Fine Alumina Powder Research Articles

Electrothermal vaporization–inductively coupled plasma mass spectrometry for determination of metal impurities in slurries of aluminium oxide

A new analytical procedure for the direct determination of metal impurities in aluminium oxide by ultrasonic slurry sampling–electrothermal vaporization–inductively coupled plasma mass spectrometry (ETV–ICP-MS) is reported. The elements studied include Cr, Cu, Ga, Fe, Mg, Mn, Na, V and Zn. Two samples of fine alumina powder (including the certified reference material NIST SRM 699) were used in this study without any additional pre-treatment. Analytical results of the external calibration using aqueous standards, the external calibration using matched matrix standards and the standard additions method are presented and compared. The most appropriate (average accuracy of 97±5.5% for sample NIST SRM 699) was the standard additions procedure of calibration. An acceptable agreement was found between the results of the slurry sampling procedure and the certified or reference values, and the repeatability of measurements was always better than 13.6%. Some other analytical figures-of-merit,e.g., limits of detection, are also presented. The absolute limits of detection (for the analysis of 15 µl samples) ranged from 0.8 to 8.3 pg. The speed and multi-element analytical potential of this technique seem to be very advantageous.

Closed die axixymmetric forging of sintered aluminium preforms

The present work deals with an investigation into the closed-die forging of Aluminium powder preforms that have been compacted and sintered from very fine aluminium powder. The sintered preforms, of the same diameter but of different lengths, have been pressed in the cold condition. It is seen that the resistance to flow of the metal into the flash gap can be expressed in terms of the normal stress developed at the junction of the flash and the body of the forging. It appears that there exists a direct relationship between the normal stress and the flash thickness.

Structural phase transformations of rare-earth modified transition alumina to corundum

The effect of rare-earth dopant on transformations of the γ→θ→α phases in fine alumina powders under vacuum was investigated by in situ neutron diffraction from 500 to 1300°C. La-doped (1 mol%) Al2O3 powders (surface area ∼170 m2/g) were prepared by an impregnation technique. Below 800°C both samples contain a dominant γ-phase. Above 1000°C, transformation of the γ-phase to the intermediate θ-phase, and then completely to the α-phase (corundum) was observed. Addition of 1 mol% of La in Al2O3 effectively shifts the α-phase formation temperature from ∼1125°C for pure alumina to ∼1250°C, probably due to the larger size of La compared to Al ions, which hinders ionic diffusion in the processes of sintering and transformation. Consequently, doping La in alumina improves the surface-area and thermal stability at high temperatures, which is important for catalytic applications.

Thermal debinding of injection moulded ceramics

Special equipment for thermal removal of binder under controlled pressure conditions and with controlled gas flow rate was built to study the debinding process of injection moulded parts. The equipment was designed to record changes in the weight of injection mouldings, and an operating system made it possible to control their weight loss by temperature. The thermal debinding process of injection moulded parts prepared from fine alumina powder and a thermoplastic binder system based on ethylene-vinyl acetate copolymer and paraffin was studied using this equipment. The influence of pressure and flow rate of various gaseous atmospheres (air, oxygen, nitrogen, carbon dioxide and vacuum) was investigated. The benefit of binder removal in an inert atmosphere with controlled weight loss rate is described.

