Determination of Particle Size Distribution by Laser Diffraction of Doped‐CeO2 Powder Suspensions: Effect of Suspension Stability and Sonication

The particle size distributions (PSDs) of metal oxide powders are often determined by analyzing suspensions of powders using laser diffraction (e.g. Malvern MasterSizer 2000). Particle agglomeration can effectively bias the resulting distribution towards “unrealistic” particle sizes. Solutions to avoid this problem must be found if a particle distribution based on the elemental or primary particle sizes is desired. In this work, the particle size distribution of doped‐CeO₂ powders was studied. These powders show a crystalline single phase structure of controlled stoichiometry as determined by X‐ray diffraction and ICP analysis. The apparent size distribution was found to be a strong function of suspension stability. Dispersant agents (PBTCA and phosphonoacetic acid) and suspension pH affected stability as characterized by zeta potential measurements. Sonication of the suspensions further enhanced particle de‐agglomeration. Finally, only the combined effect of a dispersant agent, pH adjustment of the suspension and sonication provided a primary particle size distribution. The results presented in this work can be used in the analysis of similar ceramic powders in which strong particle agglomeration is present.     

TiO2 .nH2O凝胶预处理对水热合成SrTiO3粉的影响

以TiCl4为钛源,首先制备TiO2*nH2O凝胶,然后在80℃的水热条件下制备了SrTiO3粉.利用X射线衍射(XRD)、透射电子显微镜(TEM)和红外光谱(FTIR)研究了TiO2*nH2O凝胶水洗方式、阴离子(Cl-和NO3-)以及TiO2*nH2O热处理对SrTiO3粉性能的影响.结果表明,水洗和热处理都能使TiO2*nH2O凝胶产生晶化;TiO2*nH2O的晶化程度对产物SrTiO3颗粒的粒度和粒度分布有很大影响,以非晶质TiO2*nH2O为钛源制备的SrTiO3颗粒粒度大且粒度分布宽.以结晶TiO2*nH2O为钛源制备的SrTiO3颗粒粒度小且粒度分布窄,而且可以得到纳米颗粒.水热反应液相中存在Cl-或NO3-能使产物SrTiO3颗粒粒度稍有增大.综合以上结果,TiO2*nH2O凝胶水洗对产物颗粒的影响主要是由于使凝胶产生了晶化,而由阴离子脱除产生的影响很小.因此,在不考虑阴离子对其它工程化影响(如设备腐蚀等)的前提下,可采用热处理代替水洗.     

Characterization of Powder Beds by Thermal Conductivity: Effect of Gas Pressure on the Thermal Resistance of Particle Contact Points

The thermal conductivity of ceramic powder packed beds was measured at temperatures below 100 °C for various powder sizes and compositions and under different gas atmospheres. Measurements at low pressures (down to 10 Pa) combined with a theoretical model allowed the elucidation of geometrical and thermal resistance parameters for the contact points between granules. The gap thickness and contact point size were found to be well correlated with the mean particle size. The thermal conductivities of all powders at low pressure were found to differ at most by a factor of two, whereas the solid‐phase conductivities of the powder materials differed by more than one order of magnitude. A theoretical model accounting for the size‐dependence of contact point conductivity is incorporated to rationalize this trend.     

Particle size,cohesiveness and charging effects on electrostatic and nonelectrostatic powder coating

Ten powders, differing in protein, carbohydrate and salt contents and ranging from 19 to 165 μm were coated by nonelectrostatic and electrostatic coating. Nonelectrostatic transfer efficiency (TE) increased to a maximum before leveling off with increasing particle size. Electrostatic TE either decreased or increased then decreased with increasing particle size. Powders became more free flowing as particle size increased. Since TE increases as powders become more free flowing, TE increased with particle size for both nonelectrostatic and electrostatic coating. For electrostatic coating, the effect of charge decreases with increasing particle size. Thus, the conflicting effects of ability to pick up charge and flowability caused an increase then decrease in the TE for powders coated electrostatically, and can also explain the exceptions. The average improvement in TE by electrostatic coating was 20%, with the improvement increasing as particle size decreased.     

Synthesis and characterization of micrometric ceramic powders for electro-rheological fluids

Micrometric lamellar ceramic powders of the displacive ferroelectric oxide Bi₄Ti₃O₁₂ were synthesized by co-precipitation of bismuth nitrate and ammonia titanyl solutions followed by a heat treatment. It was found that a complete thermal decomposition is reached at 1000 °C. Structural and thermal evolution of these ceramic powders were studied by X-ray diffraction, thermogravimetry and differential thermal analysis. The homogeneity in size and morphology of these ferroelectric particles are appropriate to prepare electro-rheological fluids. One of these fluids was prepared by dispersing the powders in silicone oil; the complex cluster structure formed by the particles, under an applied AC electric field, was observed.     

