Powder dispersion and aqueous tape casting of YSZ-NiO composite

Powders of YSZ and NiO were prepared by precipitation of hydroxides, calcination and wet grinding. The dispersion conditions were optimized from zeta-potential and viscosity studies. Aqueous suspensions of both the powders exhibited maximum zeta-potential values in two ranges of pH, 3 to 4 (positive) and 9 to 11 (negative), establishing the feasibility of a dispersion of both oxides together. Viscosity measurements indicated presence of effective dispersion of a 1:1.5 mixture of YSZ:NiO below a solid concentration of 25 volume percent. Aqueous tape cast slurries were formulated using PVA solution as binder and glycerol as plasticizer. The rheological behavior of these slurries (viscosity and visco-elastic parameters) was strongly influenced by slight variation in the solid content. Visco-elastic measurements revealed all the slurries to be predominantly viscous and become more elastic with increasing solid content. Defect free flat tapes could be obtained by casting the slurry with optimum flow characteristics (i.e., pseudoplastic but non-thixotropic) that could be sintered to flat sheets with 80% T.D. (theoretical density) and 20% O.P. (open porosity) at 1450°C. Upon reduction in hydrogen at 1000°C, it formed in to YSZ-Ni composite that retained the same micro structural features of YSZ-NiO.     

Preparation of Tungsten Carbide Nanopowders by Self-propagating High-Temperature Synthesis

Fine-particle tungsten carbide powders are produced from tungsten(VI) oxide by self-propagating high-temperature synthesis. The starting-mixture composition is shown to have a significant effect on the phase composition, morphology, and particle size of the WC powder. A procedure is described for recovering tungsten carbide from intermediate products using chemical dispersion. The influence of the dispersion agent on the morphology and particle size of the resultant powder is examined. The results indicate that the use of organic solvents for chemical dispersion prevents the formation of solid agglomerates, enhances the dissolution rate of impurities and defect-rich intercrystalline layers, and makes it possible to obtain tungsten carbide powders with a predominant particle size no greater than 0.05–0.2 m.     

pH controlled dispersion and slip casting of Si3N4 in aqueous media

The dispersion characteristics of commercial Si₃N₄ powder in aqueous media (deionized water) was studied as a function of pH in the range 2–11. The slip was characterized for its dispersion quality by various experimental techniques like particle size analysis, sedimentation phenomena, viscosity and flow behaviour and zeta potential analysis. The optimum dispersion was found to be in the pH region 9–11 wherein the slurry displayed minimum sedimentation height, minimum viscosity, near Newtonian flow behaviour and maximum zeta potential. The slip is highly agglomerated in the pH range 2–8 as manifested by higher sedimentation height, higher viscosity, lower zeta potential and thixotropic non-Newtonian flow behaviour. The 72 wt% (44 vol.%) Si₃N₄ slips made at pH = 10 resulted in green bodies having 53–59% of theoretical density after casting into plaster molds.     

The influence of stearic acid coating on the properties of magnesium hydroxide,hydromagnesite, and hydrotalcite powders

Hydrated filler-type flame retardants were coated with approximately a monolayer of stearic acid using a solvent technique. Compared to the uncoated powders, the BET surface area was lower, the powder packing density was improved, and the thickening effect on white oil was significantly reduced. The latter two observations are rationalized in terms of a reduction in the attractive interactions between the powder particles. The viscosity of white oil slurries containing 25 wt% solids showed shear-thinning non-Newtonian behavior. The coated powders showed significantly lower viscosities at low shear rates although the difference diminished at high shear rates. The lower viscosities shown by the coated powders indicate that the surface modification facilitated the break-up of agglomerates and the dispersion of individual particles in the fluid.     

A novel method for producing open-cell Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-ZrO<Subscript>2</Subscript> ceramic foams with controlled cell structure

A novel method was introduced to prepare open-cell Al₂O₃–ZrO₂ ceramic foams with controlled cell structure. This method used epispastic polystyrene (EPS) spheres to array ordered templates and centrifugal slip casting in the interstitial spaces of the EPS template to obtain cell struts with high packing density. Aqueous Al₂O₃–ZrO₂ slurries with up to 50 vol.% solid contents were prepared and centrifuged at acceleration of 2,860g. The effect of the solid contents of slurries on segregation phenomena of different particles and green compact uniformity were investigated. In multiphase system, the settling velocities of Al₂O₃ and ZrO₂ particles were calculated. Theory analysis and calculated results both indicated segregation phenomenon was hindered for slurries with 50 vol.% solid content. The cell struts of sintered products had high green density (61.5%TD), sintered density (99.1%TD) and homogeneous microstructures after sintered at 1,550 °C for 2 h. The cell size and porosity of Al₂O₃–ZrO₂ ceramic foams can be adjusted by changing the size of EPS spheres and the load applied on them during packing, respectively. When the porosity increased from 75.3% to 83.1%, the compressive strength decreases from 3.82 to 2.07 MPa.     

