High-pressure Spark Plasma Sintering Research Articles

Highly transparent α-alumina obtained by low cost high pressure SPS

Highly transparent pure alumina (65.4%) with an average grain size of 200nm was fabricated using high-pressure (>400MPa) spark plasma sintering. For the first time, highly transparent alumina ceramics were produced using low cost carbon fiber composite for the pressing punches. The high pressure device was also used to fabricate fully dense nanostructured alpha alumina with average grain size of 107nm. The developed low cost high pressure sintering apparatus might be applied to produce a wide range of pore free nanostructured materials.

Effect of Alumina Dopant on Transparency of Tetragonal Zirconia

Aiming to characterize the effect of alumina dopant on transparency, powders of yttria stabilized tetragonal zirconia doped with alumina (TZ‐3Y‐E) are used as starting material to fabricate transparent tetragonal ZrO2 by high‐pressure spark plasma sintering (HP‐SPS). However, low transparency of the resultant TZ‐3Y‐E specimens does not suggest a beneficial effect of alumina dopant although nanometric grains and high density have been achieved. The mechanism is analyzed by comparing with the results of as‐sintered yttria stabilized tetragonal zirconia without alumina dopant.

High-pressure spark plasma sintering of MgO-doped transparent alumina

Transparent alumina doped with MgO is fabricated by spark plasma sintering (SPS) under high pressures. An optimum sintering condition explored by changing the combination of the applied pressure and sintering temperature led to an in-line transmission close to the maximum reported for alumina in earlier studies. The effects of pressure on both grain size and transmission are strongly dependent on the sintering temperature: negligible effect at low temperatures and significant effect at high temperatures.

Effect of sintering temperature on optical properties and microstructure of translucent zirconia prepared by high-pressure spark plasma sintering

Aiming to characterize the effect of sintering temperature on transparency of zirconia, we have evaluated the optical properties and microstructure of translucent cubic zirconia prepared by high-pressure spark plasma sintering (SPS) at 1000–1200 ○C. Color centers (oxygen vacancies with trapped electrons) and residual pores were primary defects in the samples. In SPS samples, the total forward transmittance and in-line transmittance are mainly affected by color centers with a limited contribution from residual pores; in contrast, the changes in reflectance are only related to the porosity. The amounts of color centers and residual pores increase with sintering temperature that reduces the total forward and in-line transmittance of the as-sintered zirconia. Annealing in oxidizing atmosphere improves the total forward and in-line transmittance. During the annealing, the concentration of color centers decreases but the porosity increases.

Highly Infrared Transparent Nanometric Tetragonal Zirconia Prepared by High‐Pressure Spark Plasma Sintering

Highly infrared transparent tetragonal ZrO2 (3 mol% yttria‐stabilized) was prepared by means of high‐pressure spark plasma sintering (HP‐SPS). The crystallite size of the resulting dense sample was about 80 nm. In‐line transmittance for the 1.5 mm thick sample ranged between 63% and 67% in the wavelength of 3–5 μm, representing 81%–87% of the theoretic value. Meanwhile, the cutoff wavelength of tetragonal ZrO2 was superior to the values of other high‐strength transparent ceramics. This study highlighted the efficiency and simplicity of HP‐SPS for obtaining transparent tetragonal ZrO2 and the great potential of this ceramic applied for durable infrared windows.

Transparent hydroxyapatite ceramics with nanograin structure prepared by high pressure spark plasma sintering at the minimized sintering temperature

A hydrothermally processed bulky powder composed of loosely aggregated nano-sized rods was consolidated by spark plasma sintering. The use of a high pressure cell allows the application of pressure up to 500 MPa. It was found that applying of high pressure is beneficial for widening up the kinetic window for attaining dense HAp nanoceramics. The high transparency of HAp nanoceramics obtained in this study is ascribed to the high density and homogeneous nano-grained structure achieved besides the unique intrinsic optical properties of the HAp crystal itself, i.e. its low refractive index and very small birefringence. Achieving full densification at the minimized sintering temperature allows for the first time the preparation of transparent HAp nanoceramics with stoichiometric composition, i.e. avoiding the loss of structural water that commonly takes place during the conventional ways of sintering.

