Powder Processing Method Research Articles

Effects of powder processing on colloidal and microstructural characteristics of β-SiC powders

Three different powder processing methods for introducing sintering aids (yttria and alumina) into a commercial silicon carbide powder are studied: mechanical mixing, heterocoagulation and coprecipitation. The surface characteristics of the raw powders and powder mixtures are analysed by acoustophoretic measurements and compared. The effectiveness of the coprecipitation process for additivation is evaluated by the comparison of ESA measurements and by TEM observations. The powder mixtures are pressureless sintered in argon atmosphere and the microstructure of the sintered materials is analysed. The coprecipitation process allows a higher final density to be obtained in a shorter sintering time, with a finer and more homogeneous microstructure.

Powder formation by atmospheric spray-freeze-drying

Freeze-drying (lyophilization) has been one of the most useful methods for producing high quality powder from thermosensitive drugs. However, lyophilization has high capital and process costs. A new method for manufacturing powder has been developed in this work, with the aim of reducing drying time and increasing collection efficiency. Compared with other published atmospheric spray-freeze-drying technologies, the new technology combines the spray-freezing step and fluidization conveying of frozen powder using co-current flow to convey the frozen powder to the exit filter. This overcomes the difficulty of fluidizing and elutriating cohesive frozen powder from a substrate. In this process, a powder cake with uniform thickness builds up on the exit filter. Freeze-drying was then performed by continuously flowing a dry gas stream through the packed powder bed at a temperature below the eutectic point of water. The pressure in the chamber was monitored and the residual moisture of the powder was also measured vs time in the drying process. After the completion of drying, a loose powder cake was generated and a free-flowing powder was readily collected. Compared to lyophilization, the experiment indicates the content of α-helix of heat-sensitive protein (samples of bovine serum albumin) was essentially unchanged after drying (96.7% in 2.5 mM NaHCO 3 solution and 102.9% in 2.5 mM NaHCO 3 with trehalose protection (BSA:trehalose = 2:1)) and the powders show good aerosol dispersion for inhalation drug delivery (fine particle fraction FPF < 5.6 μm was 62.1% ± 2.9% for a mannitol/ciprofloxacin powder). In addition, viability of bacteria ( Bordetella pertussis avirulent strain) subjected to this powder processing method was more than 90% after drying, showing that this process has excellent potential for production of powdered biologics. The present process also offers greatly reduced process times i.e. drying was completed in 1–2 h vs 1–2 d with conventional lyophilization in a vacuum.

Copper-doped BaZrO 3 crucibles for YBCO single crystal growth

The corrosion resistance of BaZrO 3 to molten barium cuprates is controlled by secondary phases, and requires high chemical and phase purity, as well as high density for sustained flux containment. The high sintering temperatures required for solid-state derived powders is a significant obstacle inhibiting more widespread use of high density BaZrO 3. We have investigated the use of Cu 2+ doping to enable practical sintering temperatures whilst still producing corrosion resistant ceramics. Low levels of copper addition (0.2 wt.% CuO equiv.) produced densities of >97% theoretical using sintering temperatures as low as 1400 °C, as long as the dopant was added after all powder calcination steps, i.e. immediately before forming/sintering. Corrosion resistance at 0.2 wt.% CuO equiv. was equal to un-doped materials fabricated using the same powder processing method.

Dielectric Properties of CaCu3Ti4O12-P(VDF-TrFE) Composite

ABSTRACTBy using conventional solution casting method, a flexible ceramic [CaCu3Ti4O12 (CCTO)]-Polymer [P(VDF-TrFE)] composite has been fabricated. The CCTO ceramic powders with a relative uniform size were prepared by traditional powder processing method. The dielectric properties of these films with different CCTO fractions were determined. The process was optimized to achieve high dielectric constant. A dielectric constant about 510 at room temperature and 1240 at 95 °C at 1 kHz for 6 layer hot compression was obtained.

Preparing 3D Functional Gradients for SLS

The creation of three dimensional functional graded materials by powder processing methods requires the capability to arrange a sequence of two dimensional images that can be presented to a solid freeforming device. Planar stacking methods can only achieve a one dimensional gradient. We describe the acoustic control of powder metering and dispensing valves in which the flow rate and switching of powders from a capillary can be controlled using the frequency and amplitude of acoustic vibration from a computer sound card. An orchestra of such valves can be used to pattern layers of powder on the building platform which forms part of a 3-axis table in the Selective Laser Sintering (SLS) process. Patterns and gradients were prepared using copper and steel powders.

