Submicron Powders Research Articles

Behavior of Particle Size Distributions, Means and BET Values in Ideal and Non-Ideal Morphology Systems in a TEM

Abstract Historically, one of the first jobs performed by the new instrument called a “transmission electron microscope” (TEM) was the determination of particle size and distribution. Particle size is still important in controlling the properties of sub-micron and nanoparticle products. The width and average particle size can determine the light scattering properties of a pigment like TiO2, for example. The cost of installing a TEM facility and TEM sample preparation issues makes TEM analysis expensive and labor intensive. So decades ago, nitrogen BET (Brunauer, Emmett, and Teller) surface area determinations were used on sub-micron powders and pigments to minimize these costs. The BET measurement of the exposed external surface area of a material is made by detecting and measuring the amount of nitrogen given off after absorption, assuming one atomic layer of nitrogen is adsorbed.

市販高純度α‐アルミナ粉末表面上における水和物，吸着物層の分析

The surface of six different grades of commercial sub-micron high purity alpha alumina powders produced by two different processes, in-situ chemical vapor deposition (AA powders) and hydrolysis of aluminum alkoxide (AKP powders) methods are evaluated by Temperature Programmed Desorption Mass Spectrometry (TPDMS). All AA powders show almost the same quantities of H2O and CO2 desorbed molecules. On the other hand, the different grades of AKP powders show different quantities of H2O and CO2 desorbed molecules. The AKP-3000 powder quantity of desorbed H2O molecules is similar to that of the AA powders. However, the AKP-3000 powder quantity of desorbed CO2 molecules is about seven times larger than that of the AA powders. The maximum of the CO2 desorption peak is the range 230-270°C for all powders. The desorbed CO2 peak is considered to evolve from the adsorbed CO2 molecules forming hydrogen carbonyl groups by interaction with AlIV-OH groups on the α-alumina surfaces. The amount of hydrogen carbonyl groups on surface of the AKP powders is larger than for the AA powders, indicating that on AKP powder surfaces the amount of AlIV-OH groups is larger. These results show that on the surface of commercial high purity alpha alumina powder a complex hydrated layer exists by interaction between surface hydroxyl groups and CO2 adsorbed molecules, unlike commonly accepted stable alpha alumina structure.

Interface Nanochemistry in Mg-Doped Al2O3

AbstractIn this experimental paper the influence of the grain size, of the thermal history and of the nominal composition on the nanochemistry at the interfaces in α-alumina submicronic powders are analyzed. The consequences on the physical and chemical properties are also discussed. Nanocrystalline Mg-doped α-alumina powders (550 and 1650 ppm by wt MgO) were prepared by the alum process. These powders (10-400 nm) were characterized by X-ray diffraction, SEM, TEM, XPS and surface area measurements (BET). In agreement with BET analysis, SEM observations show that the grain size increases with annealing time and annealing temperature. This effect is less pronounced in the highly doped samples. The less doped samples are also less aggregated and sintering bridges between the grains are still observed. XPS shows that Mg segregates at the periphery of the grains on a depth ≤0.5 nm. The dopant segregation is stronger when the nominal amount of Mg increases, leading eventually to the formation of MgAl2O4 precipitates near the grain surface. It is possible to observe an increase of the dopant segregation with grain size in the low Mg concentration powders. Other impurities like Si or higher cooling rates also favour this Mg segregation. However, the rate of cooling is so effective that it can overshadow all the other parameters (grain size, impurities,…), suggesting that this is certainly the key parameter for obtaining a perfect sintering of these ceramics, as experimentally shown.

Plasma Pressure Compaction of Tungsten Powders

Abstract Compacts of tungsten powders were consolidated by Plasma Pressure Compaction (P2C), an electric discharge technique. The powders were a variety of commercially available grades ranging in average particle size from submicron to 12 microns. Following consolidation, the density of the compacts was measured, and the microstructure examined. Results revealed the effect of powder size, pulsed current treatment, final hold temperature, and applied pressure on final part density and microstructure development. Most important to the purpose of the study, it was found that the short cycle time of P2C did not suppress grain growth in the compacts of submicron powder. Thus, grain growth remained a consequence of full densification. Implications of these results for the development of ultra fine-grained microstructures using P2C are discussed.

