Papers by Author: J. Grabis

Abstract: Dense B₆O based materials were prepared by reactive spark plasma sintering of B/B₂O_3, B/B₂O₃/ZrO₂ B/ZrO₂, B₆O/ZrO₂ powders and parameters of the materials were determined. The reactive plasma sintering ensured preparation of dense B₆O materials at 1800 °C and pressure of 30 MPa. Additives of nanocrystalline ZrO₂ to boron precursors led to formation of ZrB₂-B₆O composite with decreased amount of B₆O. The crystallite size of the bulk materials was in the range of 18–40 nm but the size of unregular grains was in the range of 1.5–3.0 μm.

Abstract: 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.

Abstract: The influence of glycine, glycerine, ethylene glycol and citric acid fuel and their ratio to NO₃^- on formation and dispersity of Bi₂WO₆ nanoparticles prepared by combustion synthesis has been studied. The pure crystalline Bi₂WO₆ with specific surface area 24,8 m²/g and crystallite size of 28 nm was obtained by using glycerine as fuel at its ratio to NO₃^- of 0,67. The photocatalytic activity of the prepared Bi₂WO₆ in degradation of methylene blue depended on its specific surface area of samples and solution pH.

Abstract: The influence of different molten salt environments on formation of tungstates has been studied. Well crystalline and pure Bi₂WO₆ have been prepared in potassium nitrite KNO₂ (350°C), potassium chloride KCl (800°C) and mixture of potassium-sodium sulphates K₂SO₄-Na₂SO₄ (800°C), but ZnWO₄ have been successfully synthesized in sodium nitrate NaNO₃ (350°C), mixture of sodium-potassium chlorides NaCl-KCl (680°C), potassium chloride KCl (780°C) and mixture of potassium-sodium sulphates K₂SO₄-Na₂SO₄ (850°C). The higher surface area and photocatalytic activity show powders prepared at lower temperature.

Abstract: The TiB₂-TiN_0.1 C_0.9 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.

Abstract: Multi-component blends of polypropylene (PP), ethylene-1-octene copolymer (EOC) and zinc oxide (ZnO) are prepared by melt mixing using two-roll mill. Samples prepared by compression moulding have been investigated by using differential scanning calorimeter (DSC). It is found that addition of ZnO particles decreases PP melting point and increases crystallinity degree of PP. ZnO works as nucleation agent for PP by increasing quantity of crystallization centers, but at the same time decreasing structure quality. Because of the structure change mechanical properties are affected. At lower nanoparticle concentration some reinforcement effect I observed, while at higher particles concentration brittlening occurs. Crystalline structure and particles nature increases thermal stability of PP and PP/EOC blends.

Abstract: TiN and TiN/TiB_{Subscript text}2 nanoparticles with crystallite size of TiN in the range of 27–38 nm and TiB_{Subscript text}2 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 TiB_{Subscript text}2 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.

Abstract: Structure and magnetic properties of nickel oxide (NiO) nanopowders have been studied by X-ray/neutron diffraction, SQUID magnetometer, and micro-Raman spectroscopy. Our diffraction data indicate that at room temperature all NiO powders are antiferromagnetically ordered and have a rhombohedral (R-3m) phase. The SQUID magnetometry and Raman spectroscopy measurements support the presence of the antiferromagnetic ordering.

Abstract: 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.