Effect of compaction pressure on the sinterability of submicron powders of tetragonal zirconium dioxide

Conclusions We studied the effect of compaction pressure on the pore structure of the paniculate compacts obtained using two types of agglomerated submicron powders of tetragonal zirconium dioxide, on the structure evolution during the sintering process, and on the strength of the obtained material. It was established that the characteristics of the agglomerates present in the powders have a significant effect on their behavior during compaction and sintering. At a given compaction pressure, the powders having weaker agglomerates densify up to a higher density and give a more uniform distribution of pores in the preform. The low-density compacts obtained using agglomerated powders having a high specific surface area sinter faster and attain high strength levels at a lower temperature; however, the sintered materials obtained from such compacts contain several structural defects in the form of large pores and have a lower strength. The uniformity of the distribution of pore volume with respect to size (or the specific content of the interagglomerate pores) forms the main criterion of the quality of particle packing in the compacts obtained from agglomerated powders. The compacts having a low content of the interagglomerate pores give a defect-free dense and strong material after sintering. The presence of the anion impurities in the original powders leads to a decrease of density during the sintering process after the attainment of a threshold density at which formation of closed porosity occurs. Pressure sintering (HIP) forms an effective method of suppressing the decrease of density.Translated from Ogneupory, No. 2, pp. 5–11, February, 1993.     

Grain growth during the sintering of compacted submicron powders

The kinetics of grain growth in compacts made of submicron titanium powders obtained by controlled hydrolysis of titanium alcoholates was studied. It was established that inhomogeneous packing of the particles in a compact made of agglomerated powder stimulates intense grain growth even in the early stage of sintering. It is shown that a material obtained by sintering homogeneously packed compacts composed of coarse non-agglomerated particles possesses, despite a low density, a homogeneous highly disperse structure and exhibits a much higher strength than a material obtained upon sintering a finely divided but agglomerated powder to maximum density. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 4, pp. 18–22, April, 1997.     

Inhomogeneity of powder packing in compacts and strength of the ceramics obtained

Results of studying the effect of the packing inhomogeneity of the powder compact on the strength of sintered corundum ceramics are presented. In the powders used for molding powder compacts, agglomerates of various sizes were formed artificially. Under the same sintering conditions no substantial difference in the density or grain size of materials that initially differed in the inhomogeneity of particle packing were observed. Strength measurements revealed an inverse dependence of the strength on the degree of powder inhomogeneity in the starting compact. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 5, pp. 14–19, May, 1997.     

Sintering Behavior of Fully Agglomerated Zirconia Compacts

Fully agglomerated superfine zirconia powders were prepared with the coprecipitation and spray-drying method. The compaction of such powders shows no fragmentation of the agglomerates. The sintering behavior of the compacts was studied and two sintering stages were identified: densification within agglomerates at temperatures not higher than 1250°C and the removal of interagglomerate pores at temperatures above 1600°C. The interagglomerate pores are difficult to remove, and sintering between agglomerates even at 1600°C is still insignificant. Heating of the compacts at temperatures above 1600°C leads only to grain growth and the entrapping of pores in large grains.     

Unagglomerated powders for transformation-toughened structural ceramics

Results of a study of the characteristics of ZrO₂ powders stabilized by 3 mol% Y₂O₃ and fabricated by the method of ultrasound spray pyrolysis from aerosols of aqueous solutions of mineral salts are presented. Unlike powders of the same composition obtained by the conventional coprecipitation technique, aerosol powders have coarser particles with a narrower size distribution and do not contain agglomerates. The temperature of sintering coarse aerosol powders to a maximum density is not high (1450°C). Despite the larger grain size in ceramics sintered from aerosol powders, their strength is much higher than that of ceramics obtained from ultradisperse coprecipitated powders of a similar composition. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 6, pp. 5–8, June, 1997.     

Porosity measurement of fragile agglomerates

Agglomeration processes in the food and pharmaceutical industries frequently produce porous, brittle agglomerates intended for redispersion in liquids (a.k.a. “instant powders”). Typical products have a porosity ranging from =0.4 to =0.8 and a particle size between 0.2 and 2 mm. The agglomerate porosity is an important product property, as it has a major influence on the wetting behaviour. Due to their high porosity and comparatively weak internal bondings, instant agglomerates are fragile.

