Green Specimens Research Articles

Microstructures and thermal conductivities of reaction sintered SiC ceramics

Reaction sintered SiC ceramics were prepared by the silicon melt infiltration method over temperatures of 1450−1550°C. The effects of the carbon and silicon contents of the starting materials as well as the sintering temperature and time on the thermal conductivities and microstructures of the ceramic materials were studied. The thermal conductivities and microstructures of the samples were characterised using thermal conductivity measurements, X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy and mercury injection porosimetry. The results showed that sintering temperature and time as well as the carbon and silicon contents of the green specimens are the main factors affecting the microstructure and porosity of reaction bonded SiC ceramics. Increasing the reaction temperature and time decreased the porosity of the ceramics. This was due to the infiltration of the silicon melt into the ceramic specimens. The thermal conductivity and porosity of the sample sintered at 1550°C for 3 h in an argon atmosphere were 102·5 W m K−1 and 0·3% respectively.

Physical, Mechanical and Electrical Properties of W-20 wt.% Cu Composite Produced by Liquid Phase Sintering Process

In this study, the effect of sintering temperature on the properties of tungsten-copper (W-Cu) composite produced by liquid phase sintering (LPS) process has been investigated. W-20 wt.% Cu composite powders with particle size less than 1 μm was prepared by cold compaction and followed by cold isostatic pressing. The green specimens were then sintered under nitrogen based atmosphere in the temperature range of 1100°C to 1300°C. The sintering studies were conducted to determine the extent of densification and corresponding to microstructure changes. In addition, the properties of the sintered specimens such as physical appearance, microstructure evolution, mechanical and electrical properties were presented and discussed.

Preparation of Aluminum Feedstock for Green Part Specimen Using Metal Injection Molding

In metal injection molding, to identify the homogenous aluminum based feedstock is a challenging issues. In this study, a metal injection molding of aluminum feedstock which contains of high density polyethylene, stearic acid and paraffin wax as binder system was performed. The feedstock are used to produce tensile and gear shape green specimens using injection molding machine. The process ability of the metal injection molding feedstock depends on different parameters such as their binder composition and amount of metal powder used. From this study, the percentage of volume shrinkage experienced a sudden increase at the metal composition more than 50%. It also shown that, the paraffin wax content, affects the feedstock performances.

First records and description of metallic red females of Euglossa (Alloglossura) gorgonensis Cheesman, with notes on color variation within the species (Hymenoptera, Apidae)

Metallic coloration is one of the signatures of orchid bees of the genus Euglossa, with some species showing variation associated with their geographic range. Euglossa (Alloglossura) gorgonensis Cheesman exhibits color variation, ranging from mainly green specimens in the southern extreme of its range (Pacific slope of Colombia), to noticeably reddish specimens in parts of the northern known limits of its range (Pacific slope of southern Costa Rica). Here we present the first description of females from Costa Rica belonging to the reddish extreme of the color variation.

Fabrication of Transparent Spinel Honeycomb Structures by Methyl Cellulose-Based Thermal Gelation Processing

Transparent MgAl2O4 spinel of flat as well as honeycomb structure was fabricated by employing thermally induced gel casting of the slurry with 56 wt% solid loading containing 0.2 wt% of methylcellulose. The green specimens were pressureless sintered to 98%–99% of theoretical density with no open porosity at an optimum temperature of 1700°C. Final densification by hot isostatic pressing of both the specimens at the optimum temperature of 1800°C and 195 MPa pressure enabled further elimination of residual porosities and full densification resulting in theoretical density of 3.58 g/cc. The design of the honeycomb was such that it exhibited a surface area to volume ratio of 0.65 cm2/cm3 and a relative density of 0.69. The hardness of the honeycomb specimens has been found to be 13 GPa, which is at par with the solid specimens processed under identical conditions. Solid specimens of around 4 mm thickness exhibited a transmission of >80% in the visible (0.4–0.8 μm) region. Specimens were also tested according to ASTM procedures and have shown a flexural strength (σf) of 195 MPa and plane-strain fracture toughness (KIc) of 1.87 MPa·m1/2 as reported in this study.

