Particle Size Of Starting Powder Research Articles

Effect of Starting Powder Particle Size on the Thermoelectric Properties of Hot-Pressed Bi0.3Sb1.7Te3 Alloys.

P-type Bi0.3Sb1.7Te3 polycrystalline pellets were fabricated using different methods: melting and mechanical alloying, followed by hot-press sintering. The effect of starting powder particle size on the thermoelectric properties was investigated in samples prepared using powders of different particle sizes (with micro- and/or nano-scale dimensions). A peak ZT (350 K) of ~1.13 was recorded for hot-pressed samples prepared from mechanical alloyed powder. Moreover, hot-pressed samples prepared from ≤45 μm powder exhibited similar ZT (~1.1). These high ZT values are attributed both to the presence of high-density grain boundaries, which reduced the lattice thermal conductivity, as well as the formation of antisite defects during milling and grinding, which resulted in lower carrier concentrations and higher Seebeck coefficient values. In addition, Bi0.3Sb1.7Te3 bulk nanocomposites were fabricated in an attempt to further reduce the lattice thermal conductivity. Surprisingly, however, the lattice thermal conductivity showed an unexpected increasing trend in nanocomposite samples. This surprising observation can be attributed to a possible overestimation of the lattice thermal conductivity component by using the conventional Wiedemann-Franz law to estimate the electronic thermal conductivity component, which is known to occur in nanocomposite materials with significant grain boundary electrical resistance.

Phase transition between sphalerite and wurtzite in ZnS optical ceramic materials

The phase transition behavior of zinc sulfide (ZnS) ceramics consolidated via pressureless and hot press sintering has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) analyses. Two types of ZnS powders with different particle sizes and morphologies were employed to study the influence of microstructural features of starting powders on the ZnS phase transition behaviors. The present work has revealed that during sintering of ZnS ceramics, the phase transition behavior varies based on the starting powder particle size and magnitude of the applied pressure. It has been demonstrated that smaller particle sizes lead to an increased degree of “early” phase transformation from sphalerite to wurtzite at 1000 °C. Additionally, the application of uniaxial pressure during sintering can lead to a reverse phase transition from wurtzite to sphalerite while simultaneously inducing twinning, resulting in improved optical transmittance and mechanical hardness.

Corrosion and Wear Behaviour of Spark Plasma-Sintered NiCrCoAlTiW-Ta Superalloy

NiCrCoAlTiW-Ta superalloy was sintered using spark plasma sintering technique. The influence of starting powder particle size on the corrosion and dry sliding wear behaviour of sintered NiCrCoAlTiW-Ta superalloy was investigated. The nickel matrix (> 60 wt%) was varied over three different particle sizes (3–44, 45–106 and 106–150 µm). The powders were sintered at 1100 °C, heating rate of 100 °C/min and pressure of 32 MPa. The effect of particle sizes on sintered density, microhardness, corrosion and wear were reported. The results show that the sintered density of 97.48% and microhardness value of 382.88 HV0.1 were reported for the smallest starting powder. Furthermore, the microstructures of the sintered alloy revealed the presence of three major phases: γ, γ′ and the precipitated solid solution phase. There was an improvement in the corrosion response in relation to the particle size of the starting powder. This indicates that the least corrosive alloy has the least starting powder particle size. It has a corrosion rate of 0.047 and 0.056 mm/year in saline and acidic media, respectively. Also, the coefficient of friction increased with increase in powder particle size. However, the poor wear response of the alloys with bigger powder particle size could be attributed to poor adhesion of the oxide layer.

Metal injection moulding of 17-4PH stainless steel: Effects of porosity on the mechanical properties of the sintered products

17-4PH stainless steel (SS) is a martensitic precipitation hardenable alloy which normally contains approximately 15.5-18.5 wt.% Cr and 2.5-4.5 wt.% Ni and Cu. It is well known for its high impact strength and fracture toughness, and excellent corrosion resistance at service temperatures below 300 °C promoted by its Cu-rich precipitates. 17-4PH SS is widely used as a structural material in various sectors such as aerospace, medical, petrochemical, etc. The high strength of the alloy makes it difficult to machine, especially when making complex shaped products. Metal injection moulding (MIM) uses the shaping advantage to produce small-to-medium intricate products in high volumes at a relatively low cost. However, due to the nature of MIM process, the mechanical properties of the sintered products depend strongly on various metallurgical MIM variables with porosity included. Porosity is largely dependent on the starting powder characteristics, powder loading, and sintering conditions. In this study, the mechanical properties of the sintered 17-4PH SS MIM samples fabricated at different powder loadings, viz. 55, 60 and 65 vol.%, will be evaluated. The starting powder particle size distribution is -5, -15 and -45 μm. The sintered density, microstructure and hardness response of the samples are presented. These properties are influenced by the nature of the pores, i.e. size, density and distribution.

