Ilmars Zalite

Nanosized Powders of Refractory Compounds for Obtaining of Fine-Grained Ceramic Materials

Most of ceramic materials are made of powders and therefore their properties depend to a great extent on the quality of the starting powders. The powder determines the processing, sintering behavior and the subsequent formation of the microstructure, which strongly affects many of the dense materials properties. One of the ways to produce ceramic materials with fine-grained structure is the application of nanopowders. Different methods are used for the production of nanopowders. One of them is the method of plasmachemical synthesis. Different nanopowders of refractory materials can be obtained by this method. Preparation of nanosized powders of nitrides and oxides and their composites by the method of plasmachemical synthesis, as well as the characteristics and possible advantages of nanopowders were investigated.

Some sialons prepared from nanopowders by hot pressing

Investigation of fabrication of ?-sialon and ?-/?-sialon ceramics from nanopowders by hot pressing (1800 and 1910 ?C) were carried out. Samples of sialons were made by using Si3N4-AlN nanopowders composites fabricated by the method of plasmachemical synthesis. The hardness of materials from nanopowders are of HV1 = 17,6-21,9 GPa, bending strength of ?3p = 520-800 MPa and fracture toughness up to Klc = 7,1 MPa.m1/2, depending on composition and compacting temperature.

Metal oxide mineral filler containing polymer nanocomposites

Metal oxide mineral filled polycarbonate (PC) nanocomposites have been investigated. Plasma-chemical synthesis has been used to obtain metal oxide nanoparticles (MON). Melt mixing has been utilized to obtain PC/MON composites with regularly organized nanostructure. Consequently, an increasing in MON content caused a small decrease of surface and volume resistivities and an increase of nanocomposites magnetic activity.

Coating of plasma-processed nanosized powders

The homogeneous Si3N4/SiC nanocomposites withsintering additives of alumina and yttria h ave been prepared by plasma technique and combining plasma technique with chemical deposition of oxides. The powders have been characterized by XRD, TEM, Photon Correlation Spectroscopy and electrokinetic titration method. Both preparation routes allow us to produce coated particles with bimodal particle size distribution and average particle size in the range of 40–70 nm, but their surface characteristics and phase composition are different. The surface characteristics of Si3N4/SiC and Si3N4/SiC–Al2O3–Y2O3 nanocomposites prepared by plasma technique are similar to pure Si3N4 nanoparticles. The chemical deposition of oxides on the Si3N4/SiC nanoparticles leads to surface characteristics similar to pure alumina. The influence of the peculiarities of the prepared nanocomposites on their processing have been proved by hot pressing. # 2003 Published by Elsevier Ltd.

Plasma Processed Nanosized-Powders of Refractory Compounds for Obtaining Fine-Grained Advanced Ceramics

One of the ways for the production of ceramic materials with a fine-grained structure is the use of nanopowders. Different methods are used for the production of nanopowders. One of them is the method of plasmachemical synthesis. Different nanopowders of refractory materials can be obtained by this method. The preparation of nanosized powders of nitrides and oxides and their composites by the method of plasmachemical synthesis, the possibilities to receive nanopowders with different particle size and the potential advantages of nanopowders were investigated.

Spark-Plasma-Sintering (SPS) of tungsten carbide and titanium carbonitride nanopowders

The spark-plasma-sintering (SPS) method was used for sintering of different tungsten carbide (WC) and titanium carbonitride (TiCN) nanopowders obtained by the method of plasma synthesis. Dense, fine-grained monophase materials (WC and TiCN) were obtained at relatively low temperatures. Obtained results were compared with that for hot pressing (HP) method.

The Effect of Raw Components on the Densification and Properties of Nanostructured Sialon Materials / Izejas materiālu ietekme uz nanostrukturētu sialona materiālu saķepšanu un īpašībām