Activation energy of tantalum–tungsten oxide thermite reactions

The activation energy of a sol–gel (SG) derived tantalum–tungsten oxide thermite composite was determined using the Kissinger isoconversion method. The SG derived powder was consolidated using the high-pressure spark plasma sintering (HPSPS) technique at 300 and 400 °C. The ignition temperatures were investigated under high heating rates (500–2000 °C min −1). Such heating rates were required in order to ignite the thermite composite. Samples consolidated at 300 °C exhibit an abrupt change in temperature response prior to the main ignition temperature. This change in temperature response is attributed to the crystallization of the amorphous WO 3 in the SG derived Ta–WO 3 thermite composite and not to a pre-ignition reaction between the constituents. Ignition temperatures for the Ta–WO 3 thermite ranged from approximately 465 to 670 °C. The activation energies of the SG derived Ta–WO 3 thermite composite consolidated at 300 and 400 °C were determined to be 38 ± 2 kJ mol −1 and 57 ± 2 kJ mol −1, respectively.

Tantalum–tungsten oxide thermite composites prepared by sol–gel synthesis and spark plasma sintering

Energetic composite powders consisting of sol–gel derived nanostructured tungsten oxide were produced with various amounts of micrometer-scale tantalum fuel metal. Such energetic composite powders were ignition-tested and the results show that the powders are not sensitive to friction, spark and/or impact ignition. Initial consolidation experiments, using the High-Pressure Spark Plasma Sintering (HPSPS) technique, on the sol–gel derived nanostructured tungsten oxide produced samples with higher relative density than can be achieved with commercially available tungsten oxide. The sol–gel derived nanostructured tungsten oxide with immobilized tantalum fuel metal (Ta–WO 3) energetic composite was consolidated to a density of 9.17 g cm −3 or 93% relative density. In addition, those samples were consolidated without significant pre-reaction of the constituents, thus retaining their stored chemical energy.

Electrical, structural, and microstructural characterization of nanometric La 0.9Sr 0.1Ga 0.8Mg 0.2O 3 − δ (LSGM) prepared by high-pressure spark plasma sintering

The synthesis of bulk nanometric La 0.9Sr 0.1Ga 0.8Mg 0.2O 3 − δ (LSGM) was investigated using the high-pressure spark plasma sintering (HP-SPS) method. The starting precursor nanopowders were synthesized by the Pechini method. Single phase, fully dense LSGM was obtained with a sintering cycle of 5 min under a uniaxial pressure of 700 MPa at sintering temperatures as low as 850 °C. This sintering temperature is several hundred degrees lower than those typically reported for LSGM materials. A considerable reduction in the final grain size of the sintered materials was consequently observed. Fully dense samples with an average grain size as low as 65 nm could be prepared by this method. Despite the short, low temperature sintering process a single phase material was obtained. Due to the small size of the constituting grains it was possible to estimate the specific grain boundary conductivity by impedance measurements.

A novel route for preparing binary Sm–Co bulk amorphous alloys

A novel route for preparing binary amorphous alloy was proposed using Sm–Co system as an example. The alloy ingots with given compositions were used as the parent material. The amorphous intermetallic powder obtained by ball milling the parent alloy was consolidated by the high-pressure spark plasma sintering. The bulk amorphous alloy was detected and confirmed by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The present route is potentially applicable to prepare a variety of binary and multicomponent bulk amorphous alloys with adjustable compositions.

Nanoscale Effects on the Ionic Conductivity of Highly Doped Bulk Nanometric Cerium Oxide

AbstractNanometric ceria powders doped with 30 mol % samaria are consolidated by a high‐pressure spark plasma sintering (HP‐SPS) method to form > 99 % dense samples with a crystallite size as small as 16.5 nm. A conductivity dependence on grain size was noted: when the grain size was less than 20 nm, only one semicircle in the AC impedance spectra was observed and was attributed to bulk conductivity. In contrast to previous observations on pure ceria, the disappearance of the grain‐boundary blocking effect is not associated with mixed conductivity. With annealing and concomitant grain growth, the samples show the presence of a grain‐boundary effect.

Processing and Properties of Ceramic Nanocomposites Produced from Polymer Precursor Pyrolysis, High Pressure Sintering and Spark Plasma Sintering

ABSTRACTSilicon nitride/silicon carbide nanocomposites and alumina-based nanocomposites were investigated in an effort to produce materials with high structural integrity and service properties. Bulk nano-nano composites of silicon nitride and silicon carbide were processed by crystallization of amorphous Si-C-N ceramics that were consolidated in-situ during pyrolysis of a polymer precursor. This material was developed for the purpose of examining the creep behavior of covalent ceramics when there is no oxide glassy phase at grain boundaries. Si3N4/SiC micro-nano composites were sintered by spark plasma sintering (SPS), aiming at better microstructural control and improved creep resistance. Composites of alumina with diamond, silicon carbide and metal (Nb) were developed by high pressure sintering and SPS. These composites maintain microstructures with a nanometric alumina matrix and are targeted for studying the toughening mechanisms and superplastic deformation mechanisms.