Particle Size Comparison of Hydrothermally Synthesized Cobalt and Zinc Aluminate Spinels

Nanosized CoAl₂O₄ and ZnAl₂O₄ powders with particle sizes of 67 and 6.5 nm are synthesized under hydrothermal conditions at 245°C for 20 h. The precursors are reacted at different temperatures to provide intermediate phase transformations using X-ray diffraction (XRD), infrared (IR) spectra, and thermal gravity and differential thermal analysis (TG-DTA). XRD patterns and IR spectra demonstrate that the CoAl layered double hydroxide structure (CoAl-LDHs) is more stable than ZnAl layered double hydroxide structure (ZnAl-LDHs) when they are hydrothermally treated. The different thermal stability of the CoAl- and ZnAl-LDHs results in the different aluminum source, e.g., β-Al(OH)₃ for ZnAl₂O₄ vs γ-AlO(OH) for CoAl₂O₄, when the aluminate spinels are formed. The different aluminum sources lead to the particle size difference. The phenomenon is reasonably expounded based on the nucleation theory from the microscopic scale.     

Synthesis and Investigation of Submicrometer Spherical Indium Oxide Particles

Spherical indium (In) oxide in submicrometer size is considered as a practical material in photonic bandgap applications. To obtain narrow size distribution, systematic studies of different conditions such as precursor concentration, reaction temperature, and other parameters have been carried out and are reported here. Tri- or dicarboxylic acid of citric acid (CA), malic acid, and tartaric acid were introduced into the reaction system and In-precipitates of spherical shape were produced. The particle size and internal morphology were observed by scanning electron microscopy and transmission electron microscopy. X-ray diffraction and thermal analysis were performed. The reaction kinetics analysis shows that the reaction rate depends on the reaction temperature, urea, and In concentration rather than on CA.     

Preparation of Uniform Submicrometer Particles of Calcium Titanate and Lead Niobate by Replacement Reactions

Uniform submicrometer particles of calcium titanium peroxo-hydroxide were prepared by adding calcium salts to the solution of peroxotitaniun. species chelated with nitrilotriacetic acid (NTA). Similarly, corresponding leadniobium compodnd of [Pb]/[Nb] = 0.75 was obtained by introducing lead salts into the solution of chelated niobium complex with ethylenediaminetetraacetic acid (EDTA) at pH 3. Calcium ions in solutions containing a mixture of NTA-chelated Pb and NTA-chelated Nb complexes also caused the precipitation of lead niobium oxo-hydroxide of [Pb]/[Nb] = 1.5 at pH 9. The so-obtained precursor powders were converted on calcination to the corresponding composite oxides. The mechanisms of these reactions were suggested, based on the chemical, thermal, and X-ray diffraction analyses.     

Microwave Synthesis and Characterization of Spinel-type Zinc Aluminate Nanoparticles

In the present work, ZnAl₂O₄ nanoparticles have been synthesized with the aid of Zn(OAc)₂·2H₂O and Al(NO₃)₃·9H₂O as starting reagents in the presence of microwave irradiation. Besides, the effect of preparation parameters such as microwave power and irradiation time on the morphology and particle size of products was studied by SEM images. The as-prepared ZnAl₂O₄ nanoparticles were characterized extensively by techniques like XRD, TEM, SEM, FT-IR, PL, and EDS. Photoluminescence studies of the ZnAl₂O₄ nanoparticles displayed quantum confinement behavior with band gap of 3.2 eV. The XRD studies showed that pure orthorhombic ZnAl₂O₄ nanoparticles have been produced after calcination.     