Compared to the wide range of currently available particle sizing methods, few porosity-measuring methods exist, most of which are not applicable to instant agglomerates. For controlling the porosity of instant agglomerates produced in our laboratory, we used individual particle weighing and sizing. A balance with 1 μg of precision allowed reliable weighing of individual agglomerates as small as 300 μm diameter. The volume of the particles was determined using a long-distance microscope with CCD camera attached to a computer running public domain image analysis software.

Porosity measurements of agglomerated skim and whole milk powder, agglomerated pectin, agglomerated flour/maltodextrin mixture, and agglomerated rice starch/maltodextrin mixtures were carried out. The results of our measurements show, for the majority of the samples, an approximately constant porosity over a wide particle size range, with a tendency towards lower porosity for smaller particles.     

Agglomeration of fine particles subjected to centripetal compaction

Agglomeration is a common phenomenon in many processes. The mechanical properties of agglomerates strongly depend on their structures. This paper presents a numerical study of the agglomeration of fine particles down to 1 μm in size based on the discrete element method. The agglomerates were formed with particles initially generated randomly in a spherical space and then packed under an assumed centripetal force. Agglomerate structure, packing density, coordination number and tensile strength were analysed with particular reference to the effect of particle size associated with the van der Waals attraction. The results showed that both the packing density and coordination number of the agglomerates decay exponentially to their limits as agglomerate size increases. The tensile strength of the agglomerates was calculated from the simulations and shown to decrease with the increase of particle size. The strength was also estimated from the Rumpf model supported by the empirical equations formulated based on the present simulation results. The good agreement between the results from the simulations and the estimation indicates that the equations are useful to facilitate engineering applications.     

超细粉流化机理和团聚现象的探讨

在直径60mm的流化床中，以SiO2和TiO2超细粉为原料，考察了粉体物性、粉体填充状态和空气湿度等因素对超细粉流化行为和聚团性质的影响，结果表明：超细粉的流态化经历了活塞流、过渡区和聚团流化3个阶段；聚团的流化行为与大颗粒相似；初始填充状态对超细粉流化行为和聚团大小有重要影响，松填充有利于减小聚团尺寸和减少颗粒夹带，提高流化质量；并结合粉体层受力分析，对超细粉的流化机理和团聚现象进行了探讨。     

Cohesive strength of alumina powder compacts

In order to characterize the nature of the interparticle forces that causes particle agglomeration in submicron size alumina particles, eight commercial alumina powders were investigated. Since the strength of the agglomerates depends upon the interparticle forces and the packing density of the particles the Hartley model which relates the tensile strength, packing density of a powder compact, to the interparticle force has been applied. The present experimental results suggest that in the absence of any electrostatic forces (either force of attraction or repulsion between particles) van der Waals force is responsible for the agglomeration of alumina particles.     

Sinterability of Agglomerated Powders

A concept is presented which relates the sinterability of a powder compact to its particle arrangement as defined by the distribution of pore coordination numbers, i.e., the number of touching particles surrounding and defining each void space. Previous thermodynamic arguments suggest that pores will disappear only when their coordination numbers are less than a critical value. The coordination-number distribution of an agglomerated powder is discussed with respect to the size of the multiple-particle packing unit, consolidation forces, and phenomena occurring during sintering. One pertinent conclusion is that the multiple-particle packing units densify and support grain growth as sintering initiates. Grain growth and rearrangement processes decrease the coordination number of remaining pores to allow them to disappear during latter states of sintering. Porosimetry and direct observations of powder compacts of <1 μm Al₂O₃ heat-treated between 600° and 1600°C support this concept.     

Determination of the characteristics of agglomerates in aqueous suspensions using nonlinear optimization

Measurement of characteristics of particles in suspensions without dilution has a practical interest in formulation, mineral processing, material sciences and environmental technologies.These characteristics are the size, shape, and surface properties of the primary particles, and also the size, structure and the number of primary particles in the agglomerates.In this work, the multiple light-scattering model through the optical analyzer, Turbiscan MA 2000 is used to determine the mean settling velocities of monodisperse glass beads and two polydisperse samples of powders, kaolin D and alumina, differing by their particle size distribution, their shape and their surface properties.Beyond the experimental validation of theoretical and empirical predictions, the nonlinear adjustment of experimental settling data gives the number of primary particles per agglomerate and the agglomerate size. These two characteristics lead to the determination of the fractal dimension of the agglomerates. The latter was found in the range of 2.5-2.7 for all suspensions examined. The calculation of permeability and spherical factor reveals the nonspherical impermeable agglomerates.     