INSTANTANEOUS REGRESSION BEHAVIOR OF HTPB SOLID FUELS BURNING WITH GOX IN A SIMULATED HYBRID ROCKET MOTOR

An experimental investigation using a high−pressure hybrid motor analog has been conducted to provide detailed solid fuel regression rate data at realistic operating conditions for both model validation and correlation development. The 2−D motor operated at a maximum pressure of 9 MPa (1300 psi) with maximum injected gaseous oxygen mass fluxes of 420 kg/m2·s (0.6 lbm/in2·s). Hydroxyl−Terminated Polybutadiene (HTPB, R−45 M) cured with Isonate 2143L was used as the baseline solid fuel. Either carbon black powder or ultra−fine aluminum (UFAL) powder was added to the HTPB fuel during the processing stage for some tests. Both ultrasonic pulse−echo and real−time X−ray radiography techniques were used to determine the instantaneous regression rate. An array of fine−wire thermocouples was used to determine fuel surface temperatures and subsurface temperature profiles. The deduced instantaneous regression rates displayed a complex dependency on axial location and flow conditions. Close to the sample leading edge, the regression rate, governed by the mixing motion of the entering GOX flow, did not vary with time. Following this region, the regression rates exhibited strong dependency on both axial location and flow structure. When the GOX mass fluxes were below 140 kg/m2·s (0.2 lbm/in2·s), gas−phase radiative heat transfer to the fuel surface was found to play a more important role in the regression of solid fuel. Solid fuel regression rates were found to correlate well with the injected oxidizer mass flux, chamber pressure, axial location, and port height. The addition of carbon black had no effect on regression behavior. Addition of 20% of UFAL powder, however, was found to increase the mass burning rate by 70%.

The effect of porosity on the electric strength of heat-resistant concrete

In many applications where both high temperature and high electrical stress exist, little is known about the interaction of the electrical and thermal properties of the materials involved. This paper is a study of this interaction for a material which we call a concrete, and is a fine alumina powder mixed with a cement, i.e. calcium aluminate (CaOAl/sub 2/O/sub 3/) slurry, which cures in the same manner as a concrete. The porosity of the product depends upon the relative volume of the hydrated cement to the volume of the spaces between the alumina particles, and the shapes of the pores may be changed by choosing the mix of various particle sizes of alumina particles. In this study, specimens of this material were cast between uniform field electrodes in one case, and between nonuniform field electrodes in the other, cured and dried to remove residual moisture, and tested for electric strength. It is well known that the electric strength of porous materials is low and nearly the same as the electric strength of the gap between the same electrodes in air in the absence of the solid porous material. This paper shows that for this material and only at porosities <20%, as measured by a mercury porosimeter, does the electric strength become larger than that of air and become a significant function of the porosity and of the distribution of pore volume among pores of differing equivalent pore diameter. The uniformity of the field had no noticeable effect.

Liquid phase sintering of bimodal size distributed alumina powder mixtures

Fine and coarse alumina powder mixtures (non-additive specimen) and those containing the additive formed liquid phase during firing (additive specimen) were compacted and fired at 1400–1600°C. Liquid phase sintering proceeded markedly at 1400–1500°C and additive specimens had much higher relative density than non-additive specimens at 1500°C. As the liquid phase sintering proceeded, the open pore volume decreased abruptly, but the open pore size changed depending on the packing structure. The open pore size decreased in the specimens where the fine particles formed matrix structure, while it increased in the specimens where the coarse particles formed skeletal structure. At 1600°C all additive specimens having different mixing ratios of fine and coarse powders had similar microstructure and the same relative density of 97%. However, spherical large pores were formed and remained in all additive specimens even at 1600°C. The bending strength of those specimens was about 400 MPa.

Agglomerate breakdown in fine alumina powder by multiple extrusion

Repeated extrusion through dies of various diameters and die entry angles was used to determine the rate of agglomerate breakdown in a paste consisting of a fine alumina powder, carbon black and a binder of hydroxyl propyl methyl cellulose in water. It was found that three extrusion passes were enough to break up all but ∼0.4% of the agglomerates. Dies with orifices of approximately 1 mm diameter and die entry angles of 45 to 90‡ (where the elongational strain and the deformation rates were highest) were the most efficient for disrupting and dispersing agglomerates and distributing the moisture evenly. This process of deagglomeration was studied by monitoring the load required to extrude and moisture distribution during five repeated extrusion passes of each test paste. The density, agglomerate area fraction and agglomerate circularity of dried extradates were quantified and plotted.