Development and applications of ultrafine aluminium powders

Over the last 20 years or so, a variety of new technologies has been developed to produce sub-micron powders. Among the products attracting interest is nanoaluminium which is being evaluated in specialist propulsion and exothermic end-uses. This paper examines the advances made in ‘nanopowder’ production in the context of the existing aluminium powder industry where finest commercial grades have a median size of ~6 μm (one or two orders of magnitude coarser than nanopowders) and which today supplies the markets being targeted by nanopowders with coarser, but effective products. Are there genuine market opportunities for nanoaluminium and if so, how will they be produced? One the one hand there are the novel nanopowder production methods which are high yielding but generally slow and costly, while on the other, there is the very fine fraction from conventional atomising routes which generate a very low yield of sub-micron powder but which nevertheless can translate into a meaningful rate as part of the bulk production. Can conventional routes ever hope to make sufficient volumes of nanopowders cost effectively and which will be the favoured routes in future? Moreover, what of the ‘ultrafine’ size range (~0.5–5 μm) which is of more immediate potential interest to today’s powder users. This paper seeks to identify the near term opportunities for application of low volume/high value ultrafine and nano powders.

Influence of alumina powder size on mechanical properties of in-situ coated alumina fiber-reinforced alumina composites

Continuous alumina fiber-reinforced alumina-matrix composites were fabricated by hot-pressing. Fibers were coated by La-monazite, and matrix was prepared using the mean size of 7 nm and/or 180 nm alumina raw powders. It was observed that the powder size of matrix affected the properties of the composites. Composites using the nano-sized powder (7 nm) as matrix showed higher pseudo-ductility, but lower maximum strength and density than those of the composite using the submicron-sized powder (180 nm) as matrix, regardless of the sintering temperature. Woody-type fracture due to fiber pull-out from matrix could be observed for all composites regardless of the size of powder for matrix after sintering at relatively lower temperatures. Nano-sized powder supplies a looser matrix than submicron powder, therefore, it should be corresponded to higher pseudo-ductility of the composites. Combination of the nano-sized and submicron-sized powder as matrix gives composites with highest strength as well as enough ductility.

Manufacture And Morphological Characterization Of OsteoconductivePorous Scaflolds

This paper discusses the manufacturing, chemical, and physical characterization of a three-dimensional porous scaffold developed for the repair of skeletal defects. Autografts from patient donor sites are the current gold standard. However donor sites morbidity and a limited material supply restrict this treatment option. This is the main reason why the presence of synthetic biomaterials has grown significantly over the past two decades with applications in orthopedics. Bone consists of cortical and spongy regions both composed of Gas,? (PO4)4,3 (HP04, C03)1,7 (OH,C03)0,3 mineral phase. Spongy bone is a highly spongy tissue (55-70% interconnected porosity) that allows the in-growth of the soft tissues and organic cells into the bone matrix. Chemically sub-micron and high phase-purity hydroxyapatite powders (HA: Calo(P04)6(0H)2) were mixed, in a centrifuge ball mill, together with ethanol (EtOH) and hydroxypropyl-methyl-cellulose (HPMC) powders in appropriate amounts. The mixtures produced in this way were slowly dried in an oven. Dried powders were formed under pressure in cylindrical cakes and heated in an air atmosphere to the optimum sintering temperature. The raw materials and the scaffolds obtained by this technique were characterized by: XRD, laser granulometer, DSc-TGA analysis, FT-IR spectroscopy, SEM micrographs, and porosity measurements. This method produces porous HA scaffolds very similar to spongy bone from a chemical, physical and morphological point of view. The key parameters to the performance of the material for osteomimetic purposes will be analyzed and their effects studied also with reference to desirable modeling efforts. Transactions on Engineering Sciences vol 43, © 2003 WIT Press, www.witpress.com, ISSN 1743-3533

Study on the variation of morphology and separation behavior of the stainless steel supported membranes at high temperature

Palladium composite membranes were prepared on stainless steel (SUS) supports modified by nickel submicron powder and colloidal silica sols. Permeation tests of the palladium composite membranes were carried out at high temperature in order to observe the thermal stability of the membrane. The palladium composite membrane failed with formation of plenty of pinholes in the presence of hydrogen at high temperature. The failure of the composite membrane was verified by comparing the nitrogen permeance before hydrogen permeation test with that after hydrogen permeation test and comparing the H 2/N 2 selectivity for single gas permeation test with that for mixture gas permeation test. The variation of the membrane surface due to the failure of the membrane was characterized in scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS) analyses. As a result, it can be concluded that reducible metal oxides can be attributed to the failure of the composite membranes resulting from reduction of the metal oxides by hydrogen whichever position in the membrane the metal oxides are layered.

