Starting Powder Characteristics Research Articles

Influence of starting Y2O3 and Nd2O3 powders characteristics on optical properties of highly transparent Nd:Y2O3 ceramics

The present paper systematically investigated the correlation of the starting powder characteristics and optical properties of neodymium doped transparent Y2O3(Nd:Y2O3) ceramics by hot pressing without a subsequent hot isostatic pressing (HIP) treatment. It was found that the in-line transmittance of Nd:Y2O3 was improved by using coarser primary particle size of Y2O3 and Nd2O3. Additionally, an increase in the hot pressing temperature from 1500 to 1600 °C was effective for improvement on optical qualities of Nd:Y2O3 ceramics without an exaggerated grain growth. These results demonstrate the possibility of fabricating highly transparent ceramics by a simple and cost-effective powder process and sintering technique.

Manufacturing porcelain components by CIM: Viability of processing different ceramic powders

Ceramic Injection Moulding (CIM) is an advanced powder processing technology launched three decades ago which has become a growing attractive alternative for new technical applications, such as thermal management and wireless charging. Porcelain-type ceramics can allow the introduction of new feedstocks into the CIM market at a competitive cost, in comparison with the conventional alumina and zirconia feedstocks. During the CIM manufacturing process, several factors related to the starting powder characteristics have an influence on the quality and properties of the final components, such as the mixing behaviour and the feedstock flow properties, as well as injection, debinding and sintering parameters. In this work, the viability of three porcelain-based powders to successfully achieve injectable mixtures for CIM is described. The mixing behaviour and the feedstocks flow behaviour as a function of its solids loading is evaluated. In this way, the ideal attributes for a CIM porcelain powder are discussed, studying the first process stages through to the production of a sintered component by injection moulding. Finally, density, hardness, bending strength and microstructural properties of the selected porcelain are investigated and a cost-efficient ceramic feedstock is suggested for aesthetic and electrical applications.

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.

Influence of Powder Characteristics on Processability of AlSi12 Alloy Fabricated by Selective Laser Melting.

Selective laser melting (SLM) is one of the additive manufacturing technologies that allows for the production of parts with complex shapes from either powder feedstock or from wires. Aluminum alloys have a great potential for use in SLM especially in automotive and aerospace fields. This paper studies the influence of starting powder characteristics on the processability of SLM fabricated AlSi12 alloy. Three different batches of gas atomized powders from different manufacturers were processed by SLM. The powders differ in particle size and its distribution, morphology and chemical composition. Cubic specimens (10 mm × 10 mm × 10 mm) were fabricated by SLM from the three different powder batches using optimized process parameters. The fabrication conditions were kept similar for the three powder batches. The influence of powder characteristics on porosity and microstructure of the obtained specimens were studied in detail. The SLM samples produced from the three different powder batches do not show any significant variations in their structural aspects. However, the microstructural aspects differ and the amount of porosity in these three specimens vary significantly. It shows that both the flowability of the powder and the apparent density have an influential role on the processability of AlSi12 SLM samples.

Influence of powder characteristics on hot-pressed Si3N4 ceramics

In this paper, the influence of the starting powder characteristics for five commercial Si3N4 powders on sintering behavior, microstructure and mechanical properties of hot-pressed Si3N4 ceramics with and without the addition of La2O3-Yb2O3-MgO sintering additives was investigated. For the system without sintering additives, the high amount of the impurities in the starting powders could facilitate the densification process and promote ?-Si3N4 grain growth. However, large whisker-like particles (? phase) present in the initial powders inhibited the sintering process, which led to a lower density of Si3N4 ceramics. On the other hand, when the sintering additives were introduced in the beginning stage of the powder processing step, the effects of impurities content and particle morphology in the initial powders on the densification and microstructure were not apparent. There was tendency that finer powder size resulted in finer microstructure. However, a high ?-phase ratio in the initial powders could accelerate the abnormal grain growth and thus lead to better mechanical properties.

Processing, Mechanical and Optical Properties of Additive-Free ZrC Ceramics Prepared by Spark Plasma Sintering

In the present study, nearly fully dense monolithic ZrC samples are produced and broadly characterized from microstructural, mechanical and optical points of view. Specifically, 98% dense products are obtained by Spark Plasma Sintering (SPS) after 20 min dwell time at 1850 °C starting from powders preliminarily prepared by Self-propagating High-temperature Synthesis (SHS) followed by 20 min ball milling. A prolonged mechanical treatment up to 2 h of SHS powders does not lead to appreciable benefits. Vickers hardness of the resulting samples (17.5 ± 0.4 GPa) is reasonably good for monolithic ceramics, but the mechanical strength (about 250 MPa up to 1000 °C) could be further improved by suitable optimization of the starting powder characteristics. The very smoothly polished ZrC specimen subjected to optical measurements displays high absorption in the visible-near infrared region and low thermal emittance at longer wavelengths. Moreover, the sample exhibits goodspectral selectivity (2.1–2.4) in the 1000–1400 K temperature range. These preliminary results suggest that ZrC ceramics produced through the two-step SHS/SPS processing route can be considered as attractive reference materials for the development of innovative solar energy absorbers.