A Die Pressing Test for the Estimation of Agglomerate Strength

A die pressing test was developed for quick and inexpensive estimation of the agglomerate strength of ceramic powders. The critical nominal pressure ( p _c) at which contact areas between agglomerates start to increase rapidly was found from the relationship between change in sample height and applied pressure in uniaxial single-ended die pressing. A quantitative microscopic method was used for measuring the area fraction (Ψ) of agglomerates which transmits the force through the assembly. A die pressing agglomerate strength, σ_d, is defined as σ_d= 0.7 p _c/Ψ. This strength was compared with the agglomerate tensile strength obtained from single agglomerate diametral compression tests and found to be 50% higher than the latter because of multipoint loading. A suggested guideline is that the mean agglomerate tensile strength is approximately 52% of p _c determined in a die pressing test for spherical agglomerates. In addition to agglomerate tensile strength, the mean agglomerate size, the interior macropore structure of agglomerates, as well as the packing efficiencies between and inside agglomerates can be estimated by the procedure.     

The role of the van der Waals force in the agglomeration of powders containing submicron particles

A model has been developed which relates the tensile strength and packing density of powder compacts containing submicron particles to the interparticle force. Hamaker's expression for the van der Waals force and the law of corresponding states are used to develop an expression for the reduced inter-particle force R which is used to evaluate an energy parameter ?_o for a given powder on an absolute scale. The tensile strength of several carbon black and titanium dioxide powders was measured using a split-cell apparatus. Analysis of plots of R against separation distance for the carbon blacks gives an average value of ?_o = 2.1 × 10^?19 J which compares favorably with the literature value of 1.09 × 10^?19 J for the Hamaker constant. This is taken as confirmation that the van der Waals force is dominant in agglomerates of submicron carbon black particles. Similar plots for titanium dioxide powders give values of ?_o that are much higher than that attributed to the van der Waals force alone; the extra force may be due to water on the particle surfaces in the form of adsorbed layers or liquid bridges.     

Agglomeration during the Drying of Fine Silica Powders, Part II: The Role of Particle Solubility

The effect of particle solubility and the dissolution rate on agglomeration was studied by drying silica and titania particles from aqueous slurries with pH values in the range of 2–12. The agglomerate strength and strength distribution were measured by a calibrated ultrasonic force, and the strength increased as the solubility and dissolution rate increased. Two silica powders of different particle size (60 nm and 500 nm) were studied, and smaller-sized particles formed stronger agglomerates. The drying rate of the powders was varied by using spray drying and tray drying, and slower drying was shown to lead to higher agglomerate strength. The agglomerate strength of titania powder (insoluble in water) was independent of pH, whereas the agglomerate strength of silica was dependent on pH. It was concluded that the solubility and dissolution rate are important parameters that govern the strength of agglomerates.     

Sintering of Mullite-Containing Materials: II, Effect of Agglomeration

The sintering behavior of mullite powder compacts which contained soft and hard agglomerates was studied, The maximum density achieved depended on the size and packing of the agglomerates. Although the initial % total pore volume was kept constant, the presence of larger pores in the green compact, due to larger agglomerates, resulted in lower final densities after sintering. Densification rates were enhanced by the breakdown of agglomerates by grinding. The particle and agglomerate packing arrangements caused densification substages to occur. A schematic model is presented which agrees well with the observed experimental behavior.     

Compaction behaviour of agglomerated alumina powders

The effects of agglomerate properties, such as the binder type, binder content, moisture level, and agglomerate size, on a model compaction process was investigated by using green density-pressure interrelationships for a range of agglomerated alumina powders. The model compaction process involved single ended nominal uniaxial stress transmission in a cylindrical die. The influences of the sample aspect ratio, die wall lubrication, and compaction rate were also investigated. Two types of water soluble polymeric agents, a poly(vinyl alcohol) (PVA) and a poly(ethylene glycol) (PEG), were used. It was shown that certain agglomerate properties have a strong influence upon the compaction behaviour of these ceramic powders. The extent of the compaction is enhanced by using agglomerates with a low agglomerate yield point. In the PVA system, the agglomerate yield points decreased with increasing moisture content. The compaction behaviour of the agglomerates showed a rate dependency, that is, the compaction is retarded with increased pressing rate. The green densities of the compacts prepared in the unlubricated die were lower than those of the compacts prepared in the lubricated die due to the higher wall frictional forces operating in the unlubricated die.     