Effect of Al particle size on the reaction kinetics and densification of TiAl intermetallics

The present investigation deals with response of the particle size of aluminum on the reactive sintering of Ti–Al intermetallics and subsequently on their reaction kinetics and densification behavior. Aluminum powders of initial average particle size of 44 μm were milled for various durations in a planetary ball mill to produce average particles sizes of 100, 28 and 7 μm. These aluminum powders of various particle sizes i.e. 100, 28 and 7 μm were mixed with titanium powder of average particles size of 44 μm in the ratio of 1:1 corresponding to the Ti–Al intermetallic composition. The reactive sintering temperatures of the mixtures were determined by DTA and the effect of change in particle Al particle size has been determined for the activation energy ofthe self-propagating reaction. The effect of Al particle size on the sintering was determined by studying density and microstructure.     

PH值对YBCO纳米粉末制备工艺及性能的影响

采用溶胶-凝胶法制备YBa2Cu3O7纳米粉末,用XRD和SEM对粉体进行表征,结果发现PH值对颗粒大小、粉体纯度影响很大。当溶液PH值为6.5,烧结温度为800℃,烧结时间为5h时,制得的YBCO纯度高、颗粒小。原因在于当PH值为6.5时,柠檬酸能与溶液中的所有金属离子形成稳定的柠檬酸盐,且各金属离子形成的化合物溶解度高,不易随溶剂的蒸发而析出,避免了样品中成份的偏离。最终制得的粉体纯度高、颗粒分散性好,颗粒尺寸小于30nm。     

Effect of the temperature on the synthesis of (K,Na)NbO<Subscript>3</Subscript>-modified nanoparticles by a solid state reaction route

(Na_0.5K_0.5)NbO₃ (KNN)-modified nano-particulated powders, based on variations of sodium–potassium niobate, were synthesised by solid state reaction from carbonate starting materials. The nanoparticles were attained by an optimization of the raw materials particle size and particle refinement of the carbonates during their decomposition. Particle sizes between 50 and 200 nm have been obtained as a function of calcination (decomposition) temperature. The obtained powders showed a co-existence between a tetragonal phase and an orthorhombic phase. The optimization of the raw materials particle size and the particle refinement of the carbonates during their decomposition play a key role in the formation of the KNN-modified nano particles. The developed method is well suited for the production of KNN-modified nano powders at low cost for mass production.     

Synthesis of Yb-doped Ba(Ce,Zr)O3 ceramic powders by sol–gel method

This study was aimed to synthesize a ceramic electrolyte of BaCe_0.76Zr_0.19Yb_0.05O_2.975 using acetate and chloride precursors. The transparent sol was stable for more than 10 months but only 3 months for the gel. The dried gel was characterized using thermogravimetric analysis/differential scanning calorimetry, particle analyzer, X-ray diffraction (XRD), and scanning electron microscopy/dispersive X-ray (SEM/EDX) analysis. Thermal behavior analysis showed three steps of weight losses. All the processes were exothermic reactions as shown by derivative thermogravimetric analysis signal. Two groups of particle size were observed in the particle size distribution for the sample in the form of sol and powders ranging from 100 to 1,100 nm. A high purity single-phase sample with orthorhombic structure was identified by XRD. The theoretical density estimated from the unit cell parameters was 6.40 g cm⁻³. SEM of the powdered sample showed homogeneous distribution of particles and the grains size were in the range of 0.7–1.3 μm. EDX data revealed that the residue of small amount of chloride (≈0.25%) was still present in the final product of the compound.     

Influence of particle size on the electromagnetic and microwave absorption properties of FeSi/paraffin composites

The electromagnetic and microwave absorption properties of the composites employing FeSi alloy powders with different particle sizes as absorbent and paraffin as matrix were investigated. The results showed that the particle size had significant influence on the electromagnetic and microwave absorption properties of the composites in the 2-7 GHz frequency range. By decreasing the particle size of FeSi alloy powders, both the complex permittivity and permeability of the composites increased to a certain extent. In addition, the microwave absorption properties were improved, and the frequency of absorption peak shifted towards lower frequency range. In other words, the micron-grade FeSi alloy powders with smaller particle size were more suitable to be used as absorbent in measured frequency region.     

