Materials Science Forum Vols. 745-746

Abstract: The alumina-magnesia precast blocks developed in authors company have been successfully used in 90-ton LF refining with an average service life 80-85 heats to meet the smelting demand of low carbon steel and ultra low carbon steel. The observation of the residual blocks revealed that the section of the used blocks was divided into four layers from inside to the surface, i.e., slag layer, reaction layer, penetration layer and the original block. It has been found that there was a certain degree of structure spalling of the used blocks. The main factor of blocks damage may be due to the penetration and erosion of molten slag. The results of SEM microstructure analysis indicated that the calcium oxide (CaO) of high basicity slag easily penetrated into the blocks and formed CA₆ phase, and then transformed into CA₂ or C₁₂A₇ phases gradually with the decreasing content of alumina (Al₂O₃). It has been found that an annular distribution structure, and the silicon dioxide (SiO₂) was easily lost together with slag and formed SiO₂-rich part in the penetration layer due to the formation of low viscosity and low melting point phase of C₂AS. A large number of-5um spinel particles (MgAl₂O₄) were synthesized at 1600~ 1700, which could absorb FeO, MnO and other impurities in slag to form composite spinel phase. It is believed that the high melting point phases intertwined to form a dense area which was beneficial to improve the bricks' slag resistances to penetration and corrosion

Abstract: Mn-Co-Ni-O (Mn:Co:Ni=1.74:0.72:0.54, MCN) thin films with single cubic spinel structure were prepared on Si substrates by metal organic solution deposition (MOSD) method at different annealing temperatures. The effects of annealing temperature on the phase component, crystalline microstructure, surface morphology and electrical properties of the MCN thin films were studied. According to the results of x-ray diffraction pattern, the MCN thin film annealed at 650 had spinel structure. Observation with field emission scanning electron microscope (FE-SEM) on the MCN thin films showed that the grain size increased with increasing annealing temperature. The resistance measured at room-temperature was 18.143, 12.457, 2.435 and 3.141MΩ for the MCN thin films annealed at 650, 700, 750 and 800, respectively. The values of thermistor constant (B_30/85) and activation energy (Ea) were in the range of 3260-4840K and 0.28-0.42eV, respectively.

Abstract: Magnesium aluminate spinel specimens with various Al₂O₃ content were prepared. Sintering performance and microstructure of different kinds of MgAl₂O₄ spinel were studied. It is found that the sintering of materials becomes more difficult with the increase in their Al₂O₃ contents. Corundum begins to precipitate from the magnesium aluminate spinel with 85wt% Al₂O₃ at 1400. However, with further increase in the sintering temperature, the corundum dissolves into magnesium aluminate spinel (above 1600). The precipitation and solid solution of corundum hinder the sintering of materials. In alumina-rich spinel, if the content of Al₂O₃ in alumina-rich spinel materials is more and the content of CA₆ is also more. At the same time CA₆ crystal is bigger in the alumina-rich spinel.

Abstract: The microstructure of Al₂O₃-Cr₂O₃ refractories is an important factor to affect its high temperature performance. The Al₂O₃-Cr₂O₃ green bodies were prepared by hot pressing using aluminum oxide and Cr₂O₃ powder mixture by adding a binder. And then the specimens were sintered at 1650 for 4h in the electric muffle furnace. Properties of specimens with different species of Al₂O₃ powders were studied, including α-Al₂O₃ powder, ρ-Al₂O₃ powder, and fused corundum powder. Moreover, three sizes of α-Al₂O₃ powder (D₅₀=0.8 μm, 1.4 μm, 4.0 μm) were used as additive. Properties of specimens, including apparent porosity, bulk density, cold modulus of rupture, pore size distribution were tested. The morphology of sintered specimens was analyzed by the Scanning Electron Microscope. The results showed that the specimens adding α-Al₂O₃ powder had the better properties since α-Al₂O₃ powder has higher sintering activity, and it was more efficient for Al₂O₃-Cr₂O₃ solid solution. The specimens with α-Al₂O₃ had lower porosity, higher bulk density and cold modulus of rupture, and more uniform pore distribution. There were great differences in sintering activity for specimens with different particle size of α-Al₂O₃ powder. And the microstructure of Al₂O₃-Cr₂O₃ was significantly dissimilar. The specimen with the addition of the activated alumina powder of D₅₀ with the size of 1.4 μm and bimodal size distribution showed the perfect performance, including high density and high flexural strength. The experimental results showed that the microstructure of this specimen was uniform and its pore size was homogeneous. This special microstructure is beneficial for improving the slag resistance and thermal shock resistance of Al₂O₃-Cr₂O₃ refractories.

