Dao Yuan Yang

Effects of Graphite Mold on Cooling Process of Fused AZS 33# Refractory

This study analyzed effects of different thickness(TH) and thermal conductivity of graphite mold on temperature, liquid phase percentage and conductive heat flux, the cooling process of fused cast AZS 33# refractory could be simulated by using COMSOL Multiphysics software. The results show that: when the graphite mold thickness increased, surface center temperature and liquid phase area of casting decreased gradually, but conductive heat flux increased gradually, the cooling rate of casting increased, which were helpful to form fine crystals near the casting surface. When using conventional graphite mold, the optimum thickness was less than 40mm, while using the high thermal conductivity graphite mold, the optimum thickness was 50mm. So the rapid and homogeneous cooling process of casting could reduce the possibility of material crack and obtain fine crystals structure of material; comparing with conventional graphite mold, high thermal conductivity graphite mold was more beneficial to achieve uniform and fast cooling process to improve the performance and passing rate of products.

Synthesis of Cordierite with Low Thermal Expansion Coefficient

Cordierite is an excellent material with good thermal shock resistance and used at high temperature for its low thermal expansion coefficient. Cordierite ceramics were prepared by using talc, alumina and kaolin clay as starting materials. The thermal expansion coefficient, phase composition and microstructure were studied and the results showed that: in order to get samples with low thermal expansion coefficient, the optimum chemical composition was a little rich in Al2O3 compared with the theoretical composition, the optimum sintering temperature was 1350°C, and adding 10% starch as pore-forming agent could effectively decreased the thermal expansion coefficient of the samples even to 0.8×10-6/°C. The samples contained majority of cordierite phase, with trace mullite and glass, the acicular cordierite crystals in samples developed very well and there were 10% starch powder used as pore-forming agent in formula. All these were the reasons to decrease the thermal expansion coefficient of cordierite material.

Preparation of Fe Coating Al2O3 Nanometer Composite Powder and its Mechanical Properties after Hot Press Sintering

Nano-Fe particles coating Al2O3 composite powders were prepared by heterogeneous precipitation method with nanometer -Al2O3 and Fe(NO3)3•9H2O as raw materials. The composite powders were analyzed by DSC-TG, XRD,SEM and Zeta potential. Results showed that Fe coating Al2O3 nanometer composite powders were obtained in the condition of being sintered at 500°C for 30min and reduced at 700°C for 1h in H2. The coating Fe nanometer particles are in the shape of sphericity with diameter about 30nm and the dispersion of the powders is uniform. Al2O3/Fe composite ceramics were obtained by hot-pressing (30MPa). The mechanical properties of the composite were investigated after hot press at different temperatures. With the increasing of Fe content in composite ceramics, the hardness of the composite is decreased. Fracture toughness of 10mol%Fe content is 5.62MPa after sintered at 1400°C, which is increasing 57% high than that of monolithic Al2O3 ceramics.

Dynamics of Crack Healing and its Molecular Dynamics Simulation of Al2O3-MgAlON Composite

After preparing samples (3mm long×4mm wide×36mm high) of Al2O3-MgAlON composites and sintered at 1500°C for 2 h in N2 atmosphere, samples’ cracks were carved by a Vickers hardometer’s pressing head on the center of the sample surface (4 mm×36 mm). Subsequently, the cracks were healed at 1000°C-1550°C for 6 h respectively. Effects of healing temperature on sample’s strength, crack healing dynamics and its molecular dynamics simulation were investigated. The results suggested that: the optimum range of cracks healing temperature was 1300°C-1550°C, and the healing process accelerated at 1300°C, meanwhile, the strength of samples increased significantly. Cracks completely healing finished at 1550°C. The dynamics equation of crack healing was lnν = -Q/kT+lnC. Through characterizing the crack healing rate with the recovering rate of sample’s strength, the diffusion activation energy Q = 4.264 × 10-30 J•K-1 and diffusion constant C=7.359 were claimed. The result of the molecular dynamics simulation suggested that cracks healing process was caused by diffusion could be divided into five stages: passivation of crack tips, formation of salient island, crack shrinkage, generation of secondary crack, and complete healing.