Abstract Two sialon compositions (Y0,33Si10,5Al1,5O0,5N15,5 and Y0,5Si9,5Al2,5O1,0N15,0) were used to determine the effect of starting components on densification and properties of sialon materials. Plasma synthesized nanopowders (Si3N4, AlN, Al2O3, Y2O3 and 73 wt% Si3N4-27 wt%AlN nanocomposite) were used for the investigation. Materials were sintered using traditional or spark plasma sintering methods. Sintering temperature was reduced significantly, if Si3N4-AlN nanocomposite was used as one of the components. The increased amount of asialon phase and higher hardness were characteristic to materials obtained from individual Si3N4, AlN, Al2O3, Y2O3 components. Darbs veltīts dažādu izejas komponentu lomas noskaidrošanai sialonu kompozīcijas izveidē, kompakta materiāla ieguvē un ietekmē uz iegūtā materiāla īpašībām. Izgatavotas divas atšķirīgas α-sialona (Y0,33Si10,5Al1,5O0,5N15,5 un Y0,5Si9,5Al2,5O1,0N15,0) kompozīcijas kā izejvielas izmantojot augstfrekvences plazmā sintezētas Si3N4, AlN, Al2O3 un Y2O3 nanokomponentes (A sērija) un Si3N4-27 masas % AlN nanokompozītu (B sērija) ar Al2O3 un Y2O3 piedevām sastāva koriģēšanai. Kompaktu materiālu ieguvei izmantota klasiskā bezspiediena saķepināšana slāpekļa vidē (temperatūras celšanas ātrums 10 °C/min.; izturēšanas laiks 2 h) vai dzirksteļizlādes plazmas saķepināšana (temperatūras celšanas ātrums 100 °C/min.; izturēšanas laiks 5 min.) temperatūrās līdz 1700 °C. Kā ir pierādīts iepriekš, no nanopulveriem iegūto materiālu saķepināšanas temperatūra ir ievērojami zemāka nekā no submikrona izmēra rūpnieciski ražotiem pulveriem. Mūsu pētījumi liecina, ka būtiska loma sialona materiāla saķepšanā ir arī nanoizmēra izejas komponentu izvēlei. Piemēram, aizstājot atsevišķu Si3N4 un AlN nanopulveru maisījumu ar plazmā sintezētu tāda paša sastāva un dispersitātes Si3N4-AlN nanokompozītu, sialona saķepšanas temperatūra samazinās par aptuveni 100 °C. Turpretim, materiāliem, kas iegūti no atsevišķām Si3N4, AlN, Al2O3 un Y2O3 nanokomponentēm, ir raksturīgs palielināts α-sialona fāzes saturs un augstāka cietība. Tas varētu būt saistīts ar izmantoto nanopulveru veidošanās īpatnībām: iegūstot Si3N4- AlN nanokompozītu plazmā daudzas ķīmiskās mijiedarbības ir notikušas jau iegūšanas laikā - veidojas sialonu saturošas fāzes ar zemāku saķepšanas temperatūru un mazāku viskozitāti (piemēram, Si1,8Al0,2O1,2N1,8). Tāpēc paraugu, kas iegūti no Si3N4-AlN nanokompozīta, saķepšana notiek zemākā temperatūrā nekā paraugiem, kas iegūti no atsevišķām nanokomponentēm. Paraugu fāžu sastāvs ir atkarīgs no to ķīmiskā sastāva: rentgenstruktūras analīze parāda, ka α-sialonu fāzes saturs 1A paraugā ir aptuveni 50 % un paraugā 2A - aptuveni 100 %. B sērijas paraugos a-sialonu saturs ir ievērojami zemāks; tas arī nosaka materiālu īpašības, piemēram, cietību. Paraugu mikrostruktūru lielā mērā nosaka gan to ķīmiskais sastāvs, gan kompaktēšanas temperatūra. Materiālu, kas iegūti no nanopulveriem 1600 °C temperatūrā gan α-, gan β-sialonu fāžu kristalītu izmērs ir ~ 100 nm, graudu lielums ir 200-300 nm. Ievērojama adatveida kristālu veidošanās (īpaši 2A paraugam) sākas 1600-1650 °C. Vidējais adatveida kristālu diametrs ir aptuveni 200 nm un to garums ir līdz 2 μm.

Fine-Grained Si3N4-Containing Ceramics from Nanopowders

Different compositions of SiAlON materials have been prepared from separate nanopowders and their composites: Si3N4 – AlN, Si3N4, Al2O3 and Y2O3, produced by the method of plasma-chemical synthesis. Compositions have been pressure-less sintered in nitrogen medium up to 1750°C with a heating rate of 10°C/min and holding time for 2 hours or by spark plasma sintering (SPS) in vacuum at 1700°C with a heating rate of 100°C/min and holding time of 5 min. The densification behaviour of the materials at these sintering conditions depends insignificantly on the powder composition. It is possible to obtain dense ceramic materials with relatively fine-grained structure (200-300 nm) from nanosized powders at relatively low temperatures (1400 - 1600°C) with good mechanical properties.

Some SiAlONs Prepared from Nanopowders by Spark Plasma Sintering

Different compositions of α- and α-/β- SiAlON materials have been prepared from separate nanopowders and their composites: Si3N4 – AlN, Si3N4, Al2O3 and Y2O3, produced by the method of plasma-chemical synthesis. Compositions have been sintered by spark plasma sintering (SPS) method in vacuum at 1700 °C with a heating rate of 100 °C/min and holding time of 5 min. The densification behaviour of the materials at these sintering conditions depends insignificantly on the powder composition. It is possible to obtain dense ceramic materials with relatively fine-grained structure (200-400 nm) and good mechanical properties from nanosized powders at relatively low temperatures (1400-1600 °C). The holding time at applied compacting conditions is still too short to ensure the formation of α- SiAlON phase corresponding to the phase diagram. All samples consist of β- SiAlON’s of differing composition and this is the reason for relatively low hardness of samples (HV5 = 15,6-16,9 GPa).