Compression behavior of ceramic powders by inductive plasma sphero process

Strength of ceramic powders can be affected by the powders’ structure. In this study, two typical structured yttria-stabilized zirconia (YSZ) powders, agglomerated and sintered, were tested in a micro compression machine. The results showed that the sintered powders exhibited much higher compressive strength than the agglomerated powders. It was also found that the strength of the agglomerated powders was independent on the particle size due to their uniform microstructure, while the sintered powders showed decreased strength with the increasing of particle size because they had lower level of sintering. Finite element analysis was performed which indicated that thinner shells trended to sustain higher tensile stress/strain than the thicker shells, so the larger particles with thinner shells were easier to break.     

Effect of different binders on reconstitution behaviors and physical,structural, and morphological properties of fluidized bed agglomerated yoghurt powder

Abstract

Fluidized bed agglomeration is a widely used technique in the production of instant food powders. In this process, primary particles are agglomerated to give a granule-shaped product with improved instant characteristics compared to the original primary particles. Instant products are easy to use by the consumers and hence are favored over the unagglomerated particles. Depending on the type of binder used and process conditions, agglomerates exhibit different shapes, structures and instant properties. In this study, spray-dried yoghurt powders were agglomerated in a top sprayed fluid bed granulator. The influence of different binders (distilled water, ionic, and sugar-based solutions) on agglomeration growth mechanism, particle morphology, and chemical bonds were investigated. The results showed that sugar-based binders caused large particle size and porosity compared to water binders (p?<?0.05). Hydrophilic sugars also improved wettability. b* value of powders enhanced with sugar binders. Protein and stretching vibration of N–H groups were affected by acidity of water and resulted in an improvement of powder solubility. Circularity of particles decreased with the usage of hydrophilic sugar solutions while elongation increased.     

Influence of solid loading and particle size distribution on the porosity development of green alumina ceramic mouldings

Alumina ceramic mouldings with different solid contents ranging from 55 to 70 vol% and different ratios of coarse/fine powders, i.e. 0.4 μm (fine) and 3 μm (coarse), respectively, were prepared by compression moulding at 75 °C under a compressive stress of 10 MPa. The porous parameters, such as porosity, pore size and pore size distribution, of the green compacts were evaluated after removal of organic vehicles. Experimental evidence showed that the green density, as well as the sintered density, of the moulded alumina increased linearly with increased solid loading to an optimum of 65 vol% and decreased roughly linearly with increased coarse/fine ratio. Further increase in solid loading reduced particle packing efficiency, resulting in lower green and fired densities. No considerable improvement in green and sintered density of the moulded alumina was achieved by adjusting the coarse/fine ratio, which is due to the fact that coarse particles suppress the driving force of densification. The green compacts generally showed a bimodal pore size distribution character which may be the most important factor in dominating the densification of the powder compacts. The peak frequency at larger pore region is approximately 20–35 μm in diameter and at the smaller pore region is ˜50–95 nm in diameter. The larger pores are believed to be due to the presence of internal voids originating from entrapped gas and are probably caused by the removal of organic vehicles.     

Studies on the synthesis of nanocrystalline yttria powder by oxalate deagglomeration and its sintering behaviour

Nanocrystalline yttria powders were synthesized from deagglomerated yttrium oxalate. The precipitation of this oxalate was carried in two different modes viz., addition of aqueous oxalic acid into yttrium nitrate solution (forward strike) and vice versa (reverse strike) followed by ultrasonication in acetone and water. Nanocrystalline yttria was obtained by calcining the oxalate in air at 1073 K. The bulk densities, specific surface area, X-ray crystallite size, size distribution of particles as well as the quantity of carbon residue in these powders were determined. The influence of the deagglomeration medium on the powder properties was analyzed. Scanning electron microscopy (SEM) showed that these powders comprised irregular agglomerates while the high resolution transmission electron microscopy (HRTEM) revealed that the constituent units of these agglomerates were randomly oriented cuboidal nanocrystallites (20–40 nm). These powders were compacted at 120 MPa without any lubricant or binder and their sinterability was studied. Pellets with sintered density as high as 97.5% T.D. (theoretical density) could be obtained at a relatively low sintering temperature of 1873 K. Synthesis of nanocrystalline yttria powders by oxalate deagglomeration route as well as the systematic studies of their properties and sinterabilities are being reported for the first time. It was further demonstrated in this study that higher sintered densities could be obtained with less number of process steps and at a much lower compaction pressure. Samples prepared by reverse strike yielded a powder with characteristics most suitable for fabricating high density yttria bodies. 1673 K would be the optimum temperature for sintering the compacts made out of this powder.