Ints Šteins

Spectral characterization of bulk and nanostructured aluminum nitride

Spectral characteristics including photoluminescence (PL) spectra and its excitation spectra for different AlN materials (AlN ceramics, macro size powder and nanostructured forms such as nanopowder, nanorods and nanotips) were investigated at room temperature. Besides the well known UV-blue (around 400 nm) and red (600 nm) luminescence, the 480 nm band was also observed as an asymmetric long-wavelength shoulder of the UV-blue PL band. This band can be related to the luminescence of some kind of surface defects, probably also including the oxygen-related defects. The mechanisms of recombination luminescence and excitation of the UV-blue luminescence caused by the oxygen-related defects were investigated. It was found that the same PL band is characteristic for different AlN materials mentioned above; however, in the nanostructured materials (nanorods, nanotips and nanopowder) the intensity of UV-blue PL is remarkable lower than in the bulk material (ceramics). In the case of nanostructured AlN materials, excitation of the oxygen-related defect is mainly realized through the energy transfer from the host material (electron/hole or exciton processes) to the defects and this mechanism prevails over the mechanism of direct defect excitation.

Spark plasma sintering of SiAlON nanopowders

The spark plasma sintering (SPS) method was used for sintering of different SiAlON nanopowders obtained by the method of plasma synthesis. Almost entirely dense samples were obtained at 1450-1500 °C (the range of α-SiAlONs) and under 1600 °C (the range of α-/β-SiAlONs). Results of research were compared wit that obtained by the method of pressureless sintering. Samples obtained at 1700 °C have similar characteristics, differing only with the time of processing: by the SPS method the sample was obtained during 23 minutes (100 °C/min; 5 min.), but by the pressure-less sintering – during 290 minutes (10 °C/min; 120 min.). Material obtained by the pressure-less sintering contains the phase of α- SiAlON, but samples obtained by the SPS process contain almost only the phase of β- SiAlON and the reason of this effect is a specific crystallization process of nanopowders: firstly at temperatures over 1400 – 1450 °C β- SiAlON is formed and only after this the formation of α- SiAlON occurs over 1500-1550 °C. In the case of SPS process a rapid increase of temperature (100 °C/min) and short dwelling time at high temperatures (5 min.) is the reason, why the transition of β- SiAlON into α- SiAlON does not occur.

Characterisation of mullite – ZrO2 ceramics prepared by various methods

Mullite – ZrO2 ceramics was sintered from variously prepared powder mixtures -different time milled and hydrothermal synthesized. As sintering aid 8 wt. % illite clay for one part of starting mixtures was added. Two sintering routes was applied for consolidation of powder – spark plasma sintering (SPS) technique and conventional sintering reactions in air. It is shown that the structure of sintered samples for SPS was completed at 1250°C and by conventional - at 1300°C. The developed microstructure both conventional and SPS can be characterized by mullite matrix with evenly distributed ZrO2 grains. For conventionally prepared and sintered samples corundum and ZrO2 tetragonal grains are observed, but for SPS dominates ZrO2 cubic. The microstructure of ceramic samples from hydrothermal synthesized powders and consolidated by SPS is amorphous like, with xenomorfic crystals of mullite and inclusions of ZrO2 cubic grains. It is stated that additive of illite clay promotes the densification. At conventional sintering clay prevents the transformation of ZrO2 tetragonal to ZrO2 monoclinic by cooling of samples, but for SPS promotes formation of ZrO2 cubic. SPS sintered samples processed from conventionally milled powder mixture are characterized by pressure strength.

Characteristics of zirconia nanoparticles prepared by molten salts and microwave synthesis

Zirconia and yttria stabilized zirconia (3YSZ) nanoparticles were prepared from zirconia and yttria salts using molten salts (MS) and microwave (MW) synthesis. The crystalline ZrO2 and 3YSZ nanoparticles with crystallite size in the range of 3–27 nm were obtained by MS synthesis in NaCl–NaNO3 salts. The zirconia and 3YSZ powders with close characteristics were obtained by combining MW synthesis with calcination of products at 400-800 °C. The crystallite size depends upon synthesis or calcination temperature, and the precursors used. The powders prepared by MS and MW synthesis ensured manufacturing of bulk materials with relative density of 98.6% and 97.2% respectively by using spark plasma sintering at 1300 °C.

Preparation of YAG Nanoparticles and their Characteristics

The nanosized yttrium aluminium garnet powders doped with rare earth oxides are prepared by combustion synthesis using several organic fuel. Dense materials are manufactured by conventional sintering and spark plasma sintering (SPS). The combustion synthesis provides preparation of pure crystalline YAG nanopowders at ratio Y/Al = 3/5 after additional calcination at 1000 oC. The relative density of the SPS sintered samples at 1500 oC for 2 min is in the range of 95.4–98.5% depending on dispersity of powders.

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.

Reactive Spark Plasma Sintering of B-C-Ti-N Powders

The TiB2-TiN0.1 C0.9 and B4C-TiB2 composites with relative density of 95.7-98.6% were manufactured by SPS reactive sintering mixtures of amorphous boron with TiN or TiNC and carbon nanoparticles at 1700 °C and pressure of 30 MPa during 10 min. The phase composition of the materials was determined in dependence on molar ratio of Ti/B and B/C.

Characteristic and Sinterability of Alumina-Zirconia-Yttria Nanoparticles Prepared by Different Chemical Methods

The characteristics and sinterability of the Al2O3-ZrO2(Y2O3) nanoparticles produced by simple and effective microwave and molten salts methods and processed by using spark plasma sintering were studied and compared. The crystalline powders with the specific surface area in the range of 72–108 m2/g and crystallite size of 5–13 nm were obtained by calcination of samples prepared by both methods at 800 °C. The content of t-ZrO2 phase depends on concentration of Al2O3, Y2O3 and on calcination temperature but the impact of the preparation method is insignificant. The phase transition of tetragonal ZrO2 to monoclinic for the samples without Y2O3 started at 1000 °C though it was incomplete in the case of high content of Al2O3. The bulk materials with relative density of 86.1–98.7% were fabricated by the spark plasma sintering method at 1500–1600 °C depending on the content of Al2O3 and Y2O3.

Preparation of TiB2-TiN and TiN-B Powders and their Processing

TiN and TiN/TiBSubscript text2 nanoparticles with crystallite size of TiN in the range of 27–38 nm and TiBSubscript text2 in the range of 55–90 nm have been prepared by thermal plasma technique. The prepared nanoparticles and mechanical mixture of TiN with amorphous boron have been densified using spark plasma sintering and the microstructure and density of the samples were compared. The relative density of the samples with content of TiBSubscript text2 about 36 wt.% is in the range of 95.9–97.1% in dependence on the precursors. The higher relative density of the samples provided reactive sintering of TiN/B powder. The grain size of the composites in the range of 0.5–3 µm testified that spark plasma sintering intensified the grain growth in despite of the short sintering time.

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.