Precursor Influence on the Electrical Properties of Textured Bi-2212 Superconductors

Several synthetic methods, solid-state, sol-gel and polymer solution methods, have been used to prepare prereacted precursors, as well as a vitreous material obtained by melt quenching. The influence of the starting powder characteristics on the phase formation, microstructure, T c and J c of Bi-2212 textured rods prepared by directional crystallization from the melt has been analyzed. In all the cases, high transport critical current values (higher than 3000 A/cm2 at 77 K) have been obtained, independently of the precursor type. Samples obtained by the polymer route show improved T c values, associated to a lower oxygen content.

Fine-grained Al 2O 3–ZrO 2 composites by optimization of the processing parameters

A new type of Al2O3–ZrO2 composite was fabricated by optimizing processing parameters with emphasis on improving and controlling fine grain growth and the formation of compact desirable microstructures by using colloidal processing. The electroacoustic technique was used for on-line control of the stability of highly concentrated suspensions. The results gave good correlation between starting powder characteristics, rheological properties, sintering and microstructure of fabricated dense and homogeneous fine-grained Al2O3–ZrO2 composites.

Effects of Starting Powder Characteristics on Bulk Assembly of Titania

Seven nanoscale titanias synthesized from a sulfate process were investigated for rheological characterization. Alteration of synthesis parameters had resulted in variation of the powder surface area and soluble sulfate level from powder to powder. The powder differences were further corroborated in stress‐controlled rheometry where the viscoelastic yield stress of a suspension was seen to vary strongly with processing history. Capillary and torque rheometry further showed the dependence of processing history on steady‐state extrusion pressure and steady‐state mixing torque. The powders were washed to determine whether sulfate equilibration through washing could be used to circumvent the synthesis parameter differences.

The hot erosion behavior of HVOF chromium carbide-metal cermet coatings sprayed with different powders

The high velocity oxygen fuel (HVOF) sprayed chromium carbide-metal cermet coatings have shown some attractive behavior, which makes them superior to arc-sprayed iron base coatings in preventing erosion. While their hot erosion behavior and its dependence on carbide/metal matrix proportion have been investigated, the detail study of the hot erosion behavior being dependent on the spraying methods and the starting powder characteristics has been somewhat limited. This work was undertaken to understand the influence of powder type on the hot erosion behavior of HVOF coatings. A series of hot erosion tests was carried out on eight chromium carbide-metal cermet coatings using a nozzle type elevated temperature erosion tester. The morphology of specimens was examined by light microscopy and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS). The composition of starting powders and deposited coatings was analyzed using EDS. It was found that among the eight HVOF chromium carbide-metal cermet coatings tested the composite powder sprayed coatings had nearly the same composition as the starting powders, while the blend powder sprayed coatings had lower chromium and higher nickel contents than the starting powders. This means that more chromium carbide particles had lost during spraying the blend powders, as compared with spraying the composite powders whose composition almost remained the same. The composite powder sprayed coatings also showed higher microhardness and finer microstructure, lower porosity and oxide rate, which account for their higher erosion resistance than the blend powder sprayed coatings.

Microwave sintering and mechanical properties of PM copper steel

Microwave processing has gained worldwide acceptance as a novel method for heating and sintering a variety of materials from food to rubber to specialty ceramics, as it offers specific advantages in terms of speed, energy efficiency, process simplicity, novel and improved properties, finer microstructures, and lower environmental hazards. In the present paper, microwave sintering of modulus of rupture (MOR) bar samples of PM copper steel (MPIF FC-0208 composition) and the comparative evaluation of the mechanical properties using both microwave and conventional sintering techniques has been reported.The starting powder characteristics and the processing details of copper steel bar samples sintered in a conventional furnace and in an in house modified commercial microwave oven has been covered at length. In this study, the sintering temperature used typically ranged between 1100 and 1300°C, soaking time ranged from 5 to 20 min, and the atmosphere was controlled using flowing forming gas (mixture of 95% N2 + 5% H2). Microwave sintering resulted in higher sintered density, higher Rockwell hardness (HRB), and higher flexural strength as compared with conventional sintering. The improved mechanical properties of microwave sintered samples can be mainly attributed to the evolution of distinct porosity distribution, primarily consisting of small, rounded, and uniformly distributed pores as against large, angular and non-uniformly distributed pores observed in the case of conventional sintering.