Preparation and Growth Mechanism of Hexagonal MgAl2O4 Spinel Fibers

Hexagonal MgAl2O4 spinel fibers were synthesized at 1500oC for 6h in nitrogen atmosphere. The structure and morphology of the fibers were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The fibers grew via vapor- liquid- solid mechanism. The twin mechanism was suggested to play a key role in the formation process of the hexagonal fibers.

Riser Design of Cast System of Fused Zirconia-Alumina-Silica

Fused cast Zirconia-Alumina-Silica material (AZS) is the key refractory to glass furnace. In order to reduce production cost and optimize production process, the influences of riser position and riser size on temperature gradient, solidification time, residual melt modulus, solidification fraction, and cooling rate of cast system were studied by the finite element simulation method according to the brick size of 600 mm × 400 mm × 300 mm. It turned out that it would be more efficient in feeding when riser located at the center of maximum surface of a brick or when the height of riser is 250 mm-270 mm, while the volume ratio of brick to riser is 2.95-2.74, on condition that the size of riser upper surface is 450 mm × 450 mm and the bottom 150 mm×150 mm.

Grain Refinement Mechanism of MgAlON Composites Prepared by Spark Plasma Sintering

The paper developed a computing model to analyze the grain refinement mechanism of MgAlON composites sintered by spark plasma sintering (SPS). Using alumina, magnesia and aluminum as raw materials MgAlON composites were prepared by SPS. Based on the current assignment principle, the heating rate at different positions in Al grains was analyzed, and the particle radius during the sintering process could be calculated. The results suggested that necks could be formed between MgAlON particles with electric current heating rapidly to reduce particle radius, the shorter vertical distance from the interface was, the larger heating rate and growing rate of the neck had; the necks between particles be formed dynamically in the sintering process, so the big particle radius would be getting smaller and smaller continually so that we could get a large amount of micro particles distributed uniformly in the sintered MgAlON composites, and the resulted grain refinement was more obvious.

Current Distribution Model and Control Principle of MgAlON Composites by Spark Plasma Sintering (SPS)

The current distribution had remarkable effect on the microstructure and properties of MgAlON composites in the process of spark plasma sintering (SPS). MgAlON composites, using alumina, magnesia and aluminum as starting materials, were prepared by SPS. Based on the current distribution method, the current distribution model on samples and single grain was established, and the basic electric current control principle was analyzed. It was found that the current conveyance by sample or single Al grain could be expressed by a formula including B and I.In the formula, B was the Al volume fraction and I was the total current. When the Al volume fraction was controlled within 12%, the current conveyance by sample increased significantly with the Al addition increasing. Meanwhile, the current conveyance by single Al grain decreased significantly. The sintering speed and properties of MgAlON composites by SPS would show a significantly change.

Research the Performance of Alumina Insulation Material by Freeze-Drying

The traditional method of preparation alumina insulation material includes the addition of pore-forming agent, direct foaming, foam impregnation and gel-casting. In this experiment, α-alumina as raw material, silica fume as an additive, Combination freeze-drying method, add pore-forming agent and direct foaming successfully prepared low-density, high strength, low thermal conductivity of alumina insulation material. Change the particle size of pore-forming agent can be get different properties of the sample. The SEM photograph was clearly observed that the hole wall dense uniform, α-alumina particles sufficient contact, no significant ice sublimation hole left. There are also the reasons of the sample with higher value of bending strength and compressive strength. This can make a control of porosity, as well as pore size, pore shape and pores space topology of alumina insulation material.

Electronic Binding Energy Changes of Elements in Al2O3-MgAlON Composite during the Crack Healing Process

Using Al, MgO and Al2O3 as raw materials, Al2O3-MgAlON composite was synthesized in N2 atmosphere by hot-press sintering at 900°C. Cracks prepared on the sample surface were healed at 1200°C×6hrs. Phase composition and element chemical state of sample were studied before and after healing. The results showed that: after healing process Al and MgO phase disappeare, and AlN and MgAlON phase appear, all kinds of elements in Al2O3-MgAlON material change their chemical state after the healing treatment, the changed electronic binding energy is Al 0.05ev, Mg 0.08ev, O 2.58ev or N 1.02ev respectively, and the chemical composition and crystal structure of MgAlON phase also change, all these change are helpful for crack healing process of Al2O3-MgAlON material.