Nanosized Powder Compacts Research Articles

Nanopowders in dynamic magnetic pulse compaction processes

A theoretical study of the magnetic pulse compaction of nanosized powders is carried out. The uniaxial pressing and the radial (biaxial) compaction of powders using z- and θ-pinch setups are analyzed. The objects of study are two model systems that correspond to alumina-based nanopowders. System I corresponds to powders not inclined to strong aggregation and system II corresponds to powders in a strong aggregate state. Owing to the inertial effects under the radial pressing of conductive shells, high pressures are reached that exceed the initial “magnetic pressure” several times. A dimensionless number is found for the z-pinch that determines the process dynamics; its value range is established where most effective, “resonance,” conditions are realized.

Fabrication of Nanocrystalline Dense Silicon Nitride Ceramics

Since the properties of the bulk ceramics are dependent on the grain size the ceramic materials with nanoscale grain size is of interest. Sintering of nanosized powder compacts to full density with minimum grain growth is a difficult task to achieve due to its high surface energy. Two step sintering methode was developed to enhance densification with minimum grain growth for silicon nitride ceramics with liquid phase. Starting with nano-sized silicon nitride powder two step sintering methode gives rise to a very fine-grained b-Si3N4 matrix with large agglomerated Si2N2O grains due to its high surface oxygen content. Addition of Y2O3 shifts the composition point to a primary phase field with no Si2N2O, gives rise to b-Si3N4 with nano scale grain size and near full density. Carbothermal reduction method was employed to reduce the oxygen content in nano-sized silicon nitride powder to give nanocrystalline dense silicon nitride ceramics without Si2N2O formation. Use of SPS was effective to suppress the grain growth and to give near full density. Microstructural development and mechanical properties will be reported.

Fluorescence emission enhancement of transparent Nd:YSAG ceramics by Sc2O3 doping

Up to 40% substitution of scandium for aluminum in YAG ceramics has been achieved, and the transparent Nd-doped Y3ScxAl(5−x)O12(Nd:YSAG) ceramics were successfully fabricated by sintering nanosized powder compacts under H2 atmosphere. It was found that 40% scandium substitution for aluminum can significantly enhance the light-emission intensity around 1064 nm of the Nd:YSAG ceramics by 2-3 times as compared with Nd:YAG single crystal without Sc2O3 doping when pumped with the same 808 nm diode laser. In addition, the material was found to have prolonged fluorescence lifetime.

Cold isostatic compaction of nano-size powders: Surface densification and dimensional asymmetry

Cold isostatic pressing (CIP) is often used in the compaction of nano-sized powders. For technological reasons, however, uniaxial pressing prior to CIP takes place. This paper reveals the first quantitative measurements of density gradients within and the asymmetric sintering response of nanoscale zirconia compacts formed by (i) simple uniaxial compaction and (ii) specific ratios of uniaxial and CIP pressure. We find that CIP forms an exterior “skin” of higher but variable surface density and decreases the width of the density distribution. It does not eliminate density gradients; nonuniform shrinkage still occurs during sintering. The high- and low-density zones (the moving and fixed ram ends, respectively) that form during uniaxial compaction are reversed during CIP. Considering both density distribution width and spring-back cracking, the “best” uniaxial-CIP pressure combination is 1–20 ksi for this particular powder and an L/D of 1.0. The greater final compaction of the low-density zone during CIP causes relatively large variations in final dimensions (nearly 400 microns) in spite of the smaller density distribution width. The usually neglected uniaxial pressing step has definite technological impacts on the production of nanostructured components via compaction.

Synthesis and dynamic compaction of ceramic nano powders by techniques based on electric pulsed power

Studies on the production of nanometer-sized powders by the exploding wire method were conducted. The aim of the work was to attain a particle distribution with a high yield of particles below 20 nm in size. Powders of alumina and titania were generated during the experiments by maintaining a definite partial pressure of oxygen inside the explosion chamber. The particles are spherical in shape with specific surfaces up to 100 m 2/g. The output capacity is on the order of 1 kg/h with an energy consumption of 2 kWh/kg. Alumina powders crystallize in the gamma and delta structures, titania powders consist of anatase and rutile. The second part of the studies deals with the compaction of nano-sized powders by application of pulsed electro-magnetic power. For the compaction of nano-sized Al 2O 3 powder pressure pulses in the range from 1 GPa to 5 GPa, lasting 3 – 300 μs, were applied. The simply shaped bulk samples maintained a nano-crystalline structure and reached 62 to 83 % of the theoretical density.