Effect of internal binder on microstructure in compacts made from granules

This paper focuses on the influence of granule characteristics on the microstructure of the compact. Alumina granules were prepared with a poly-acryl acid (PAA) or a poly-vinyl alcohol (PVA) as binders by a spray-drying method. Observation with a liquid immersion method shows a significant difference in the binder distribution. Very uniform and non-uniform distributions were noted for the PAA and PVA binders, respectively. PVA binder segregated at surface and subsurface of the granules. The compression strength and deformation behavior were examined on a single granule with a micro-compression testing machine. The granule with the PAA binder has a low yield stress. In the forming process, the relative density of the compact body started to increase at low pressure. Homogeneous internal structures were noted in the green compact at all pressures examined. The granule with PVA binder showed higher yield stress. The internal structure of the compact was inhomogeneous, and large interstices were often observed between granules in the green compact also.     

Powder Processing for the Fabrication of Si3N4 Ceramics: I, Influence of Spray-Dried Granule Strength on Pore Size Distribution in Green Compacts

The effect of spray-dried granule strength on the micro-structure of green compacts obtained by isostatic pressing was quantitatively analyzed. The fracture strength of single granules of Si₃N₄ powder made with ultrafine A1₂O₃ and Y₂O₃ powders was measured directly by diametral compression. It was found that fracture strength increased notably with the increasing relative density of the granule and the decreasing size of agglomerates in suspension before spray-drying. Even when green bodies were prepared at an isostatic pressure of 200 MPa, intergranular pores, which negatively affected densification of the sintered bodies, occurred between unfractured granules. The volume and size of these pores in the green compacts increased with the increasing fracture strength of the granules. In the case of closely packed granules, an isostatic pressure of 800 MPa was required to completely collapse the intergranular pores. A simple equation was derived to calculate the isostatic pressure necessary for complete collapse of intergranular pores in the green compacts, and it was determined that granule strength must be kept as low as possible to obtain uniform green compacts.     

Fabrication of high-density magnesia using vacuum compaction molding

High-density magnesia was fabricated using vacuum compaction molding, and effects of forming pressure and sintering temperature on bulk density, apparent porosity, diameter shrinkage ratio, volume shrinkage ratio, pore size distribution, cold compressive strength, and thermal shock resistance of the magnesia samples were investigated. There were two ranges of pore distribution in samples that were formed via conventional compaction molding, and these ranges were about 350–2058 nm and 6037–60527 nm. It was considered that the range of larger pores mainly influenced the densification of magnesia. Using vacuum compaction molding, large size pores were removed, and high-density magnesia (with a density greater than 3.40 g cm^?3) was easily prepared when forming pressure was higher than 200 MPa and sintering temperature was higher than 1600 °C. Magnesia samples prepared via vacuum compaction molding showed better performance compared to that of samples prepared via conventional compaction molding.     

Influence of Slurry Parameters on the Characteristics of Spray-Dried Granules

A systematic study has been performed to determine how the characteristics of granules prepared by spray drying aqueous alumina slurries are influenced by processing parameters: binder type (PEG Compound 20M or PEG-8000), solids loading (30 or 40 vol%), ammonium polyacrylate deflocculant level (0.35-1.00 wt%), and spray-dryer type. Correlations between slurry rheology and granule characteristics have been made, and a model for granule formation is presented. The packing density of the primary particles within the granules is lower for slurries with higher yield stress and is dependent on the slurry solids loading. Granules prepared using 0.35 wt% deflocculant (0.14 mg/m²), which correspond to high slurry yield stress, are of solid morphology, whereas higher deflocculant levels result in hollow granules that contain a single large open pore or crater. The degree to which particles are able to rearrange during drying influences the final granule density and is determined by the strength of the floc structure, as indicated by the slurry yield stress. When the yield stress is low, a crater may form from the inward collapse of the surface of a forming granule when the particle packing density in a droplet continues to increase after the droplet size becomes fixed by the formation of a rigid shell, leaving an internal void with internal pressure lower than that of the surrounding atmosphere.     