Abstract: PbS quantum dots (QDs) were prepared through a two-phase approach and an aqueous method respectively. The reaction conditions of the two-phase approach were mild and highly controllable, but the as-prepared QDs were capped with oleic acid. Although the PbS QDs prepared by the aqueous method did not exhibit excellent photoluminescence properties, it was more facile and environmentally friendly, and furthermore the resulting PbS QDs were more promising in biological labeling without ligand exchange. We have investigated the absorption and luminescent properties of QDs prepared by the two methods. The results showed that the absorption shoulder of oil soluble PbS QDs was at 580nm and water soluble PbS QDs was at 600~700nm. And the emission peaks in the photoluminescence spectra were at 755nm and 970nm, respectively. XRD analysis confirmed that the PbS QDs were cubic phase. The TEM results showed that the size of both types of PbS QDs was smaller than the excitons Bohr radius (18nm).

Abstract: Joining of the high strength graphite to itself has been realized using silicon resin YR3370 with SiO₂ powders as joining materials. The joining strength of the joints is strongly affected by adding SiO₂ powders and joining temperature. Adding SiO₂ filler can effectively reduce holes and cracks in the joining interlayer. Therefore it can enhance the joining strength. The maximum shearing strength of the joints is 21.1MPa, which is equal to 52.8% of the strength of the graphite substrate. This value is obtained at the joining temperature of 1200°C. The thickness of interlayer is about 25μm. The joining interlayer is uniform and dense, and the contact at the interfaces is good.

Abstract: 5wt%B₂O₃-doped (Li,K,Na)NbO₃ (abbreviated as LKNN-B5)-based lead-free piezoelectric ceramics were prepared via solid state synthesis. The electrical humidity sensitive properties of LKNN-B5 ceramics were examined and compared with pure LKNN ceramics. The results revealed thatB₂O₃ doping enhanced electrical stability of LKNN-based ceramics in relative humidity (RH) range of 33-95%. The capacitance change was suppressed from 105% down to 43% when doped with B₂O₃. The average absorption time was reduced from 5.5 min to 1.5 min, and desorption time decreased from 8.5 min to 2.0 min. The enhanced electrical stability of LKNN-B5 series was attributed to the dense structure induced by liquid phase sintering.

Abstract: The specific fracture energy of alumina-spinel refractory castables was studied by the wedge-splitting method in this paper. The influence of synthetic spinel on the specific fracture energy of aluminamagnesia refractory castables was investigated. In-situ formation spinel was replaced by sintering spinel as the synthetic spinel. The experimental results indicated that the specific fracture energy and the compressive flesural mechanical properties had the same changing trend with the increase of the synthetic spinel, but the thermal shock resistance of the materials showed the opposite trend. However, the thermal shock resistance was closely associated with the specific fracture energy according to one theory raised by Hasselman.

Abstract: This paper presented the application of microwave-discharge plasma apparatus which was used to evaluate the catalytic properties of ZrB₂-based ultra-high temperature ceramics in the simulated real service environment by the wall temperature response method based on the heat balance principle. The results showed that the material composition had a significant influence on the catalytic properties of ZrB₂-based ultra-high temperature ceramics, and the catalytic activity of ZrB₂-SiC composites with Cr addition had been increased significantly. The relationship between catalytic properties of ZrB₂-based ultra-high temperature ceramics and surface composition was discussed in detail. The composition optimization was considered to be a very effective way to inhibit the recombination reactions of dissociated atoms on the surface of ultra-high temperature ceramics.

Abstract: Sheet nanocarbon-Al₂O₃ composite powders were produced by vibratory milling using expanded graphite and Al₂O₃ powders as raw materials. The effect of different vibratory milling time on phase compositions and microstructure of the composite powders were investigated by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM). Then sheet nanocarbon-Al₂O₃ composite powders were used as additives to produce low carbon Al₂O₃-C refractories which had a carbon content of 6%. The results showed that about 50nm thickness sheet nanocarbon and Al₂O₃ composite powders were produced via 15h milling with expanded graphite to Al₂O₃ powders weight ratio of 1:2. With the increasing of milling time, the (002) diffraction peaks of graphite wear off gradually and nanosheet was desquamated from expanded graphite; It is beneficial to improve the bulk density, cold modulus of rupture (CMOR) and thermal shock resistance of Al₂O₃-C refractories when adding certain sheet nanocarbon-Al₂O₃ composite powders. The sample containing 0.5% sheet nanocarbon had a CMOR of 18.51MPa after 1000 °C heat processing in N₂ atmosphere, while sample without adding any sheet nanocarbon was only 12.35MPa; The residual cold modulus of rupture (CMOR_TS) of sample containing 1% sheet nanocarbon was 4.22MPa after thermal shock test, while that of sample without nanocarbon was only 2.18MPa.