Fracture Behavior of Alumina-Titania-Monazite Composites

Fracture behavior was investigated in the Al₂O₃-TiO₂(3 wt%)-LaPO₄(25 wt%) composite ceramics. To improve the fracture toughness of alumina ceramics, TiO₂ and LaPO₄ as a second phase were introduced. The samples were made by conventional powder processing method. Green compacts were sintered at 1600℃ for 2 h in air. Fracture toughness was tested using Indentation Strength Bending(ISB) method. From the bending test, enhanced fracture toughness was found in the composite, compared to the pure and TiO₂-doped alumina. The main factor of the enhancement of fracture toughness seems to be attributed to the weak interphase role of the LaPO₄ as a particulate type.

Dry Powder Processing of Fibrous Fumed Silica Compacts for Thermal Insulation

This study proposes a new dry powder processing method to produce fiber reinforced fumed silica compacts for highly efficient thermal insulation. First, mechanical forces such as shear stress and compression were applied into a fumed silica powder (BET SSA 300 m2/g) and glass fibers (3 mm length, 11 μm diameter) using an attrition type milling apparatus without any media balls. In the resultant powder mixture, fumed silica porously coated glass fiber composites were successfully prepared. SiC particles were also integrated as opacifiers. Then, the fibrous fumed silica porous compact was formed by dry pressing of the powder mixture and exhibited fracture strength of 1.58 MPa at room temperature and thermal conductivity of 0.0282 W/mK at 400°C, when its porosity was 80.1%.

나노 구리 분말의 냉간정수압 공정에 대한 치밀화 거동 해석

In the study, a hybrid constitutive model for densification of metallic powders was applied to cold isostatic pressing. The model is based on a pressure-dependent plasticity model for porous materials combined with a dislocation density-based viscoplastic constitutive model considering microstructural features such as grain size and inter-particle spacing. Comparison of experiment and calculated results of microscale and nanoscale Cu powders was made. This theoretical approach is useful for powder densification analysis of various powder sizes, deformation routes and powder processing methods.

Evolution of the microstructure of disperse ZnO powders obtained by the freeze-drying method

Freeze-drying, as a cryochemical powder processing method is applied in the synthesis of ZnO submicrometer to nanosized powders. The process involves rapid freezing of the sprayed precursor solution, drying under vacuum by sublimation of the solvent and salt decomposition to oxide by thermal treatment. Calcination of dehydrated Zn(NO 3 ) 2 was performed through destruction of the primary crystal structure, i.e. through accumulation of different defect complexes and based on this, the formation of a new state of the ZnO crystal lattice. An analysis of the microstructure evolution of zinc oxide particles in the temperature range from 548 to 898 K is described. The research was performed using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), X-ray powder diffraction (XRPD), electronic paramagnetic resonance (EPR) and infrared spectroscopy (IR). It was shown that the least (“primary”) coherent scattering region (580 Å) and the most defective state were ZnO obtained at the lowest calcination temperature ( T c =548 K). Increase of the calcination temperature ( T c >548 K) favors uniting of eight closest “primary” domains into “secondary” ones. Unification is accompanied by a pronounced increase in microstrain in “secondary” domains, whose size practically does not depend on the calcination temperature in the region 573–898 K.

Influence of powder precursors on reaction sintering of Al 2TiO 5

Al 2TiO 5 has been obtained by reaction sintering of precursor powders synthesized by three powder processing methods. A field activated sintering technique has been used for the process, with 100 °C/min heating rate and short holding plateaus (minutes) at the maximum temperature. SEM, XRD, and density measurements were performed after sintering.

Effect of processing on the properties of tin oxide-based thick-film gas sensors

Semiconducting tin oxide-based powders were synthesized via three different chemical precipitation methods and also obtained from a commercial source. The precursor powders were characterized using X-ray diffraction (XRD), particle size and surface area measurement techniques. Sensor elements were produced from calcined powders using thick-film screen printing technology. The influence of various powder processing methods, sintering conditions, microstructure and moisture on the carbon monoxide gas detection capabilities, and the response and recovery times of the sensors produced from the above four powders are discussed.