Processing Problems of High Alumina Body

Excessive firing shrinkage and secondary re-crystallization cannot be controlled if alumina powder produced from Bayer's process is exclusively used to develop dense alumina body. High alumina bodies having low firing shrinkage and controlled microstructure can, however, be developed using alumina powder from Bayer's process and fused alumina grains with MgO in the form of magnesio-aluminate hydrate (MAH) as sintering aid. By introducing fine alumina powder pre-calcined at an optimum temperature, a part of fused alumina grains can be replaced. The possibility of formation of bodies involving the entire range of graded fused alumina and fine technical alumina powder has been explored.

Development of an S-band rf window for linear colliders

We report on our research and development of an S-band (2856 MHz) alumina rf window for a high power transmission which exceeds 100 MW. We have studied the mechanism of rf breakdowns of alumina disks theoretically with a computer simulation of multipactor phenomena, and also experimentally with a test facility based on a resonant ring. It was confirmed that at relatively low power transmission (< 40 MW) the multipactor process is pronounced. However, for discharge breakdowns at a higher power transmission (> 50 MW), impurities within alumina due to sintering binder materials and internal voids were shown to be mostly responsible. We have developed a technique to address those problems by using disks made from high-purity (99.9%) fine alumina powder, and by treating them with a hot isostatic pressing (HIP) method. An experiment at a resonant ring has shown that the new rf window based on this technique can successfully withstand a maximum peak transmission-power of 310 MW with a pulse width 2.5 μs at 10 Hz repetition rate, and a maximum average transmission-power of 300 MW with pulse width 2.5 μs at 50 Hz.

Use of coupling agents as a route to improvement of the compressibility of fine zirconia and alumina powders

In order to improve the compressibility of fine zirconia and alumina powders, powders were surface-treated with aluminate, silane and titanate coupling agents. The surface modification reduced both powder/powder friction and powder/die-wall friction, which increased the density of the compacts. At 2% additions, the effectiveness of the coupling agents on density increase was in the order, silane > titanate > aluminate. In zirconia systems with different titanate concentrations it was found the optimum amount of coupling agent could be approximated by the amount required for monolayer coverage on the powder surface.

アルミナ微粉末のホットフォージ焼結

Fine alumina powder prepared by an ammonium alum method was encapsulated in a stainless steel mold with the dimensions of O.D. 22φ∼I.D. 10φ×30hmm. The mold containing alumina was preheated at 1200 or 1300°C for about 8 minutes, and then immediately press-forged under uniaxial compressive load. The particle size of the starting powder (0.15μm) grew to about 0.2 or 0.3μm during the preheating, and a modarately dense polycrystal with the grain size of 0.2 or 0.3μm was obtained by the masstransport mechanism promoted during the deformation. The hot-forged polycrystalline alumina, however, did not show satisfactory hardness, because of some stored residual stress around the grain boundary. A polycrystalline alumina with excellent hardness was obtained after annealing the hot-forged body at a relatively law temperature of 1000 to 1100°C for 24 hours. The densification mechanisms of alumina powder during the hot-forging and annealing processes were investigated through microstructural observations by TEM and SEM.

Removal of organic vehicle from moulded ceramic bodies by capillary action

The removal of paraffin wax by capillary flow from ceramic mouldings consisting of a coarse and a fine alumina powder was studied. Several ceramic powders were selected for the powder bed and, as expected, the maximum extraction occurred when a coarse ceramic body was combined with a fine powder. In general, only 15–30% of the available pore space in the powder was used for extraction. Permeability estimates suggest that flow in the powder bed controls the rate of extraction when the surrounding powder is significantly finer than that of the body.

住宅における通風時の室内外の流れ場の可視化

A visualization of internal and external flow in single dwelling model with cross ventilation is conducted in wind tunnel. A fine alumina powder is used as tracing agent and a light sheet obtained by spreading a laser beam is used as a light source.The flow patterns in vertical sections and horizontal sections are photographed with various parameters of opening's position and room numbers in the model. Useful data are obtained for designing the natural ventilation systems through openings in this study.