Effect of titanium alkoxides on the properties of barium-strontium-calcium titanate powders obtained by a sol-gel method

This is a study of the effect of titanium alkoxides on the properties of barium-strontium-calcium titanate powders produced by a sol-gel method. Sols were synthesized using titanium tetraethoxide, titanium tetraisoproxide, and titanium tetrabutoxide. Xerogels of titanium oxide doped with barium, strontium, and calcium in a proportion of 60:30:10 were obtained by drying the sols at a temperature of 70°C. Thermogravimetric analysis, along with differential scanning calorimetry, IR spectroscopy, and x-ray phase analysis, were used to study the changes in the xerogel and the crystallite dimensions in the powder induced by thermal processing. The morphology and specific surface area of the powders were determined by scanning electron microscopy and low-temperature physical adsorption-desorption of nitrogen. These studies lead us to recommend sols based on Ti(OC₃H₇)₄ with a crystallite particle size in the powder of ∼63 to 40 nm for creating homogeneous ceramic materials from barium-strontium-calcium titanate.     

Raman scattering studies on nanocrystalline BaTiO3 Part II—consolidated polycrystalline ceramics

BaTiO₃ dense ceramics with different grain sizes from 5.6 µm down to 35 nm were thoroughly studied by Raman spectroscopy. The temperature characteristics of optical phonons were compared with those obtained for powders. The micrograined ceramic revealed the well‐known spectrum profiles and transitions, typical for bulk BaTiO₃. On the other hand, the Raman spectra obtained for a nanograined ceramic with an average grain size of 35 nm revealed a tetragonally distorted pure BaTiO₃ phase showing a diffused phase transition behaviour with respect to temperature. Abnormality of phonon damping characteristics for the nanograined ceramic was demonstrated through comparison with powders with various crystallite sizes and the micrograined ceramic. The Curie temperature of the nanograined ceramic was estimated to be 105 °C from the temperature characteristic of a sharp peak at 307 cm⁻¹, which is one of the most specific tetragonal features for bulk BaTiO₃. In the present study, local stabilization of the tetragonal phase in ultra‐fine grains was experimentally demonstrated from comparison between the Raman spectroscopic results for powders and ceramics prepared through microemulsion‐mediated synthesis. Rather long phonon mean free paths can exist even in such ultra‐fine grains, but the phonon characteristics originating from various grains are diffused mainly because of the effect of internal stress. Copyright © 2007 John Wiley & Sons, Ltd.     

Fully dense nano-composite energetic powders prepared by arrested reactive milling

Highly reactive metastable nano-scale composites of aluminum and metal oxides have been produced by arrested reactive milling (ARM), and their combustion performance has been preliminarily evaluated. Aluminum powder has been milled with powders of MoO₃ and Fe₂O₃. The prepared composites are powders with particle sizes in the 1–100 μm range. Each individual particle comprises a fully dense, nano-scale mixture of the chemical reagents. These composites belong to a novel class of energetic materials characterized by an intimate mixing of reactive components on nanometer to atomic scale. Reactive components can be metal/metal oxide pairs or combinations of other materials capable of highly exothermic reactions such as B–Ti or B–Zr. High-energy milling of these components leads to mechanical initiation of the reaction. Highly reactive composites are obtained by arresting this process immediately before the initiation would occur if milling were allowed to proceed. An experimental parametric study of reactive milling in the Al–MoO₃ and Al–Fe₂O₃ systems was conducted to establish at which milling times the reaction is spontaneously initiated under various conditions. Samples of nano-composite powders were synthesized by arresting the milling process, and characterized using electron microscopy, X-ray diffraction, and particle size analysis. Ignition temperatures of the materials were determined at heating rates in the range of 300–3000 K/s using an electrically heated filament. Activation energies of ignition were determined to be 152 ± 19 and 170 ± 25 kJ/mol for the Al-MoO₃ and Al-Fe₂O₃ nano-composites, respectively. The activation energy obtained for the Al-Fe₂O₃ nano-composite is consistent with a previously reported value for the Al-Fe₂O₃ thermite reaction. Combustion tests were conducted in a constant volume pressure vessel in argon for both Al-Fe₂O₃ and Al-MoO₃ and compared to respective blends of initial powders and to partially milled powders. The nano-composites showed higher respective reaction rates. Linear burning rates measured in an open channel of 2.5 × 2.5 mm cross-section were also higher for the ARM-prepared powders compared to partially milled materials.     