聚氨酯分散体粒径及粒径分布与粒径控制

介绍了聚氨酯分散体粒径与粒径分布概念、测定粒径的方法,分析探讨了分散体内在因素和外部因素对粒径的影响以及聚氨酯分散体的粒径控制。     

Fischer–Tropsch Synthesis: Morphology, Phase Transformation and Particle Size Growth of Nano-scale Particles

An unpromoted ultrafine iron nano-particle catalyst was used for Fischer–Tropsch synthesis (FTS) in a CSTR at 270 °C, 175 psig, H₂/CO = 0.7, and a syngas space velocity of 3.0 sl/h/g Fe. Prior to FTS, the catalyst was activated in CO for 24 h which converted the initial hematite into a mixture of 85% χ-Fe₅C₂ and 15% magnetite, as found by Mössbauer measurement. The activated catalyst results in an initial high conversion (ca. 85%) of CO and H₂; however the conversions decreased to ca. 10% over about 400 h of synthesis time and after that remained nearly constant up to 600 h. Mössbauer and EELS measurement revealed that the catalyst deactivation was accompanied by gradual in situ re-oxidation of the catalyst from initial nearly pure χ-Fe₅C₂ phase to pure magnetite after 400 h of synthesis time. Experimental data indicates that the nucleation for carbide/oxide transformation may initiates at the center of the particle by water produced during FTS. Small amount of ?′-Fe_2.2C phase was detected in some catalyst samples collected after 480 h of FTS which are believed to be generated by syngas during FTS. Particle size distribution (PSD) measurements indicate nano-scale growth of individual catalyst particle. Statistical average diameters were found to increase by a factor of 4 over 600 h of FTS. Large particles with the largest dimension larger than 150 nm were also observed. Chemical compositions of the larger particles were always found to be pure single crystal magnetite as revealed by EELS analysis. Small number of ultrafine carbide particles was identified in the catalyst samples collected during later period of FTS. The results suggest that carbide/oxide transformation and nano-scale growth of particles continues either in succession or at least simultaneously; but definitely not in the reverse order (in that case some larger carbide particles would have observed). EELS-STEM measurement reveals amorphous carbon rim of thickness 3–5 nm around some particles after activation and during FTS. Well ordered graphitic carbon layers on larger single crystal magnetite particles were found by EELS-STEM measurement. However the maximum thickness of the carbon (amorphous or graphitic) rim does not grow above 10 nm suggesting that the growths of particles are not due to carbon deposition.     

浅谈重灰粒度的控制与大颗粒的消除方法

影响重灰粒度的因素很多,在实际生产过程中化合水温度的控制,粗盐中钙、镁离子的含量等对重灰粒度都有影响,如何保证一水碱的结晶显得尤为关键。通过重灰生产过程中出现一水碱结晶的粗、细情况以及在生产过程中采取的措施进行相应的探讨,对稳定重灰生产有一定的指导意义。     

影响纳米碳酸钙制备过程的因素分析

Cubic shaped CaCO3 particles with mean size of 30-40 nm were prepared by intermittent carbonation process without any additives. It was found that the flow rate of CO2 has no distinct influence on the particle size in the range of 30-120ml·min-1 under conditions of 13℃ and stirring rate of 680r·min-1. A further increase of flow rate makes the particles larger. When CO2 flow rate is 70ml·min-1, a high agitation rate is in favor of the reduction of the crystal size in the range of 70-680 r·min-1. When flow rate is 120 ml·min-1, the particles prepared at agitation rates of 680r·min-1 and 280r·min-1 have similar sizes, while products prepared at 90r·min-1 have larger size.     

Effect of Synthesis Techniques on Crystallite Size and Morphology of Lithium Aluminate

γ-LiAlO₂ is considered as a candidate tritium breeder material for fusion reactors because of its thermophysical, chemical, and mechanical stability at high temperatures and its favorable neutron irradiation behavior. Crystallite size and shape could, however, alter these features. In this study the mean crystallite size, the crystallite size distribution, and the morphology of γ-LiAlO₂ are correlated to the synthesis procedure. The characterization techniques were powder X-ray diffraction and scanning electron microscopy.     