Sinter-coating method for the production of TiN-coated titanium foam for biomedical implant applications

Titanium foams for biomedical implant applications were produced by space holder technique. Novel sinter-coating method was used for the production of TiN layer on foams in order to enhance wear and corrosion resistance. In literature, Ti compacts were sintered in vacuum or argon atmosphere and then expensive and complex coating processes were carried out on sintered specimens. In this study, the titanium foams were sintered in N2 atmosphere at 1200°C. Biocompatible TiN layer was coated on foam surface by gas nitriding simultaneously during sintering. Sinter-coating combines sintering and TiN coating in one step by sintering the green specimens in N2 atmosphere. This reduces cost of operation and time. In addition, alkali treatment was used to activate surface of foams for partially induce a bioactive bone-like apatite to enhance bioactivity. Alkali treatment was performed by soaking the foams in NaOH solution and then heating at 600°C. After alkali treatment, foams were immersed into simulated body fluid to induce a bone-like apatite layer on foam surface.

Effect of heat treatment on mechanical properties of low alloy steel foams

Effect of heat treatment on compressive properties of low alloy steel foams (Fe–1.75 Ni–1.5 Cu–0.5 Mo–0.6 C) having porosities in the range of 47.4–71.5% with irregular pore shape, produced by the space holder-water leaching technique in powder metallurgy, was investigated. Low alloy steel powders were mixed with different amounts of space holder (carbamide), and then compacted at 200MPa. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were sintered at 1200°C for 60min in hydrogen atmosphere. Sintered compacts were heat treated by austenitizing at 850°C for 30min and then quenched at 70°C in oil and tempered at 210°C for 60min. In this porosity range, compressive yield strengths of as-sintered and heat treated specimens were 28–122MPa and 18–168MPa, respectively. The resultant Young’s moduli of the as-sintered and heat treated specimens were 0.68–3.12GPa and 0.47–3.47GPa, respectively. The heat treatment enhanced the Young’s modulus and compressive yield strength of the foams having porosities in the range of 47.4–62.3%, as a consequence of matrix strengthening. However, the compressive yield stress and Young’s modulus of the heat treated foam having 71.5% porosity were lower than that of the as-sintered foam’s, as a result of cracks in the structure. The results were discussed in light of the structural findings.

Weibull modulus and fracture strength of highly porous hydroxyapatite

Porous hydroxyapatite (HA) is used in a variety of applications including biomedical materials such as engineered bone materials and microbe filters. Despite the utility of the Weibull modulus, m, as a gauge of the mechanical reliability of brittle solids, there have been very few studies of m for porous HA. A recent study of porous HA that included the current authors (Fan, X., Case, E.D., Ren, F., Shu, Y., Baumann, M.J., 2012a. Journal of the Mechanical Behavior of Biomedical Materials. 8, 21–36) showed increases in m for porosity, P, approaching PG, the porosity of the green (unfired) specimen. In this paper, 18 groups of highly porous HA specimens (12 groups fabricated in this study and 6 groups from Fan et al., 2012a) were analyzed with P values from 0.59 to 0.62, where PG=0.62. The partially sintered HA specimens were fractured in biaxial flexure using a ring-on-ring test fixture. The fracture strength decreased monotonically with decreasing sintering temperature, Tsinter, from 4.8MPa for specimens sintered at 1025°C–0.66MPa for specimens sintered at 350°C. However, the Weibull modulus remained surprisingly high, ranging from 6.6 to 15.5. In comparison, for HA specimens with intermediate values of P, from about 0.1–0.55, the Weibull modulus tended to be lower (ranging from about 4 to 11) than the highly porous specimens included in this study.

Mechanical properties of sinter-hardened Cr–Si–Ni–Mo based steel foam

Highly porous sinter-hardenable Cr–Si–Ni–Mo based steel foam for automotive applications was produced by space holder method. Steel powders were mixed with binder (polyvinylalcohol) and space holder (carbamide), and compacted. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were then sintered at temperatures between 1100°C and 1250°C for sintering times of 15, 30 and 45min. In addition, the steel foams were sinter-hardened to enhance mechanical properties. Sinter-hardening combines sintering and heat treatment in one step by increasing the post-sintering cooling rate. This reduces the cost of operation and makes powder metallurgy more competitive. Effects of sinter-hardening process parameters on compressive strength, Young’s modulus, hardness and energy absorption of the steel foams were investigated.