纳米钙钛矿LaxSr1-xCoO3复合氧化物的制备和表征

采用微乳液法和自燃烧法制备Sr掺杂的LaCoO3钙钛矿氧化物的陶瓷粉末,并对该钙钛矿氧化物结构进行了XRD,IR,及ICP-AES表征,从而将微乳液法与自燃烧法进行了对比,并研究了起始原料配比对产物粒径的影响。实验结果表明：在反应温度、反应时间、煅烧条件等一定的条件下采用适宜的原料配比,可制得平均粒径为12—20nm且粒径分布较均匀的LaxSr1-xCoO3。     

Microstructure and mechanical characteristics of nanodiamond-SiC compacts

Superhard nanodiamond-SiC ceramics are prepared by infiltrating liquid Si into porous nanodiamond compacts under pressure. Synthesized samples are 2.2 mm thick and 3–4 mm in diameter. The effect of particle size of dynamically synthesized nanodiamond powders on silicon infiltration and SiC phase formation is studied. It is established that silicon does not penetrate into the pores of nanodiamond powders if the original particle size is smaller than 0.5–1.0 μm. The critical pore size for infiltration is 100–200 nm. A study of the microstructure of the samples showed the presence of the nanometer-and submicron-scale SiC phase. The ultrasound velocities are measured in the prepared compacts, and the elastic moduli are calculated. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 4, 2004, pp. 734–736. Original Russian Text Copyright ? 2004 by Ekimov, Gromnitskaya, Mazalov, Pal’, Pichugin, Gierlotka, Palosz, Kozubowski.     

Functional Nanostructured Silica Powders Derived from Colloidal Suspensions by Sol Spraying

The preparation of silica powders derived from sols with a variety of primary particle sizes from 4 to 100 nm and the influences of preparation conditions such as drying temperature and initial concentration of precursor on the final powders morphology were investigated for the first time. The prepared silica powders have spherical morphology, and the final diameter (between 0.1 and 2 um) was controlled by changing the concentration of starting colloidal suspension. The preparation of silica based luminescent powders was investigated by adding europium ion as the dopant. The Eu-doped silica powders showed a visible-luminescence with wavelength of 613 nm, a red emission, under excitation of 394 nm light source. The effects of Eu-doping concentration, operation temperature and primary particle size of silica sols on the luminescence were also investigated.     

Model for milling of powders

A model is proposed for mechanical milling of powders that relates the applied energy to average particle size D in the powders. It is shown that the milling energy is consumed for the rupture of interatomic bonds in crystalline particles and for the creation of an additional surface during powder fragmentation. The appearance of microstrains ɛ retards powder fragmentation. Average particle size D after milling decreases with increasing milling time t and decreasing particle size in the initial powder or its mass M. The calculated results are compared to the experimental data obtained on a tungsten carbide WC powder.     

Properties of PZT/PZT ceramic piezocomposites

A method for obtaining PZT/PZT ceramic piezocomposites is developed. Samples of piezocomposites with a volume fraction of components from 0 to 100% are prepared and investigated. Various types of PZT type piezoceramics powders and milled PZT piezoceramics particles, as well as presintered piezoceramic granules, are used as matrix and filling components, respectively. Sets of complex elastic, dielectric, and piezoelectric parameters of piezocomposites are measured. The microstructure of the obtained ceramic composites is investigated.     

Effect of Particle Size on the Photocatalytic Activity of Nanoparticulate Zinc Oxide

In this study, a three-stage process consisting of mechanical milling, heat treatment, and washing has been used to manufacture nanoparticulate ZnO powders with a controlled particle size and minimal agglomeration. By varying the temperature of the post-milling heat treatment, it was possible to control the average particle size over the range of 28–57 nm. The photocatalytic activity of these powders was characterized by measuring the hydroxyl radical concentration as a function of irradiation time using the spin-trapping technique with electron paramagnetic resonance spectroscopy. It was found that there exists an optimum particle size of approximately 33 nm for which the photocatalytic activity is maximized. The existence of this optimal particle size is attributable to an increase in the charge carrier recombination rate, which counteracts the increased activity arising from the higher specific surface area for a sufficiently small particle size.     

Sintering characteristics of LiZn ferrites fabricated by a sol-gel process

Ultra-fine powders of LiZn ferrites with composition of Li_0.4Zn_0.2Fe_2.4O₄ were synthesized by a sol-gel process. X-ray diffraction pattern reflected that the synthesized powders were single spinel structure, with crystallite size of about 35 nm. Then the powders were granulated, pressed and sintered at different temperatures. The sintered samples were investigated by means of characterizing microstructures and magnetic properties by scanning electron microscope and B-H analyzer, respectively. The results indicate that when compared with a traditional ceramic process, the sol-gel process could slightly bring down the sintering temperature of LiZn ferrite whereas the microstructures are not homogeneous in the sintered samples. The sintering mechanisms of LiZn ferrites sintered at 1360 °C were studied, which reveal that during sintering, solid mass transfer is dominant in the LiZn ferrites fabricated by a traditional ceramic process while in the gel-derived ferrites, gas mass transfer is dominant.