Porous mullite ceramics with low thermal conductivity prepared by foaming and starch consolidation

Porous mullite ceramics were prepared from an industrial grade mullite powder by foaming and starch consolidation. The viscosities of the original suspensions and the foamed ones with solid loading of 62.5 and 67.5 wt% were measured. After the steps of forming and drying, the green bodies were sintered under different temperatures from 1,200 to 1,600 °C for 2 h. The influence of solid loading of suspension and sintering temperature on the porosity and compressive strength was evaluated. The sintered mullite ceramics, with porosity from 86 to 73 vol% and corresponding compressive strength from 1 to 22 MPa, contained a multi-modal microstructure with large spherical pores and small pores on internal walls. Thermal conductivity measurement carried out by the transient plane source technique at room temperature resulted in values as low as 0.09 W/mK. In addition, the relationship between thermal conductivity and porosity was discussed in detail.     

Effect of Organic Binder Segregation on Sintered Strength of Dry-Pressed Alumina

The influence of binder systems on the structure and properties of alumina ceramics was examined. Two types of binders were used to prepare alumina granules by spray drying: poly-acrylic acid (PAA) and poly-vinyl alcohol (PVA). PVA segregated to the surface and subsurface of the granules, whereas PAA was homogeneously distributed. Compaction tests on a single granule showed that the PAA binder provides lower yield stress of granules than PVA. In the die compaction process, the relative density of the compact body started to increase at a lower pressure, and a more homogeneous internal structure was observed in the green compact prepared with the PAA binder than PVA. The homogeneous structure resulted in a sintered body with a strength of 580 vs 485 MPa when PVA was used.     

Particle and Granule Parameters Affecting Compaction Efficiency in Dry Pressing

The slope of density versus log (pressure) represents the efficiency of compaction forming. A high maximum particle packing density of powders increased the pressing efficiency and the compact density. A high granule density favors a high compact density, whereas, a low granule density favors an increase in the pressing efficiency and the elimination of the intergranular pores; an optimum clearly existed. At a very low forming pressure, the compact density using granules containing a binder was lower; but at a higher forming pressure, the binder acted as a lubricant and assisted particle sliding and rearrangement which increased the pressing efficiency and compact density.     

Effect of diluents on the porous structure of crosslinked poly(methyl methacrylate) beads

Summary Copolymer beads based on methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDM) were prepared by suspension polymerization technique. Two different solvents, namely toluene and cyclohexane, were used as the diluents in the polymerization system. It was found that toluene produces pores of radii 2 to 10 nm corresponding to the interstices between the nuclei. In the case of cyclohexane as the diluent, formation of large pores with a broad size distribution from 10 to 1000 nm were observed. These sizes correspond to the spaces between the microspheres and the aggregates forming the copolymer beads. The results indicate that, as in porous styrene-divinylbenzene copolymers, the solvating power of the diluent present during the network formation is mainly responsible for the structural characteristics of porous MMA/EGDM copolymer beads.     

Use of X-ray tomography to study the porosity and morphology of granules

X-ray computed tomography (XRCT) is a technique that uses X-ray images to reconstruct the internal microstructure of objects. Known as a CAT scan in medicine, it has found wide application for whole-body and partial-body imaging of hard tissues (e.g., bone). A modern tabletop XRCT system with a resolution of about 4 μm was used to characterize some pharmaceutical granules. Total porosity, pore size distribution, and geometric structure of pores in granules produced using different conditions and materials were studied. The results were compared to data obtained from mercury porosimetry. It was found that while XRCT is less precise in the determination of total porosity in comparison to mercury porosimetry, it provides detailed morphological information such as pore shape, spatial distribution, and connectivity. The method is nondestructive and accurate down to the resolution of the instrument.Tomographic images show that the pore network of individual granules comprises relatively large cavities connected by narrow pore necks. The major structural difference between granules produced at different conditions of compaction and shear is a reduction in the pore neck diameter; the cavity size is relatively insensitive to these conditions. Comparison of pore size distributions determined from tomographic images and mercury porosimetry indicates that mercury intrusion measures the pore neck size distribution, while tomography measures the true size distribution of pores ca. 4 μm or larger (the instrument resolution).     

Effect of forming pressure on densification behavior of nanocrystalline ITO powder

Since ultrafine nano-particles easily produce agglomerates, they result in an inhomogeneous particle packing structure within a green body. Nanocrystalline indium tin oxide (ITO) powder was prepared, and the compaction behavior of the powder was investigated by using an Hg porosimetry and a scanning electron microscope (SEM) as a function of the applied cold isostatic pressing (CIP) pressure. In addition, the sintering behavior of the compact was monitored with a thermo-mechanical analyzer (TMA), and the relationship between the inhomogeneity in the green body and the sintering behavior was examined. An increase in the forming pressure enhanced the homogeneity of the powder compact by crushing the larger agglomerates. The first generation agglomerate compact showed a two-step densification process in the shrinkage rate curve of TMA. The two-step densification behavior is explained by the elimination of two kinds of pores, which are small inter-crystallite pores and large inter-first generation agglomerate pores.     