Effect of nickel powder particle size on the microstructure and thermophysical properties of spark plasma sintered NiCrCoAlTiW-Ta superalloy

The spark plasma sintering technique (SPS) was used to consolidate admixed elemental powder in order to fabricate NiCrCoAlTiW-Ta superalloy. Nickel which is the key element that formed the matrix powder particle size was varied in the range of 3-44, 45-106 and 106-150 μm. The effect of varying the starting powder particle size, on the formation of intermetallic phases was investigated. Also, important thermophysical properties in the development of superalloys for high temperature applications were investigated by using laser flash machine, in the range of 100-800 °C. The constituents of the microstructure for the sintered alloy includes, the gamma (γ) matrix phase (Ni), precipitated intermetallic gamma prime (γ’) phase Ni3(Al, Ti) and the precipitated solid solution strengthening elements (Cr and W). There was an increase in the density with decreasing powder particle size. Also, the averave grain size increased with increasing powder particle size. The thermophysical properties obtained, which include thermal conductivity and diffusivity increase with increasing temperature. Non-linear behaviour and inflection points were observed as the temperature increases due to short range order and disorder phenomenon which is associated with superalloys. The powder particle size has little or no significant effect of the thermophysical behaviour as the pattern observed were almost the same.

Microstructural investigation and performance evaluation of slip-cast alumina supports

In this work we successfully obtained slip-cast alumina supports with tubular shape. It was investigated the influence of both the starting powder particle size and heat treatment program on the pore structure and water permeability of the prepared materials. This study is supported by a series of experimental tests, including Archimedes method, mercury intrusion porosimetry, scanning electron microscopy, and cold crushing tests. We observed that the heat treatment temperature exhibited a more significant effect on the porosity than the sintering time. It was noticed that, in a general way, the higher the sintering temperature, the smaller the porosity and the larger the apparent density of the prepared materials. In addition, the raise of the sintering temperature from 1100 to 1500°C increased the cold crushing strength of the tested samples. This behavior was not observed for F2, which seems to be related to the poor sinterability of the starting alumina powder used in its preparation. Among the samples prepared in this work, F2 showed the highest water permeability, followed by F3 and F1. The water permeation behavior of these materials is discussed on the basis of their mean pore size, porosity, genus per unit volume, and pore network tortuosity.

Effect of starting powder particle size and heating rate on spark plasma sintering of Fe[sbnd]Ni alloys

The effect of starting powder particle size and heating rate on spark plasma sintering of FeNi alloys was investigated, with the particle powder size varying from 3 to 70 μm and heating rate from 50 to 150 °C/min. The effect of the starting powder particle size was more obvious when comparing 3-FeNi and 70-FeNi at all heating rates, with the former having better density and hardness than the latter. Sintered densities close to theoretical (≥99%) were achieved for a heating rate of 50 °C/min for the different starting particle size powders, and decreased with increasing heating rate. The average grain size of alloys sintered at 150 °C/min was ∼34% smaller than those sintered at 50 °C/min. The porosity content of the sintered samples increased with increasing heating for the same particle size. The shrinkage rate depends on both heating rate and particle size. At a particle size of 3 μm and a heating rate of 50 °C/min, three peaks were observed indicative of the phenomena responsible for good densification. As the heating rate increases, only two peaks and one peak are observed at heating rates of 100 and 150 °C/min, respectively. This suggests that, unlike high heating rates, the longer processing time at low heating rate allows the three phenomena to take place. The hardness measurement revealed a steady decrease with increasing heating rate. At a heating rate of 150 °C/min the particles were well packed but no typical dimple structure of a ductile material was observed. However, for samples sintered at 50 and 100 °C/min a typical dimple fracture morphology was observed.

Dielectric properties of spark plasma sintering AlN-W composite ceramics

In order to develop AlN composites suitable for high average power electronic tube, AlN-W materials were prepared by spark plasma sintering. The effects of manufacture parameters on dielectric loss tangent and permittivity constant were investigated, which include powder-mixed method, milling time of high-energy ball milling, starting powder particle size, sintering temperature and holding time and adding amount of the conductive second phase. The results showed that AlN-W materials sintered at the temperature of 1700°C holding for 5 min with 10 vol.% W showed the best dielectric loss tangent larger than 0.81 at the frequency 1 kHz-1 MHz. In addition, magnetic stirring mixed powder and lower sintering temperature led to the better properties because of the higher porosity. The samples sintered from the starting AlN powder with smaller particle size also had the better properties.

Microstructure and properties of Cu–Al–Ni shape memory alloy strips prepared via hot densification rolling of argon atomized powder preforms

Abstract In the present work, Cu–Al–Ni SMA strips with grain size less than 100 μm were successfully prepared via a novel powder metallurgy route involving hot densification rolling of unsheathed sintered powder preforms prepared from argon atomized pre-alloyed Cu–Al–Ni powder. It has been shown that the hot rolled Cu–Al–Ni strips exhibited a very small grain growth during heat treatment at 950 °C up to 4 h. This behavior was attributed to the pinning effect of nano-sized alumina segregated on the grain boundaries. It was established that the grain size of the finished Cu–Al–Ni strips is governed by the starting powder particle size, and the mechanism of this aspect has been discussed. The finished heat-treated Cu–Al–Ni alloy strips consisted of elongated shape grains, with average grain size approximately 27 μm. The heat treated strips were fully martensitic in nature consisting of self-accommodated β ′ 1 martensite along with small amount of γ ′ 1 martensite. The heat treated strips had an extremely good combination of average fracture strength (∼626 MPa) and fracture strain (∼13.5%). The two-way shape memory strain was found approximately 0.23% after 25 training cycles at 4% applied training strain.