Influence of the precipitation medium on the properties of fine alumina powders

Fine alumina powders have been synthesised by a homogeneous precipitation of aluminium sulphate by urea in water-glycerol, water-ethylene glycol and in pure water, followed by thermal decomposition of the hydroxide at 600°C. Alumina powder was also prepared by the conventional ammonia method for comparison. The powders were characterised by their particle size distribution, BET surface area, tap density and particle morphology (SEM). Point of zero charge of the powder was determined by the potentiometric method. The surface areas ranged from 43 m2/g to 105·5 m2/g, the particle size less than 0·5 micron from 50% to 12% and the point of zero charge from 7·05 to 7·85. The precipitation medium has been shown to have profound influence on the particle characteristics as well as on the surface charge of fine aluminium oxide powders.

Micropore Distribution Change of Heated Powder Compacts Using Binary Alumina Powder Mixtures

The sintering behavior of bimodal alumina powders was investigated with reference to the change of open pore size distributions. Avarage particle sizes of fine and coarse alumina powders were 0.52μm (f particle) and 6.3μm (c particle). The coarse powder was spherical and polycrystalline. The open pore size distributions in green compacts were narrow for specimens with mixing ratios c:f=4:6-1:9, but were broad for c:f=9:1-5:5. When sintered at 1400°-1600°C, the open pore size increased with c:f=9:1-4:6, remained unchanged with c:f=3:7-1:9, and decreased with c:f=0:10. It is considered that the growth of pores was caused by the difference of sintering rates between tight and loose-packed parts, and also between c and f particles.

Dual phase vitreous enamels Part 1 Preparation, application, and properties

Abstract Vitreous enamel coatings are prone to impact damage as a consequence of their low toughness. Previous attempts to improve this situation by adding fine aluminium powder particles have not been entirely successful because of the very high levels of contiguous porosity that result from the reaction of molten aluminium with hydroxyl ions. A new processing route is described that overcomes this problem and leads to high quality coatings having greatly improved impact resistance, excellent adhesion to steel substrates, and enhanced high temperature oxidation behaviour.MST/1015

Dual phase vitreous enamels Part 1 Preparation, application, and properties

Vitreous enamel coatings are prone to impact damage as a consequence of their low toughness. Previous attempts to improve this situation by adding fine aluminium powder particles have not been entirely successful because of the very high levels of contiguous porosity that result from the reaction of molten aluminium with hydroxyl ions. A new processing route is described that overcomes this problem and leads to high quality coatings having greatly improved impact resistance, excellent adhesion to steel substrates, and enhanced high temperature oxidation behaviour.MST/1015

An easy modification of epoxy adhesive agents for low-temperature use

Various epoxy adhesive agents mixed with fine alumina powder have been used at low temperatures as an alternative to Stycast 2850GT.

Liquid break-up in gas atomization of fine aluminum powders

Formation of powder particles has been studied in aconfined design atomizing nozzle. Liquid metal (AA 2014) is presented in the form of a thin film to the atomizing gas in this type of operation. Upon contacting, the film breaks up into large droplets of diameters up to 500 /im (primary break-up). These droplets undergo further disintegration in flight to produce the powder (secondary disintegration). Photographs taken using a high-speed flash indicate that this takes place by stripping. It is proposed that the fine range of the particles are the products of stripping break-up, while the coarse range are stable particles obtained when stripping stops and the remaining particle becomes too small to undergo further disintegration. Size distribution curves often contained two or more peaks providing support for different formation mechanisms for fine and coarse particles. All particles were dense and single droplets except for the very large ones (>55 μm)which had satellites of fine particles on the surface and showed porosity in some cases. Particles <30 μm in general were fully spherical, whereas larger ones also showed oblong features. No evidence was found for small particles agglomerating to produce large ones.