水柱成型法制备重整载体

以拟薄水铝石（PB）和海藻酸钠（ALG）为原料,采用水柱成型法制备了纯度较高、孔结构适宜的重整载体γ-氧化铝。对拟薄水铝石进行了筛选;考察了成型时间对钠离子脱除的影响;研究了不同种类拟薄水铝石复配比例对载体物性的影响;对载体负载催化剂进行了评价。实验结果表明：当成型时间为30 min时,氧化铝中钠离子质量分数可以降低至1.9×10-5,海藻酸钠未引入钠;选用合适的拟薄水铝石复配比例,可以制备孔容为0.568 cm3/g、比表面积为217 m2/g、强度为47 N、表观密度为0.564 g/mL的γ-氧化铝。以γ-氧化铝为载体制备重整催化剂用于评价,其液体收率、芳烃收率与辛烷值均优于国外同类产品,应用前景良好。     

Packaging of Sol–Gel-Made Porous Nanostructured Titania Particles by Spray Drying

Porous titania granules (1–20 μm) were made by spray drying titania solutions with and without peptization. An unpeptized titania slurry was prepared by hydrolyzing titanium tetraisopropoxide, while a peptized sol was prepared by adding nitric acid to the slurry. The characteristics of the granules depended on the feed solution properties. In particular, granules from the unpeptized slurry showed a bimodal pore size distribution (PSD), with a rough surface attributed to hard aggregates; granules from the peptized sol showed a monomodal PSD, with a smooth surface of well-packed nanoparticles. The effect of dopants (alumina and zirconia) also was investigated. Doping slowed the crystallite growth of the titania granules, as well as the anatase-to-rutile phase transformation. All of the peptized granules exhibited a monomodal PSD of intraparticle pores, from 2 to 15 nm. However, the PSD of the pure titania granules collapsed at 600°C, mainly by phase transformation.     

Al2O3纳米线薄膜的制备及其生成机理研究

研究了一种简单新颖的制备Al2O3纳米线薄膜的电化学方法。以阳极氧化法在高纯铝片表面制备多孔阳极氧化铝的工艺为基础,在草酸电解液中通过一次阳极氧化过程,将阳极氧化铝孔壁溶解形成Al2O3纳米线,均匀覆盖在阳极氧化铝有序孔道的上方,得到一种特殊结构的纳米线薄膜。使用扫描电镜、高分辨透射电镜和X-射线能谱仪对纳米线的形貌、结构及成分进行了表征分析。并结合扫描电镜对不同反应时间长度的产物形貌来观察纳米线的生成过程。探讨了Al2O3纳米线薄膜的生成机理。     

Structure and properties of dense periclase refractories prepared from laminated granules

Conclusions On the basis of laminated granules we prepared dense periclase refractories with a higher thermal-shock resistance than that of ordinary articles prepared by sintering solid granules. The technical properties are determined by the degree of development of a fragmentary structure which depends on the size of the granules, the composition of their outside layer, and the composition of the bonding mass in the batch.With an increase in the dimensions of the laminated granules and addition to the composition of their outside layers of yttrium oxide or lanthanum zirconate and to the composition of the batch of solid grains of periclase of a smaller fraction in the articles, complex elements are produced which form the fragments of the structure. This increases the thermal-shock resistance of the goods. The porosity of the articles is reduced with a reduction in the amount of dispersed part of the batch and its additional sintering with the formation of closed pores. The strength of the articles is reduced with a rise in the amount and size of the channel pores, and there is its increase during the formation of additional bonds in the area of contacts between the bond and the aggregates of coarse grains.Translated from Ogneupory, No. 3, pp. 9–13, March, 1984.     