Theoretical and Experimental Study of Barium Zinc-Cadmium Tantalate-based Microwave Dielectrics

ABSTRACTSingle-phase Ba(Cd1/3Ta2/3)O3 ceramics have been produced using conventional powder processing methods. In our initial investigations, 2wt% ZnO powder was added to act as a sintering aid since a high-density ceramic was not formed from solid-state diffusion alone. The resulting Ba(Cd0.327Zn0.006Ta2/3)O3 material sintered at 1550° C exhibits a dielectric constant of ∼33 and loss tangent of &lt;5×10−5 at 2 GHz. In our more recent work, we have used boron as a sintering aid to facilitate sintering at temperatures as low as 1300° C, enhance the structural quality and improve the microwave properties of Ba(Cd1/3Ta2/3)O3 dielectrics. TEM results indicate that the liquid sintering mechanism is an important factor for boron concentrations exceeding 0.5wt%, while a point defect mechanism plays the dominant role at lower boron concentrations. The presence of superstructure peaks and splitting of the (220) and (214) peaks in X-ray diffraction spectra are direct evidence for the distortion from cubic symmetry as a result of Cd and Ta ordering on the B-site.Ab-initio electronic structure calculations within the local density functional approximation have been used to give insight into the unusual properties of this class of materials. In both Ba(Zn1/3Ta2/3)O3 and Ba(Cd/3Ta2/3)O3, the conduction band maximum and valence band minimum are composed of mostly weakly itinerant Ta 5d-and Zn-3d/Cd-4d levels, respectively. The covalent nature of the directional d-electron bonding in these high-Z oxides plays an important role in producing a more rigid lattice with higher melting points and enhanced phonon energies, and possibly inherently lower intrinsic microwave loss than comparable ionic materials.

Structural and Electrical Investigations of Ferroelectric Lead Strontium Titanate Thin Films and Ceramics

ABSTRACTLead strontium titanate (PbxSr1-xTiO3) (x=0.3–1.0) ceramic targets were prepared by the conventional powder-processing method. Thin films of these compositions were deposited on platinized silicon substrates by pulsed laser deposition technique. X-ray diffraction studies of the ceramic targets showed that the lattice structure changes from tetragonal to cubic phase with the increase of Sr content in PbTiO3. Raman spectroscopic studies of PbxSr1-xTiO3(PST) ceramics and thin films showed that the soft mode decreases to lower frequency and finally disappear at around 60–70 at% Sr content, which confirms the tetragonal to cubic phase transition at room temperature. Dielectric constant measured for PST thin films was in the range of 900–1500 at 1 MHz, with maximum value obtained for PST30 thin film. The loss tangents at room temperature were in the range of 0.07–0.1 for PST thin films with different compositions.

Preparation and Fracture Behavior of Alumina Platelet Reinforced Alumina-Monazite Composites

Fracture behaviors were investigated in the laminated alumina composite ceramics. To improve the mechanical properties of alumina ceramics, templated grain growth (TGG) technique of α-Al 2 O 3 platelet and LaPO 4 as a second phase were introduced. Textured samples were made by the lamination of green tapes through the tape-casting method, while random samples were made through the conventional powder processing method. Green compacts were sintered at 1600°C for 2 h in air. Fracture toughness was tested using indentation strength bending (ISB) method. Indentation on the sample surface was produced by Vickers indent at various loads. From the bending test, enhanced fracture toughness was found in the textured composite, compared to the random composite.

Effect of the powder precursor characteristics in the reaction sintering of aluminum titanate

Aluminum titanate precursor powders, containing 2.5 wt.% MgO additive, were prepared by two powder processing methods, the hydrolysis of alkoxides and coprecipitation of inorganic salts. The hydrolysis-derived precursor powder showed a higher chemical homogeneity than the coprecipitated-derived precursor powder. Dilatometric analysis, accomplished with XRD and microscopy, was a very good method to study both phase transformation and sintering processes. Different phase transformation paths of TiO 2 and Al 2O 3 were observed in the two powders. The formation of aluminum titanate solid solution began at ∼1220°C in both, but in the hydrolysis-derived powder compact the aluminum titanate grains grew faster, leading to incomplete reaction and a non-uniform microstructure of large grains with small residual inclusions.