Synthesis and characterization of ZnS:Cu,Al phosphor prepared by a chemical solution method

A novel and simple synthesis route for the production of ZnS:Cu,Al sub-micron phosphor powder is reported. Both the host and activator cations were co-precipitated from an ethanol medium by mixing with a diluted ammonium sulfide solution. The co-precipitated ZnS:Cu,Al was in cubic zinc blende structure after an intermediate-temperature furnace annealing. Strong photoluminescent and cathodoluminescent (CL) emission were observed, which was attributed to the 3d 10-3d 94s 1 radiative transition at those copper sites. At an accelerating voltage of 1 kV, the CL intensity of the co-precipitated ZnS:Cu,Al sample was recorded 94% of the commercial reference phosphor with the same composition made by high temperature solid-state-reaction method. The particle size of the co-precipitated phosphor powders was found to be controllable simply through adjusting the reactant concentrations. The particle size of the annealed samples was measured by dynamic light scattering, which showed a mean particle diameter between 200 and 700 nm depending on the co-precipitation conditions.

Sol-gel synthesis of sub-micron titanium-doped chromia powders for gas sensing

Cr1 − xTixO3+x/2, with x = 0.05,0.1 and 0.2, powders have been synthesised by an emulsion gelation route starting from aqueous chloride solutions. Gel particles were produced by de-anionisation of a water-in-oil emulsion using an immiscible organic amine solution. After calcination to produce the oxide, the powders were characterised in terms of particle size distribution, crystallinity, surface composition, particle shape, surface area, anion contamination and doping uniformity. By optimising the processing variables it has been possible to produce un-agglomerated, spherical, uniformly doped powders with a narrow particle size distribution. The mean particle size can be controlled within the range 0.3 to 3 microns and the particles are porous and nano-crystalline. The powders have been used to fabricate thick film sensor structures and show good response to combustible gases in air.

Sintered Tantalum and Niobium Sub-micron Powders of Solid Electrolite Capacitors

ABSTRACTThe results of x-ray study and electrical measurements are brought together in order to understand a difference in performance and failures of solid electrolyte capacitors made of sintered tantalum and niobium sub-micron powders. It is shown that bulk Ta and Nb exhibit different abilities for the formation and decomposition of oxygen solid solution during manufacturing steps which may be critical for stability of metal/oxide interface of capacitor.

Low Cost (La,Sr)MnO3 Cathode Material with Excellent Electrochemical Properties

The main objectives of H. C. Starck's internal development program for powders used in SOFC applications are to meet the cost targets of the customers as well as the performance requirements during cell operation. Optimization of LSM-powder engineering process resulted in very homogenous powders with good crystallographic properties. The morphological properties can be adapted to customer requirements, the primary particle size can be varied in the range of few micrometers down to submicron powders. Electrochemical performance tests of electrolyte supported single cells with cathodes based on these LSM-powders were conducted at the IWE, Universitat Karlsruhe (TH). The single cells were characterized with respect to performance and long term behavior (> 1000 h) including load and thermal cycling. Current densities of 0.18 A/cm 2 (750°C), 0.43 A/cm 2 (850°C) and 0.7 AJcm 2 (950°C) were achieved at a cell voltage of 0.7 V. The long term stability and thermal cycling behavior was acceptable for mono-layer cathodes; and performance and degradation were in good agreement with the standard type cathode compositions and microstructure.