Chemically produced nanostructured ODS–lanthanum oxide–tungsten composites sintered by spark plasma

High purity W and W–0.9La 2O 3 (wt.%) nanopowders were produced by a wet chemical route. The precursor was prepared by the reaction of ammonium paratungstate (APT) with lanthanum salt in aqueous solutions. High resolution electron microscopy investigations revealed that the tungstate particles were coated with oxide precipitates. The precursor powder was reduced to tungsten metal with dispersed lanthanum oxide. Powders were consolidated by spark plasma sintering (SPS) at 1300 and 1400 °C to suppress grain growth during sintering. The final grain size relates to the SPS conditions, i.e. temperature and heating rate, regardless of the starting powder particle size. Scanning electron microscopy revealed that oxide phases were mainly accumulated at grain boundaries while the tungsten matrix constituted of nanosized sub-grains. The transmission electron microscopy revealed that the tungsten grains consist of micron-scale grains and finer sub-grains. EDX analysis confirmed the presence of W in dispersed oxide phases with varying chemical composition, which evidenced the presence of complex oxide phases (W–O–La) in the sintered metals.

Formation of High-Strength Zr-Nb-Cu-Ni-Al Alloys by Warm Extrusion of Gas Atomized Powders

ABSTRACTRecently developed Zr-based metallic glass composites containing a ductile phase demonstrate improved mechanical properties such as high strength combined with good ductility compared to the glass monoliths. Zr-Nb-Cu-Ni-Al amorphous powders with bcc phase precipitates were obtained by high pressure gas atomization. Formation of the bcc phase in the amorphous matrix strongly depends on the material composition and cooling rates during solidification. Melt spinning using various wheel speeds selected to simulate the cooling rates during gas atomization was used to define a specific composition for gas atomization. Gas atomized powders were consolidated by warm extrusion. Various processing conditions including starting powder particle size, extrusion temperature and extrusion ratio were examined to obtain materials having various microstructural features. Structure and thermal stability of consolidated bulk metallic glass composites as well as selected mechanical properties will be discussed.

Hot pressing and vacuum sintering of fine titanium nitride powder

Increasing the specific surface of titanium nitride from 18 to 90 m2/g lowers the initial recrystallization temperature of loosely poured powder from 1300 to 600°K. The temperature at which blanks attain practically 100% density in the hot pressing of finely divided titanium nitride (a starting powder particle size of 0.05–0.07 μm) is 1600°K, which is 500–700°K below the temperature level of full sintering of relatively coarsegrained powders (a particle size of about 0.5 μm). At hot-pressing temperatures above 1800°K a fall in the density of sintered compacts is observed, which is apparently attributable to the beginning of nitrogen evolution from the nitride and also to the formation of microcracks. In vacuum sintering without a plasticizer, crack formation lowering the density of specimens by 3–4% is characteristic of the whole sintering temperature range. The grain size in hot pressing and vacuum sintering is practically the same, being determined chiefly by the sintering temperature and time. At the maximum specimen density the maximum grain size is 20 μm.

Production technology and properties of large-sized titanium filters

1. A study was made of a new process for the hydrostatic pressing of large-sized filter elements from titanium powder. 2. Designs of a pilot-plant hydrostatic pressing unit and a hydrostatic installation based on a standard hydraulic press have been developed. 3. The effects of compaction pressure and starting powder particle size upon the microstructure and properties of sintered filter elements were investigated. 4. The statistical shape characteristic of the titanium powder particles and pores in filters was evaluated by stereometric metallography techniques. The relation between the particle shape factor and the properties of filter elements was established.

Effect of technological factors on the strength of a sulfidized iron-graphite material

1. The strength of iron-graphite powder metallurgical materials after sintering is governed not only by the charge composition but also by a number of technological parameters. Varying the degree of alloying of the iron particles with carbon enables the strength properties of a sulfidized iron-graphite material to be changed by a factor of nearly two; decreasing the starting powder particle size by a factor of four produces a 30% increase in strength; lowering the porosity from 30 to 1% increases the strength characteristics of the material four to five times. 2. The strength of a material can be changed appreciably by alloying its interparticle contacts. Adding zinc sulfide to an iron-graphite material causes alloying of its contacts with zinc and raises its integral hardness by almost 30%. 3. The starting porosity determines the character of strengthening and rupture of a material in compression. During the compression of porous materials, densification and increase in the area of contacts per unit sectional area (geometric factor) constitute the main mechanism of strengthening, but with nonporous materials strengthening is due to the work-hardening of particles and interparticle contacts (physical factor). Rupture may take place in both the contacts and particles. 4. Raising the test temperature from 20 to 600°C lowers the strength of ZhGr3Tss4 material by a factor of 2–2.5, but produces a twofold increase in its ductility properties.