超细晶复合粉体,继而进行了放电等离子烧结(SPS),获得了致密的超细晶W-40Cu块体复合材料,着重研究了烧结温度对复合材料组织和性能的影响.结果表明,随着烧结温度升高,材料的致密度、硬度和电导率也随之升高;在950℃烧结5 min的W-40Cu复合材料,W颗粒尺寸约300～500 nm,相对致密度达98％,显微硬度HV为287,电导率为17.9 MS/m.     

Dense sub-micron-sized ZrC-W composite produced by reactive melt infiltration at 1200 °C

ZrC-W composite was produced by reactive infiltration of Zr₂Cu into WC preform at 1200 °C. The WC reacted completely with Zr in the alloy to form 61.8 vol% ZrC and 38.2 vol% W. The infiltrated composite reached a relative density of 98.3% and average grains sizes of about 0.5 μm. The flexural strength and the fracture toughness of the ZrC-W composite was improved by the addition of W.     

A new route for preparing Mo-10wt.%Cu composite compacts

In this work, high pure Mo-10 wt% Cu composite powders with a ultrafine particle size are prepared by a two-step reduction process (first carbothermal reduction and then hydrogen reduction). Ammonium heptamolybdate and copper nitrate trihydrate powders are used as molybdenum and copper source, respectively. The mixtures of raw materials are calcined at 400 °C firstly to form the composite oxides which are then reacted with insufficient carbon black at 1050 °C for 2 h. The as-prepared powders are further reduced by hydrogen at 750 °C for 2 h to obtain the ultrafine Mo-10 wt% Cu composite powders. The experimental results show that the residual carbon of the Mo-10 wt% Cu powders can be decreased to 0.015 wt%, and the composite powders have an average particle size of 200 nm. The sintering behavior of ultrafine MoCu powders and the properties (vickers hardness and thermal conductivity) of samples after sintering are investigated. High sintering temperature is beneficial to increase the density of the compact. At 1200 °C, the relative density of the MoCu compacts is 98.8%. The vickers hardness and thermal conductivity of the Mo-10 wt% Cu composites sintered at 1200 °C for 3 h are 233 HV and 130 (W/m·k), respectively.     

Electrical and thermal characterization of Sm doped ceria electrolytes synthesized by combustion technique

Nanocrystalline samarium doped ceria electrolyte [Ce_0.9Sm_0.1O_1.95] was synthesized by citrate gel combustion technique involving mixtures of cerium nitrate oxidizer (O) and citric acid fuel (F) taken in the ratio of O/F = 1. The as-combusted precursors were calcined at 700 °C/2 h to obtain fully crystalline ceria nano particles. It was further made into cylindrical pellets by compaction and sintered at 1200 °C with different soaking periods of 2, 4 and 6 h. The sintered ceria was characterized for the microstructures, electrical conductivity, thermal conductivity and thermal diffusivity properties. In addition, the combustion derived ceria powder was also analysed for the crystallinity, BET surface area, particle size and powder morphology. Sintered ceria samples attained nearly 98% of the theoretical density at 1200 °C/6 h. The sintered microstructures exhibit dense ceria grains of size less than 500 nm. The electrical conductivity measurements showed the conductivity value of the order of 10⁻² S cm⁻¹ at 600 °C with activation energy of 0.84 eV between the temperatures 100 and 650 °C for ceria samples sintered at 1200 °C for 6 h. The room temperature thermal diffusivity and thermal conductivity values were determined as 0.5 × 10⁻⁶ m² s⁻¹ and 1.2 W m⁻¹ K⁻¹, respectively.     

Vacuum sintering of WC-8 wt.% Co hardmetals from WC powders with different dispersity

The effect of sintering temperature and particle size of tungsten carbide WC on phase composition, density and microstructure of hardmetals WC-8 wt.% Co has been studied using X-ray diffraction, scanning electron microscopy and density measurements. The sintering temperature has been varied in the range from 800 to 1600 °C. The coarse-grained WC powder with an average particle size of 6 μm, submicrocrystalline WC powder with an average particle size of 150 nm and two nanocrystalline WC powders with average sizes of particles 60 and 20 nm produced by a plasma-chemical synthesis and high-energy ball milling, respectively, have been used for synthesis of hardmetals. It is established that ternary Co₆W₆C carbide phase is the first to form as a result of sintering of the starting powder mixture. At sintering temperature of 1100-1300 °C, this phase reacts with carbon to form Co₃W₃C phase. A cubic solid solution of tungsten carbide in cobalt, β-Co(WC), is formed along with ternary carbide phases at sintering temperature above 1000 °C. Dependences of density and microhardness of sintering hardmetals on sintering temperature are found. The use of nanocrystalline WC powders is shown to reduce the optimal sintering temperature of the WC-Co hardmetals by about 100 °C.     