Study of high temperature wetting and infiltration for optimising liquid phase sintering in low alloy steels

Liquid phase sintering is commonly used in powder metallurgy to improve physical properties through densification enhancement. With the aim of combining the advantages of liquid phase sintering and the use of promising alloying elements such as Mn and Si, liquid promoters with complex compositions were designed to provide a low melting point to form a liquid phase below the common sintering temperatures. The properties of these liquid phases were characterised in terms of contact angle, spreading evolution and infiltration. Using a Krüss drop shape analysis system, both wetting angle experiments and infiltration experiments were performed by changing the substrate characteristics from sintered to green iron specimens respectively. The discussion is based on the different features found for these liquids compared with copper, which is a well known liquid phase former used for improving the properties of low alloy steels. Simulations of the thermodynamic and kinetic processes taking place were performed by combining ThermoCalc and DICTRA software analysis.

Gene Signatures Revisited

Gene Signatures Revisited

The effect of strontium doping on densification and electrical properties of Ce0.8Gd0.2O2−δ electrolyte for IT-SOFC application

The effect of strontium doping on densification and ionic conductivity of gadolinium-doped ceria (GDC) was investigated. Doped (Sr-GDC) and un-doped GDC green specimens were subjected to dilatometric measurements to evaluate their sintering behavior and to identify the sintering temperature regimes. XRD spectra show the crystal structure of the sintered samples to be cubic. Strontium doping has exhibited a relatively larger grain size as is evident by the microstructural characterization. AC impedance analysis exhibited a threefold increase in ionic conductivity for Sr-GDC (0.072 S/cm) in comparison to GDC (0.028 S/cm) samples which can be attributed to improved density and increased grain size, resulting in enhancement of total conductivity. Additionally, strontium doping to GDC lattice not only increases the oxygen vacancies but also decreases the lattice binding energy, leading to increase in oxygen ion mobility which is also confirmed by the lower activation energy exhibited by the Sr-GDC formulation. Our experimental results established that co-doping is very effective in identifying new materials with remarkably high ionic conductivity with substantial reduction in the cost for solid oxide fuel cell application.

Development and properties of aluminium silicon alloy fly ash composites

Aluminium (Al) silicon alloy fly ash composites were developed using powder metallurgy technique. Aluminium silicon alloy powder was homogenously mixed with various weight percentages of fly ash (5–15%) and uniaxially cold pressed at pressures ranging between 200 and 515 MPa, and the green specimens were sintered at temperatures between 575 and 625°C. The various properties of the Al alloy fly ash composites were studied and compared with that of base alloy. The density of Al alloy fly ash composites was lower than that of the base alloy. The sintered density of the Al alloy fly ash composites and Al alloy slightly decreased when compared to green density. The hardness of the Al alloy fly ash composites was higher than that of base alloy and it increased with the increase in weight percentage of fly ash content upto 12 wt-%. Compressive strength of the composites was lower than that of base alloy and it decreased with increasing weight percentage of fly ash. The electrical resistivity and corrosion rate of the composites were higher than that of base alloy and they increased with increasing weight percentage of fly ash. Scanning electron microscope was used to examine the microstructural characteristics of the composites. X-ray diffraction pattern was used to identify various phases present in Al alloy fly ash composites.

Manufacturing porous multi-channel ceramics by laser gelling

This article describes a novel layer manufacturing process for forming a ceramic part with porous multi-channel architecture by laser gelling under low laser energy. The process involves bonding silica powder by gelled silica sol after exposure by a CO 2 laser. Lower laser energy density of 0.8 J/mm 2 is required to produce ceramic parts by “gelling effect”. Therefore, the geometrical deflection and thermal distortion can be reduced after laser scanning. The inner porous structures were supported by ceramic slurries to prevent sagged deflection and to enhance dimensional accuracy due to optimal slurry suspension. The flexural strength of the green specimen was 4.7 MPa, while that of the gelled specimen was 12.5 MPa after heat-treatment at 1200 °C for 1.5 h. The proposed process has potential for fabricating complex interconnected porous ceramics for tissue engineering applications.