Fabrication of nano-structured sub-micron metal powders by ultrasonic atomisation

Fabrication of nano-structured sub-micron metal powders by ultrasonic atomisation

Electroless deposition of copper onto alumina sub-micronic powders and sintering

Abstract This work has been initiated in order to find new routes for the preparation of 3D interconnected ceramic–metal composites, usually obtained by direct metal oxidation or liquid infiltration of a ceramic preform. The system under investigation is the alumina–copper system. The key point of this new route was the preparation of very fine composite powders, which allowed the material to be densified by natural sintering. The first part of the presentation deals with the preparation of the composite powders, which was achieved using an electroless process. Influence of experimental parameters controlling the deposition of a copper layer on a sub-micronic alumina powder was investigated. The second part deals with the processing of the composite. A statistical design methodology was applied to determine the main parameters controlling the shrinkage. For some conditions, a new in situ Cu–CuAlO2 composite is formed, which presents an interesting microstructure with CuAlO2 acicular grains.

Cerium-ion-doped yttrium aluminum garnet nanophosphors prepared through sol-gel pyrolysis for luminescent lighting

Luminescent Y3Al5O12: Ce3+ [yttrium aluminum garnet (YAG):Ce3+] nanoceramics (5–50 nm) have been prepared through a facile sol-gel pyrolysis route. A blueshift of about 720 cm−1 in the reflectance spectra near the absorption edge can be observed for YAG:Ce3+ nanoparticles (having an average particle size around 5 nm) with respect to the submicron powders (∼0.9 μm). Furthermore, the scope of application for this important luminescent system can be extended by isostructural substitution with heavier cations such as Ba2+. This substitution giving rise to an oxygen-vacancy defect complex, that exhibits certain size-sensitive exciton-impurity energy-transfer property in YAG:Ce3+ nanoparticles.

Production of sub-micron powders by chemical reduction from aqueous solutions

Production of sub-micron powders by chemical reduction from aqueous solutions

Ceramic nano-composites made from submicron powders coated by a sol-gel method

Ceramic nano-composites made from submicron powders coated by a sol-gel method

Preparation of (Pb, La)(Zr, Sn, Ti)O3 antiferroelectric ceramics using colloidal processing and the field induced strain properties

Abstract A two-step wet chemical method using colloidal processing was developed for the preparation of (Pb, La)(Zr, Sn, Ti)O 3 antiferroelectric ceramics. The precursor B site was prepared by coprecipitation and precursor A site was introduced in the form of aqueous solution. Dried gel calcined at 600°C for 2 h formed single perovskite phases. Submicron powders were obtained by planetary ball milling. Average grain size about 4 μm and relative density of 98% were achieved from disc specimens sintered at 1100°C for 2 h. An antiferroelectric double polarization hysteresis loop with antiferroelectric to ferroelectric phase revert electric field E AFE-FE =4.3 kV/mm and a strain curve with maximum longitude strain of 0.30% under 6 kV/mm electric field were measured from composition of (Pb 0.97 La 0.02 ) (Zr 0.65 Sn 0.25 Ti 0 10 )O 3 .

Non-isothermal sintering of barium titanate nano-powders of different origination

Experimental investigation of non-isothermal sintering was carried out using the barium titanate of different dispersity: submicron powder (∼ 150 nm), and two nanosized powder (∼ 25 and 50 nm), obtained by thermal decomposition of barium titanyl-oxalate. Present study has shown the strong correlation of powder sintering behavior and preparation method, as well as particle size distribution. Final sintering temperature of nano-powders, was found to decrease for 70–100°C in comparison with the submicron powder. Applying a method of rate-controlled sintering, the full dense ceramics with a grain size around 0.15 microns had been obtained. The dielectric properties of such ceramics are quite high to serve a good prerequisite for thin dielectric layer's manufacturing.

Ceramic nanocomposites obtained by sol–gel coating of submicron powders

MgAl 2O 4–ZrO 2 nanocomposites were fabricated by conventional sintering of composite powders obtained by sol–gel coating of a submicron spinel powder. In the composite powder the zirconia grains remain narrow sized and completely tetragonal even after being heat treated at temperatures where a free xerogel is completely monoclinic. The sintered material exhibits a dense, fine and highly homogeneous microstructure. The zirconia nanoparticles are located at both inter- and intragranular positions and exhibit heteroepitaxial relationships with the surrounding crystals. Tetragonal zirconia seems to be stabilised by an interface effect. Both the scale of the microstructure and the fraction of intragranular grains were controlled by adjusting the mean grain size of spinel grains before coating and sintering conditions.