In situ observation of evolution of internal structure of alumina during sintering by swept-source OCT

In situ observation of sintering of ceramics is important to understand the sintering behavior, including the development of internal structures, and to fabricate ceramics with superior properties. However, there has been no studies thus far on high-speed in situ observation of the internal structure of green and sintered bodies at high temperatures with micrometer-scale resolution. Here, we applied swept-source optical coherence tomography (OCT) to observe the evolution of the internal structure in Al₂O₃ green bodies during sintering. In situ OCT observations revealed that internal structure changes during sintering, such as the development of density distribution and growth of coarser pores. We could also obtain the sintering shrinkage ratio and relative density from the OCT images. The OCT observation of the internal structure during sintering of green bodies with added or stacked granules of different primary particle sizes confirmed that inhomogeneous regions developed as the densification progressed at high temperatures. These results indicate that in situ observation of the sintering behavior of ceramic green bodies using swept-source OCT is a novel technique that facilitates real-time observation of the evolution of the internal structure during sintering.     

Porous Ceramic Granules of Hydroxyapatite

A technology for fabrication of porous spherical granules from hydroxyapatite is reported, and the structure and properties of granules prepared under different process conditions are described. The granules prepared range in size from 50 to 2000 m and contain open pores of diameter mainly 2 – 3 nm. The granules can be used to heal defective bones and as vehicles for targeted drug.     

Reaction forming of silicon carbide ceramic using phenolic resin derived porous carbon preform

SiC ceramics were prepared with porous carbon preforms derived from phenolic resin by a reaction-forming method. The effects of the structure of the preform pores and the infiltration process on the properties of SiC ceramics were investigated, and components with complex shapes were fabricated by combining this process with stereolithography (SLA). Dense SiC ceramics were obtained from carbon preforms with high apparent porosities, but SiC ceramics with many macrodefects resulted from a carbon preform with an apparent porosity of 39%. The infiltration of molten silicon into the preform pore channel was accelerated under vacuum pressure, resulting in an increase in the depth of the Si infiltration. The growth of SiC was predominantly controlled by carbon diffusion at the last stage of the reaction. The extended grain growth caused the SiC grains to coalesce and some free Si was enveloped in the SiC grains. SiC components with complex geometries were fabricated by combining reaction forming with SLA. The geometry was controlled by SLA.     

Direct Observation of Internal Structure in Spray-Dried Alumina Granules

The internal structure of spray-dried alumina granules was characterized by optical microscopy by immersing them in a liquid having a refractive index close to that of alumina. This method provides a unique technique for the detailed analysis of the internal structure of spray-dried granules.     

Control of internal (2D and 3D hexagonal) mesostructure and particle morphology of spherical mesoporous silica particles using the emulsion and solvent evaporation (ESE) method

In this study we demonstrate the opportunities controlling internal structure as well as exterior morphology of surfactant templated mesostructured materials through the newly developed emulsion and solvent evaporation (ESE) method. In particular, we consider the control of synthesis temperature and map the influence upon both internal structure and surface morphology of particles templated by the temperature sensitive Pluronic block copolymer F127. Furthermore, we vary compositions, by adding poly(propylene glycol) acting as a swelling agent, as well as by controlling the moisture content. Both of these are having an impact on the internal mesostructure as well as the pore size. Apart from probing internal structure by scattering techniques, the accessibility of the mesoscopic pores of these materials are investigated by measuring the adsorption of a cationic dye, Janus Green B, into the materials. This method shows that accessibility varies dramatically with internal structure.Further, by carefully controlling the moisture content when using the cationic surfactant C₁₆TAB as template, a well ordered 3D hexagonal closed packed (P6₃/mmc) material with large surface area as well as pore volume was prepared. This further indicates the versatility of the new preparation technique.     

Porous carbon material based on petroleum pitch

The samples of granulated (2–4 mm) porous carbon material with unimodal and bimodal poresize distributions were prepared. The sample with a unimodal porous structure was prepared via the mixing of soot and petroleum pitch followed by carbonization (the first stage). At the second stage, the carbon material obtained was ground to a particle size smaller than 0.2 mm, mixed with petroleum pitch, and also subjected to carbonization. The porous structures of these materials were studied by scanning electron microscopy and mercury porosimetry. It was established that the predominant pore size (20–1000 nm) of the samples with the unimodal structure depends on the used soot grade. At the second stage of the process, a considerable volume of pores in a range of 4–12 μm is formed in the production of bidisperse material. As compared with a sample with the monodisperse structure, porous carbon material with a bimodal distribution of pores was characterized by a lower crushing strength of granules and comparable specific surface areas, total pore volumes, and ash contents.