Particle size distribution control and related properties improvements of tungsten powders by fluidized bed jet milling

Fine grinding process of different particle size tungsten powders was carried out by fluidized bed jet milling. The results showed that the jet milling treatment caused deagglomeration of tungsten powders, which led to particles sufficient dispersion and narrow particle size distribution. Grinding gas pressure of 0.70 Mpa made the particles achieve high speed which promoted the particles collision contributing to particle dispersion and shape modification. For tungsten powder with particle size of 3 μm FSSS, a higher packing density with 5.54 g/cm³ was obtained, compared with the 3.71 g/cm³ of the original powder. For tungsten powder with particle size of 1 μm FSSS, the big agglomerates disappeared and the particle size distribution become narrower, while small aggregates about 2–3 μm still exist after the jet milling process. For tungsten powder with particle size of 5 μm and 10 μm FSSS, different medium diameter particle size and narrow particle size distribution of monodisperse tungsten powders can be produced by the optimized jet milling parameters. More important, the effective dispersion, favorable shape modification and precise classification have been achieved in the simple process.     

Colloidal processing of lead lanthanum zirconate titanate ceramics

Well-dispersed aqueous slurries of fine ceramic powders with high solids loading are often required for various shape forming techniques such as slip and tape casting in order to fabricate advanced ceramics with a dense and uniform microstructure. Colloidal processing of lead lanthanum zirconate titanate (PLZT) powders was conducted at various pH using ammonium polymethacrylate as a dispersant. Suspensions were characterized by viscosity and zeta-potential measurements. The effect of pH on polymer adsorption and the rheological behavior of the slurries were investigated and stabilization mechanisms discussed. Through optimization of the dispersant concentration and pH, solids loadings of the suspensions up to 50 vol.% with a relatively low viscosity were obtained.     

Bioactivity and mechanical properties of CaSiO3/high-density polyethylene (HDPE) composites prepared by a new surface loading method of CaSiO3 powder

CaSiO₃/high-density polyethylene (HDPE) composites with flexibility and biocompatibility were prepared by a new surface loading method. CaSiO₃ powder was synthesized by coprecipitation method with heating at 1300 °C for 2 h. The obtained α-CaSiO₃ powder was sieved to 45-75 μm (sample M) and 75-150 μm (sample C). Fine powder sample (sample F) was prepared by grinding the powder being the average particle size of 2.9 μm. These powders were sprinkled on the melted HDPE sheets heated at 160, 180 and 200 °C. The amounts of sprinkled powder were only <0.1 vol.% but the ratios of surface coverage area were >50% in all the samples. Apatite formation in simulated body fluid (SBF) was observed by soaking for 5 days in sample F while within 1 day in samples M and C. The sample M retained flexible properties of HDPE together with excellent biocompatible properties.     

Evolution of manufacturing parameters in Al/Ni3Al composite powder formation using blending and mechanical milling processes

Aluminu–matrix composites produced by Ni₃Al intermetallic particles are increasingly used in aerospace and structural applications because of their outstanding properties. In manufacturing of metal–matrix composites using powder metallurgy blending and milling are important factors. They control the final distribution of reinforcement particles and porosity in green compacts which in turn, strongly affect the mechanical properties of the produced PM materials. This paper studies different conditions for producing composite powders with uniform dispersion of Ni₃Al particles in aluminum powders and improved physical and mechanical properties. The results indicated that an intermediate milling time for fabrication of composite powder, better than prolonged and shortened ones, causes better microstructure and properties. It was shown that addition of 5 vol.% Ni₃Al particles, produced by 15 h mechanical alloying to aluminum powders, and then 12 h blending operation provides an appropriate condition for producing Al–Ni₃Al composite powder.     

Properties of tape-cast Y-substituted strontium titanate for planar anode substrates in SOFC applications

This work evaluates the use of fine-grained yttrium-substituted strontium titanate powders for the preparation of planar anode supported solid oxide fuel cells. Starting from a submicron-sized powder of Y-substituted strontium titanate Sr_0.895Y_0.07TiO₃ (SYT), which was synthesised via spray pyrolysis followed by a grinding process, suspensions of high solid concentration were prepared by steric stabilisation. From these suspensions, tape casting slurries of up to 25 vol% were produced and further processed to ceramic green tapes using the doctor blade technique. The rheological behaviour of the slurries was investigated in dependence on the content of solids and organic additives. Furthermore, the binder burnout and sintering behaviour of the green sheets were characterised. After firing, crack-free substrates of high planarity were obtained. The achieved properties of the sintered tapes such as density, porosity, warping, mechanical strength, and electrical conductivity were determined in dependence on sintering temperature.     