沉降终速与颗粒粒度测定

讨论了颗粒体在静止流体中的运动规律;分析了沉降终速在测定颗粒粒径大小及其粒度分布中的应用;介绍了粉末粒度分布测试仪的操作。     

Effect of Precursor Particle Size on the Densification and Crystallization Behavior of Mullite

Stoichiometric mullite, 3Al₂O₃·2SiO₂, has been prepared from a variety of colloidal and solution precursors. In order to change the level of mixing, the size of the silica particles has been varied while keeping the alumina (boehmite) particle size constant. The effect of varying particle size on the crystallization and densification behavior has been studied. Densification behavior was characterized by measuring the bulk density of pellets as a function of heat treatment temperature and by dilatometry. Phase development was examined by thermal analysis and X-ray diffraction. Results showed that the formation of crystalline phases including mullite inhibited densification. The formation of mullite was controlled by the initial segregation of alumina and silica in the gel. A heat treatment time-temperature profile designed to optimize the densification of the colloidal powders was investigated in order to compare the ability of the different gels to densify.     

Sol-Gel法合成锌铝尖晶石的影响因素研究

以Zn(NO3)2·6H2O,Al(NO3)3·9H2O为原料,柠檬酸为络合剂,采用sol-gel法制备了尖晶石型ZnAl2O4粉体.得到干凝胶后在不同温度下(600～ 800℃)进行热处理.探究pH值对凝胶的影响以及煅烧温度对粉体相组成和显微结构的影响.用热分析,X射线衍射分析(XRD),扫描电镜(SEM)等手段对制备的锌铝尖晶石粉体进行了表征.结果表明:当煅烧温度为600℃时,ZnAl2O4开始生成,随着煅烧温度的提高,ZnAl2O4晶粒逐渐长大、发育完整,其他物相含量减少,粉体趋于单一尖晶石晶体结构;随着溶液pH值的增加,ZnO含量增加,ZnAl2O4含量减少,当pH值为3时,得到纯度较高的ZnAl2O4粉体.     

Dependence of the Crystal Structure on Particle Size in Barium Titanate

The effect of the sample particle size on the crystal structure and the Curie temperature of BaTiO₃ powder has been investigated in the particle size range 0.1 to 1.0 μm. The transformation from tetragonal to cubic symmetry occurs at a critical particle size of 0.12 μm at room temperature, and the Curie temperature drops below room temperature at the critical particle size.     

Adhesion Induced Flow of a Soft Polyester-Polydimethylsiloxane Copolymer Substrate Over Micrometer and Submicrometer Size Spherical Particles: Observations of Anomalously Large Menisci, Interparticle Bridging and Particle Encapsulation

Observations made with a scanning electron microscope (SEM) provided direct evidence for a soft polyester-polydimethylsiloxane block copolymer substrate undergoing extensive surface-force-induced plastic deformation upon contact with micrometer or submicrometer size spherical particles. Anomalously large menisci were detected at the particle /substrate interfaces. Moreover, the substrate material appeared to bridge or encapsulate the particles. The heights of the contact menisci between the 2.2 micrometer radius polystyrene beads and the substrate were found to be approximately 0.4 micrometers; those between 3.6 micrometer radius glass spheres and the substrate were approximately 0.5 micrometers. The heights of the observed menisci were found to be large compared with the values calculated using Tabor's analysis (D. Tabor, J. Colloid Interface Sci. 58, 2 (1977)) based on the elastic model proposed by Johnson, Kendall and Roberts (K. L. Johnson, K. Kendall, and A. D. Roberts, Proc. R. Soc. London A. 324, 301 (1971)). These results suggest that the surface-force-induced tensile stresses may have exceeded the elastic limit of the substrate or that the surface material has an unexpectedly low surface modulus.     

悬浮聚合法制备墨粉的粒度控制及成粒机理分析

研究了含有微细炭黑粒子混和单体悬浮聚合产物的粒径及形貌特性.分析了炭黑添加量、超声分散搅拌作用及分散剂、助分散剂对产物粒径的影响,并分析了形成炭黑颗粒镶嵌于聚合物表面的原因.结果表明在超声分散作用下,添加适量的炭黑及Span80有助于混和单体相的稳定,从而利于产物粒径的减小和较光滑表面的形成;搅拌作用存在一临界值;分散剂及助分散剂是影响产物粒径的重要因素,对粒子形状也有影响.炭黑粒子与水相和油相的表面能及体系粘度随反应进程的变化是导致炭黑粒子镶嵌于聚合物表面结构的原因.     