Precursor powder influence on melt processing of high critical currentBSCCO rods

The influence of the starting powder characteristics on the phaseformation, microstructure and Jc (77 K) of Bi-2212 monoliths fabricated bythe LFZ method has been analysed. Attractive transport critical current values(ca 90 A in ≈1.8 mm diameter rods) have been measured onsamples obtained from practically any of the precursors studied, that isprereacted 2212, mixtures of 2201 and sub-micrometre sized CaCuO2 or,alternatively, a mixture of the corresponding commercial oxides andcarbonates. The latter method reduces significantly the processing procedurerelated to the use of chemically derived starting precursors or to theattainment of the 2212 phase in the precursor at a high purity level. Theselection of particular precursor mixtures affects the final texturedsuperconductor properties only to a limited extent. These appear to be relatedto the influence of the precursors on both, the melting and solidificationprocesses, associated in turn with their modification of the reaction pathwaytowards 2212 formation.

Fabrication and characterisation of Al 2O 3 porous bodies by hot isostatic pressing

Porous ceramic filters have applications in many industrial areas. The major requirements for filter materials are high porosity, high mechanical strength and uniform pore size. However, conventional fabrication methods cannot produce materials with both high porosity and high strength, since an increase in porosity usually results in a decrease in strength. A method has been reported to obtain porous materials of high strength and high porosity by hot isostatic pressing (HIP), because the high pressurised gas prevents shrinkage of open pores and enhances bridging of particles. In the present study several commercial Al 2O 3 starting powders were uniaxially and isostatically pressed and the green compacts were directly treated under different HIP conditions. The microstructure (grain size and shape, density and pore size distribution) of the HIP products was evaluated and then compared to that of the traditionally sintered ones in order to investigate the influence of starting powder characteristics, forming process and thermal treatment parameters on the characteristics of the final materials. © 1997 Elsevier Science S.A.

Microstructure‐Property Relations of Solid Oxide Fuel Cell Cathodes and Current Collectors: Cathodic Polarization and Ohmic Resistance

Microstructure, cathodic polarization, and ohmic resistance on the cathode side of ‐based solid oxide fuel cells have been studied for the intermediate temperature operation range between 700 and 900°C. Starting powder characteristics, powder calcination temperature, and sintering temperature strongly influence the final microstructure of cathodes. Electrochemical performance depends on these processing parameters as well as on the cathode thickness and the contact spacing of current collectors. A decrease in effective electrode area occurs both on the microscopic level with coarse and inhomogeneous cathode microstructure and on the macroscopic level with a wide contact spacing of the current collectors. The smaller effective electrode area causes inhomogeneous current density distribution and results consequently in higher ohmic losses originating from the electrolyte and higher cathodic polarization. These losses are evaluated using cathodes with different microstructures and on the mole percent electrolyte. The influence of current path constrictions on the ohmic and nonohmic losses is demonstrated using Pt current collectors of different geometric spacings.

Sintering of ultradisperse powders based on zirconium dioxide (review)

We consider the effect of the starting powder characteristics (purity, grain size and shape, size distribution, sintering aids content, etc.), green compact microstructure (density and porosity distribution), and processing parameters (including temperature, exposure time, rate of heating or cooling of the medium) on sintering of ultrafine ZrO2-based powders. We discuss various sintering techniques: hydrothermal sintering, microwave sintering, hot pressing, sinter—forging, sinter-HIP, and gas-pressure sintering.

Superconducting composite wires and tapes

Abstract The optimization of superconducing Ag Bi-Sr-Ca-Cu-O composites towards high critical currents (Jc) has been studied using the powder-in-tube method, together with appropriate cold rolling-annealing sequences. Both monofilament and multifilament wires and tapes have been produced. The effect of starting powder characteristics and other important processing parameters on Jc in external magnetic fields has been analyzed in relation with their microstructure.

Physical and mechanical properties of YBa2Cu3O7-δ superconductors

The physical and mechanical properties of YBa2Cu3O7−° superconductors are examined. These properties are related to powder preparation method, powder characteristics, sintering behaviour and sintered microstructure. The sintering atmosphere and sintering schedules affect the final microstructure very strongly and determine, in conjunction with starting powder characteristics, the sintered density. The mechanical properties such as Young's modulus, bend strength and critical stress intensity factor (fracture toughness) are measured and related to microstructure as determined by electron microscopy. Control of microstructure by careful powder selection and sintering schedule is seen as key to optimizing the physical and mechanical properties of the material. Finally attention is drawn to fabrication techniques and how these must be optimized in order to realize the mechanical properties which are necessary if these are to be useful as engineering materials. Comparisons between fabrication techniques show that uniaxial powder pressing suffers from limitations in terms of specimen complexity and densification whereas the favoured route, termed viscous processing, gives a more homogeneous microstructure, higher strength and allows near theoretical density to be achieved.