Preparation of barium titanate powders and colloidal processing in a strong (9.4 T) magnetic field

Hydrothermally prepared BaTiO₃ powders were tested inside a 9.4 T magnet. 5 vol.% and 15 vol.% BaTiO₃ suspensions were slip casted inside a 9.4 T magnet and the sintered (1400 °C) compacts were XRD analysed. Texture degrees of 0.25 and 0.43 were calculated according to the lotgering method. 15 vol.% suspensions did not yield texture. High powder loadings may give colloidally stable suspensions with low viscosities. However, agglomerates inhibit the rotational freedom of individual particles in the strong magnetic field which resulted in not or weakly textured BaTiO₃ at 15 vol.%.     

Effects of paste composition and heat-treatment conditions on the microstructure of polycrystalline CdS<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

We have studied the effects of flux content, synthesis procedure, and grinding conditions of CdS_1?x Se _x powders for paste preparation on the composition and microstructure of films produced by screen printing. The results demonstrate that, to obtain single-phase polycrystalline CdS_1?x Se _x films with well-formed grain boundaries, the flux content of the paste must lie in the range 10–12 wt %, and the solid solution with necessary doping impurities should be presynthesized. The powder must be ground to a particle size no greater than 6 μm.     

Characteristics and sintering behaviour of 3 mol % Y2O3-ZrO2 powders synthesized by reaction in molten salts

Yttria-stabilized tetragonal zirconia powders have been synthesized by reaction in molten salts at 450 °C. To obtain a Y₂O₃-ZrO₂ solid solution powder, it was necessary to use yttrium salts (not yttrium oxide directly) in molten NaNO₃-KNO₃ eutectic. The agglomeration state of the powders depended on their washing conditions. An alcohol-washed powder containing soft agglomerates led to a fine-grained and high-density sintered body.     

Flow properties of spray dried alumina granules using powder flow analysis technique

Commercially available alumina powders having an average particle size of <1 μm were spray dried to granules from aqueous slurries of different solid loadings. Spray drying is a one step granulation technique for obtaining the free flowing granules with desired sizes and morphologies from slurries. These free flowing spray dried granules on subsequent compaction processing results in the powder compacts with high densities and strength. The spray dried granules were evaluated for their flow properties in terms of cohesion index, compaction coefficient, powder flow speed dependency and caking properties using a powder flow analyzer attached to a texture analyzer. Spray-dried granules exhibited the cohesive index value of ～6.5 compared to a value of 16 for the commercially available powder. It is observed that a good correlation can be elucidated with respect to the powder particle size, surface area and morphology. The cake height ratio and compaction coefficient also complimented the above results. Powder flow analysis is an effective method to correlate the flow properties of the ceramic powders with the morphology, granule size and their distribution.     

Preparation of highly concentrated aqueous hydroxyapatite suspensions for slip casting

This paper reports the preparation of highly concentrated aqueous hydroxyapatite (HA) suspensions for slip casting of dense bone implants. The dispersing behaviour of HA powders in aqueous media was monitored by viscosity and zeta potential analyses as a function of pH of the slurry. The rheological properties of concentrated aqueous hydroxyapatite suspensions have been characterized with varying pH, NH₄PAA concentration and solids loading. The intrinsic pH of the suspension was found suitable for slip casting. The optimum dispersant concentration is 0.75 wt.% for 75 wt.% solid loading. A stable suspension with 75 wt.% solid was suitable for slip casting with viscosity of 0.36 Pa s at 100 s⁻¹. Finally, crack-free and dense microstructures have been obtained successfully with a grain size of 2–5 μm.     

Synthesis and characterization of silicalite powders and membranes with micro–meso bimodal pores

Silicalite sols containing silicalite agglomerates of 150–380 nm in size were synthesized by hydrothermal synthesis for 0.5–3 days. Silicalite powders and supported silicalite membranes containing micro-meso bimodal pores were prepared by the sol–gel method using these silicalite sols. The silicalite powders contain intracrystalline zeolitic pores (0.54 nm) and intercrystalline mesopores of about 3–4 nm in diameter. For the silicalite powders the mesopore size decreases and mesopore surface area increases with increasing silicalite agglomerate size as a result of a change of the shape of silicalite agglomerates from round to more faceted one. Continuous silicalite thin films of thicknesses ranging from 3 μm to 12 μm were made on α-alumina by the sol–gel dip-coating method. The supported silicalite membranes also contain both zeolitic pores and mesoporous intercrystalline pores. The single gas He permeance of the 3 μm thick α-alumina supported silicalite membrane was found to be from 2.7 × 10⁻⁶ to 3.3 × 10⁻⁶ mol/m² s Pa. These bimodal pore zeolite powders offer the potential as catalysts and sorbents with improved efficiency. The bimodal pore zeolite membrane can be used as support for zeolite and other membranes and as compact packed-bed reactor for chemical reaction.     