Effect of SiC Submicrometer Particle Size and Content on Fracture Toughness of Alumina–SiC "Nanocomposites"

A simple model was developed which describes the effect of SiC submicrometer particles on the fracture toughness of alumina–SiC "nanocomposites." This effect was attributed to the change in the fracture mode observed in alumina on adding SiC submicrometer particles, which was suggested to be a result of both matrix weakening and grain boundary strengthening. The model suggests that the increase in fracture toughness should be obtained only for small additions (less than 5 wt%) of SiC.     

Ferroelectrics of Ultrafine Particle Size: I, Synthesis of Titanate Powders of Ultrafine Particle Size

Two methods used to synthesize high-purity ferroelectric titanate powders in controlled, narrow size distributions, with average particle diameters <1000 A, were: (1) isothermal pyrolysis of barium titanyl oxalate or mixed calcium-barium titanyl oxalates as low as 550° and 825°C, respectively, average particle sizes depending strongly on the pyrolysis temperature; and (2) hydrolysis of titanate esters in barium hydroxide. Using solvent media of controlled polarity, high-purity stoichiometric BaTiO₃ was obtained with average sizes as small as 100 A. Factors affecting stoichiometry and particle size are discussed in terms of assumed reaction mechanisms.     

Microwave‐Assisted Hydrothermal Synthesis of Submicrometer Willemite Phase Zinc Silicate and Its Zinc Ion Release Behavior

As an important trace element in human bone, zinc (Zn) has a great influence on bone metabolism. The aim of this work was to prepare a Zn‐containing material that can release Zn ions. For this purpose, submicrometer willemite phase zinc silicate (Zn₂SiO₄, ZS) with poor crystallinity was synthesized at 110°C via the microwave‐assisted hydrothermal (MH) method. Under the MH condition, the growth of ZS was consistent with “multi‐core growth” mechanism. Moreover, the influences of the reaction temperature and the reactant concentration on the final products were investigated in detail. The inductively coupled plasma atomic emission spectroscopy (ICP) data indicated that the poor crystallinity ZS could successfully release Zn for at least 28 d as soaking in the simulated body fluid (SBF). Without the replenishment of SBF, ~333 μM Zn was released from ZS synthesized at 110°C (ZS‐110). If SBF was periodically replenished once a day, more Zn was released from ZS‐110, and the parascholzite phase calcium zinc phosphate hydrate (CaZn₂(PO₄)₂·2H₂O) formed during the soaking process of ZS‐110. Under the concentration of 6.25 mg/mL, the extract of ZS‐110 was proved to be nontoxic by assessing with mouse osteoblast cells (MC3T3). Therefore, the poor crystallinity ZS has potential to be incorporated into the orthopedic reconstructive materials as a source of Zn ions, which can improve the bioactivity of the materials.     

Dependence of Compaction Efficiency in Dry Pressing on the Particle Size Distribution

The compact densification with pressing pressure (compaction efficiency) was determined to be sensitive to the particle size distribution. For the three types of alumina powders used in this research, the compaction efficiency increased with increasing particle size. It has been demonstrated that if the compact density versus log (pressure) has a linear relationship for any two types of powders, so do the blends of the two powders. A model is proposed which can predict the compaction efficiency of a binary particle system based on the Furnas particle packing model and consider the packing efficiency as a function of forming pressure. The composition of the binary mixture at which the highest density is obtained under high pressures is also the composition having the largest compaction efficiency. When coarse particles were added to this composition, the compaction efficiency slowly decreased, and when fine particles were added, the compaction efficiency rapidly decreased. For a continuous particle size distribution, the highest compaction efficiency is related to the average value of -log (porefraction).