Sinterability and electrical properties of ZnO-doped Ce0.8Y0.2O1.9 electrolytes prepared by an EDTA-citrate complexing method

The effect of ZnO addition on the sinterability and ionic conductivity of Ce_0.8Y_0.2O_1.9 is investigated. Ce_0.8Y_0.2O_1.9 is prepared using an EDTA-citrate complexing method in order to further improve its electrical properties. Using a ZnO content over 1 mol %, the sinterability of Ce_0.8Y_0.2O_1.9 is significantly improved by reducing the sintering temperature from 1500 to 1350 °C and a relative density of above 95% was achieved. The highest ionic conductivity of 0.0516 S cm⁻¹ was obtained at 750 °C for (YDC)_0.99(ZnO)_0.01 sintered at 1350 °C. Pure YDC sintered at 1500 °C, on the other hand, yielded 0.0289 S cm⁻¹.     

Fabrication of SiC particulate reinforced AZ91D composite by vacuum-assisted pressure infiltration technology

The AZ91D Mg matrix composites reinforced by SiC particulate with the sizes of 11 μm, 21 μm and 47 μm were successfully fabricated respectively by vacuum-assisted pressure infiltration technology. Microstructures and particulate distributions were analyzed with scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The coefficient of thermal expansion (CTE) measurements was performed from 75 °C to 400 °C at a heating rate of 5 °C/min. The results show that the uniform distribution of SiC particulate in metal matrix and density over 98% in theoretical density of composites were fabricated. Only MgO phase was detected at the interface and no brittle phases of Al₄C₃ and Mg₂Si were discovered. The desirable coefficients of thermal expansion of composites were achieved. The intensity of dislocation generation nearby SiC particulate increases significantly with the increasing of SiC particulate size. Therefore, this technology is a potential method to fabricate Mg matrix composites reinforced by SiC particulates with the desirable microstructures and CTE.     

Sinterability and microwave dielectric properties of nano structured 0.95MgTiO3-0.05CaTiO3 synthesised by top down and bottom up approaches

0.95MgTiO₃-0.05CaTiO₃ (MCT) nano powders were synthesised using sol-gel method and high energy ball milling (HEBM). Synthesised powders were characterised using X-ray diffraction analysis to ensure phase purity and HRTEM to determine the fine microstructural features like particle size, interplanar spacing, etc. The powder pellets were heat treated to study the sinterability and microwave dielectric properties and these properties were then compared with the microwave dielectric properties of micron sized sample. Nano powder synthesised using HEBM shows better dielectric properties, sinterability and gets densified to 90% of theoretical density (TD) at 1200 °C/2 h. Dielectric resonators prepared using chemically synthesised nano powder showed poor sinterability and microwave dielectric properties, but, dielectric properties of HEBM samples were very near to that of solid state synthesised samples. Sintered HEBM powders retain the microwave dielectric properties almost to the same level as the solid state synthesised powder with considerable lowering of sintering temperature.     

Properties and fast low-temperature consolidation of nanocrystalline Ni-ZrO2 composites by high-frequency induction heated sintering

Nanopowders of Ni and ZrO₂ (11 nm and 90 nm, respectively) were synthesized from NiO and Zr by high energy ball milling. A highly dense nanostructured 2Ni-ZrO₂ composite was consolidated at low temperature by high-frequency induction heat sintering within 2 min of the mechanical synthesis of the powders (Ni-ZrO₂) with horizontal milled NiO + Zr powders under 500 MPa pressure. This process allows very quick densification to near theoretical density and prohibits grain growth in nano-structured materials. The grain sizes of Ni and ZrO₂ in the composite were calculated. Finally, the average hardness and fracture toughness values of nanostructured 2Ni-ZrO₂ composites were investigated.     

Fabrication of W-20wt.%Ti alloy by pressureless sintering at low temperature

A powder metallurgical technology of low temperature and pressureless is used to fabricate a W-20wt.%Ti alloy using milled TiH₂ powders and micro-sized W powders. The microstructure of the milled TiH₂ powders and the bulk W–Ti alloy were studied. It is indicated that TiH₂ nanoparticles with the size of 8 to 15 nm were obtained after milling for 48 h and the decomposition temperature decreased from 520.2 °C to 395.5 °C. The W-20wt.%Ti alloy prepared at 1200 °C for 80 min had a relative density of 97.8% which was composed of α-Ti, W and β(W/Ti) solid solution. A preparation mechanism of the W–Ti alloy is also proposed based on the experimental results.