Effects of reinforcement distribution on low and high temperature tensile properties of Al356/SiCp cast composites produced by a novel reinforcement dispersion technique

A major challenge for full utilization of the potentials of SiC_p reinforced metal matrix composites is the uniform dispersion of very fine SiC particles in the matrix alloys. In this study, a novel method for gradual in situ release of properly wetted SiC particles with average size of less than 3 μm in the liquid metal was employed which greatly overcame this challenge. SiC particles were injected into the melt in three different forms, i.e., untreated SiC_p, milled particulate Al-SiC_p composite powder, and milled particulate Al-SiC_p-Mg composite powder. The resultant composite slurries were then cast in either a fully liquid state (stir casting) or semisolid state (compocasting). Subsequently, the effects of the type of the injected powder and the casting method on the microstructural and mechanical characteristics of the cast composites at room temperature and 300 °C were investigated. The results demonstrated that distribution of SiC particles in the matrix were greatly improved by injecting milled composite powders instead of untreated SiC particles into the melt. Also casting the composite slurries in a semisolid state instead of fully liquid state slightly improved the distribution. The ultimate tensile strength, yield strength and elongation at room temperature of Al356/5 vol.% SiC_p composite manufactured by compocasting of the (Al-SiC_p-Mg)_cp-injected melt were increased by 113%, 90% and 135%, respectively, compared to those of the composite manufactured by stir casting of the untreated-SiC_p injected melt. The improvements in these properties at 300 °C were about 100%, 103% and 129%, respectively. Almost all the composite samples retained more than 90% of their strengths at 300 °C, whereas the monolithic samples lost more than 25% of their strength at this temperature. The composites manufactured by compocasting of (Al-SiC_p-Mg)_cp-injected melts exhibited a typical ductile fracture surface with equiaxed dimples at both room temperature and 300 °C.     

Synthesis of γ-Al2O3 nanopowders by freeze-drying

This work studies the synthesis of γ-Al₂O₃ nanopowders by a freeze-drying method. Aqueous solutions of Al₂(SO₄)₃·18H₂O were used as precursors to Al concentrations of 0.76, 1.00 and 1.40 M. Homogeneous spherical granules with diameters ranging from 1 to 100 μm have been obtained. These porous granules are constituted by soft agglomerates of nanoparticles with primary particle size lower than 20 nm. The microstructure of the agglomerates largely depends on the freezing kinetics. After drying amorphous aluminium sulphate powder is obtained that decomposes at 825 °C leading to the formation of γ-Al₂O₃. Physicochemical study of the freeze-dried powders is performed through particle size distribution and zeta potential measurements. The characterisation of the powders is evaluated considering the influence of processing parameters such as the salt concentration, the freezing rate and the thermal treatment for the synthesis and the dispersing conditions of the obtained powders. By adjusting the dispersing conditions a minimum particle size <30 nm is measured, thus confirming that granules can be easily dispersed into nanoparticles.     

Preparation of tetragonal zirconia powders by a solid state reaction: Kinetics, phases and morphology

Powders of tetragonal (t)ZrO₂ have been prepared by a solid state reaction between sodium metazirconate and sodium metaphosphate. The reaction temperatures and times have been varied between 450 and 550°C and 5 and 75 h, respectively. Zirconia powder, mostly in thet andt′ phases, is obtained. The yield of ZrO₂ powder increases monotonically with time at all reaction temperatures according to a phase boundary controlled kinetics. The fraction oft phase also increases with time at 450°C and 500°C but goes through a maximum at 550°C, the highest temperature employed. A maximum of 55% of the precursor monoclinic zirconia (used to prepare sodium meta zirconate) is converted tot phase at 500°C/75 h. The ZrO₂ powder consists of crystallites of size 9–25 nm agglomerated into particles having average size between 2 and 4μm. The agglomerates have a breaking strength of 100 MPa. A hydrothermal treatment is found to break the agglomerates into smaller sizes. Grinding the powder in a mortar and pestle converts only 12% of thet phase into monoclinic, indicating that substantial fraction of the tetragonal phase is the non transformable varietyt′. Heating experiments also confirm this.     

Fracture Model of Fine Bulk Powder Structures

ABSTRACT

Using several line powders (particle size 15-65 μ m). the rheological parameters of tensile strength ( σ_f,mb) and plastic deformation coefficient (Y) were experimentally measured at ambient and elevated temperatures. In order to be aeratable. the rheological parameters of a specific powder should satisfy the equation of σ_f,mb = 0.11Y^{0 89.}The formation of agglomerates or dead zones can be predicted. By introducing the “Quasi porous solid body model,” the theoretical derivation of the criterion characteristic curve was accomplished using solid fracture theory.