Improvement in physical and mechanical properties of aluminum/zircon composites fabricated by powder metallurgy method

Metal–matrix composites (MMCs) are known as the most useful and high-tech composites in our world as well as aluminum (Al) as the best metal for producing these composites. Combining aluminum and zircon (ZrSiO 4) will yield a material with the best corrosive resistance and mechanical properties like strength at high temperatures. Also, the abrasive wear behavior of these composites will be improved. In the present investigation, a study on aluminum/zircon composites has been carried out. Micro-structures of these composites in powder metallurgy conditions show different size distribution of zircon with different proportions in the composite. Also, there is a case-study about density and compressive strength and hardness of aluminum/zircon composites. The green specimens prepared by isostatic pressing of prepared powders with different zircon percentages, were sintered at two temperatures. These specimens were then investigated by different physical and mechanical testing methods to observe in which conditions the best properties would be obtained. The most improved compression strength was obtained with the specimen including 5% of zircon sintered at 650 °C.

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.

Phase transformation behaviours of porous NiTi SMA fabricated as hollow and solid cylinders by SHS

Porous NiTi shape memory alloys were fabricated as hollow and solid cylinders by self-propagating high temperature synthesis for use as implant material. The green compacted specimens were synthesised with different ignition technical, high voltage electric arcs. The pore morphologies, microstructures and phase transformation behaviours of the synthesised products at different preheating temperatures were studied. It was seen that the B2(NiTi) and B19′ (NiTi) phases were predominant phases. It was also seen that porosity decreased with the increase in preheating temperature and the orientation of the combustion channels was affected by the ignition location. Phase transformation temperatures and activation energies were not changed, but the changes at enthalpy and entropy increased with increasing porosity.

A low-cost vacuum casting equipment for aluminium alloys

Some of the advantages inherent in the vacuum casting technique such as improved mechanical properties and elimination of shrinkage have been replicated in a low-cost design. Melting and vacuum requirements of the equipment were determined based on a furnace capacity of 15 kg of aluminium. Clay-based refractory and insulating bricks were used for furnace construction. The vacuum system was made from locally available materials and designed for efficiency required for vacuum casting. The equipment was used to melt and cast small-sized specimens using the lost-wax technique and compared with green sand and ceramic mould cast specimens. Tensile and shrinkage tests were carried out on all specimens. The results of the tensile tests were 123, 98 and 113 MPa for the vacuum cast, green sand cast and ceramic mould cast specimens, respectively. Furthermore, shrinkage defect which is common to both green sand and ceramic mould casting was eliminated in vacuum casting. The vacuum casting equipment cost an equivalent of about U.S $1000.00 (One thousand dollars) to construct and can be easily scaled-up for large-sized castings by increasing furnace capacity and the size of the moulding flask.

Fabrication of Tailored Powder Structures by Electrophoretic Deposition and Sintering

To enable sintering net-shape capabilities, a novel procedure for the fabrication of net-shape functionally graded composites by electrophoretic deposition and sintering are developed. The initial shape of the green specimen produced by electrophoretic deposition is designed in such a way so that the required final shape is achieved after sintering-imposed distortions. The shaped components can be metals, ceramics, polymers, and their combination as long as the material is in the form of powders. Through adjustment of an externally applied electric field and the shielding of electrical filed, the particles in the slurry are selectively deposited onto the porous mold which defines the desired geometry. By changing the slurries’ composition, the deposited component’s composition can be precisely tailored.

Difference in Thermotolerance Between Green and Red Color Variants of the Japanese Sea Cucumber, Apostichopus japonicus Selenka: Hsp70 and Heat-Hardening Effect

We studied thermal tolerance limits, heat-hardening, and Hsp70 to elucidate the difference in thermotolerance between two color variants of the sea cucumber Apostichopus japonicus. Green and Red variants occupy different habitats and have different aestivation responses to high temperature in summer. In the absence of heat-hardening, the variants showed no difference in the temperature at which 50% of the individuals died: Green, 31.49 degrees C; Red, 31.39 degrees C. However, Green specimens acquired higher thermotolerance than Red after a prior sublethal heat exposure. After 72 h of recovery from a heat-hardening treatment (30 degrees C for 2 h), the survival of Green variants was more than 50% and that of Red was less than 5% when they were treated at 33 degrees C for 2 h. Levels of mRNA and protein for Hsp70 were significantly higher in Green than Red after the heat shock of 30 degrees C, and the stability of hsp70 mRNA of Green was significantly higher than that of Red. Our findings suggest that within the same species, different variants that have similar thermal limits in the absence of heat-hardening can acquire different thermotolerances after a prior sublethal heat shock. The difference in induced thermotolerance between Green and Red is closely related to the expression pattern of Hsp70, which was partly governed by the stability of hsp70 mRNA.