High TC superconducting powders synthesis from aerosol

Abstract Spray pyrolysis, as one of the novel powder processing methods, has been applied for the synthesis of 2223 phase of the Bi-based high T c superconducting oxide ceramic materials. The process is performed through heterogeneous chemical reactions in dispersed system (aerosol) formed from common nitrates solution ultrasonically with the resonant frequency of 1.7 MHz. Synthesis through aerosol enables generation of ultrafine multicomponent particles with improved compositional homogeneity provided by higher surface reaction and the absence of compositional segregation. Consequently, spherical, solid, slightly agglomerated submicronic particles (below 400 nm) with the uniform particle size and the particle size distribution are produced. The obtained particle morphology is discussed in terms of precursor chemistry and processing parameters.

Calcining influence on the powder properties of hydroxyapatite.

The effect of different calcination temperatures on the powder characteristics and the sintered density of synthetic hydroxyapatite (HA) powders, produced using two different processing routes, was examined. Powders were produced by either drying, milling and sieving an as-precipitated HA or by spray-drying a slurry of precipitated HA. Calcining the two powders at temperatures between 400 and 1000 degrees C did not significantly affect the powder particle size. The specific surface areas of the two powders, however, were reduced from 70-80 m2/g for a calcination temperature of 400 degrees C to approximately 5-7 m2/g for 1000 degrees C. Analysis of the surfaces of the HA powders using scanning electron microscopy (SEM) illustrated the coarsening and subsequent sintering of the sub-micron crystallites that constitute a powder particle as the calcination temperature increased, corresponding to the decrease in surface area of the powders. The sintered densities of the final ceramics were not significantly affected by calcining the powders. Microhardness measurements of ceramics prepared from powders calcined at different temperatures showed no significant variations with calcination temperature or powder processing method. The results of this study have illustrated that for applications where HA may be used in powder form, for example in plasma-spraying and for the production of HA-polymer composites, calcining the HA will significantly affect the powder properties, namely the surface area and morphology of the powders. For applications requiring HA in a dense ceramic form, for example as granules or blocks, calcining the powders does not significantly affect the properties of the final ceramic.

Thermal and mechanical properties of aluminum titanate–mullite composites

The thermal and mechanical properties of aluminum titanate–mullite composites prepared by a gel-coated powder processing method, namely a mullite precursor gel coating aluminum titanate particles (containing 2.5 wt% MgO), were investigated. A microstructure with both elongated and equiaxed mullite grains was observed in the composite samples. Both the mechanical strength and the thermal expansion coefficient increased with the mullite amount. The mechanical strength showed a strong dependence on the grain size of aluminum titanate with an exponent of −3. Inhibition of aluminum titanate grain growth due to the presence of the mullite phase enhanced the mechanical strength of the composites. The thermal shock resistance tended to decrease as the mullite content increased. The thermal stability of aluminum titanate was improved in the present composite system.

Evaluation of Rule‐of‐Mixtures Predictions of Thermal Expansion in Powder‐Processed Ni–Al2O3 Composites

Ni‐Al2O3 composites containing 5 to 80 vol% Ni were produced using powder processing methods and their thermal expansion behavior was studied using dilatometry. By varying composition and relative particle sizes, either interpenetrating‐phase or particle‐reinforced microstructures were obtained. Experimentally determined values of the mean thermal expansion coefficient were compared with predictions made using two different rule‐of‐mixtures (ROM) formulations. Experimental results for interpenetrating‐phase composites were in reasonable agreement with linear ROM estimates. Certain particle‐reinforced microstructures that contain processing‐induced damage did not correlate as well with ROM predictions. By accounting for this damage in ROM models, better agreement with experimental observations was obtained. However, linear ROM predictions were reasonable over the entire composition range.

Effect of SiO2 additive on the mechanical and dielectric properties of pzfntu ceramics

Composite of Pb(Zr0.58Fe0.20Nb0.20Ti0.02)0.994U0.006O3 (PZFNTU) ceramics with SiO2 additive has been prepared by the conventional powder processing method. Microhardness, flexural strength, and the dielectric properties of the sintered material have been studied. It is found that a small amount of SiO2 improves the flexural strength, the microhardness and the dielectric constant of the sintered PZFNTU ceramics. It is also found that the fracture mode changes from intergranular to essentially transgranular with the small amount of SiO2. For large amount of SiO2 additives, the sintered ceramics are more porous and the strength and hardness values are also low. For ceramics with small amount of SiO2 additives, the structure is purely perovskite while for large amount of SiO2 additives, pyrochlore phases are detected.