Fabrication And Properties Of Porous Silicon Nitride Ceramics By Freeze-drying Process

Due to the excellent properties of low thermal conductivity, low density, high permeability, high specific surface area, high temperature resistance, corrosion resistance and so on, porous ceramics are widely used in chemical industry, metallurgy, petroleum, textile, pharmaceutical, aerospace and other fields. Freeze-drying method, as a novel type for preparation of porous ceramics with special microstructure, has attracted the interest of scholars both at home and abroad. In this experiment, with the study of the properties of water base Si3N4 ceramic slurry, the experiment determined the type of dispersant, p H value of slurry and milling time. Then, the decompression-drying schedule, the ingredients and the sintering schedule were determined. The experiment also systematically studied the influence of the content of dispersant, binder and solid phase on physical properties, mechanical properties, microstructure, dielectric properties and pore structure of porous Si3N4 ceramics. Besides, the experiment studied the mechanism of forming-pore. Furthermore, the experiment successfully controlled and optimized the microstructure and properties of porous ceramics by adding glycerol and silica sol.Firstly, the experiment studied the rheological properties and Zeta potential of the ceramic slurry by adding the dispersant and changing the p H value. Then the experiment determined the optimum dispersant of PAA-NH4 and optimum p H of about 10. When the content of dispersant was 0.8 wt%, the ceramic slurry had the smallest viscosity value; when the p H was about 10, the slurry had the largest Zeta potential.Secondly, with the analysis of freeze-drying principle and dynamics, the experiment designed and manufactured the freeze-drying mold. Besides, the experiment determined the parameters of the process, including milling time of 12 h, low temperature storage of 5h, vacuum-drying of 55 h, the experimental formulation of Si3N4: Y2O3: Al2O3 for 92:6:2. Finally, the samples were sintered at 1700 oC under 0.3 MPa air pressure for two hours with the heating rate of 5 oC/min.Then, the effects of different volume contents of dispersant, binder and solid phase on physical properties, mechanical properties, microstructure properties and dielectric properties of porous ceramics were investigated. The experimental results showed that when the amount of the dispersant was 0.8 wt%, the porosity of the sample was about 63% and the maximum flexural strength of the sample was 33 MPa. The pore size distribution showed that the quantities and size of the large pores decreased with the increasing content of binder. The quantities of the small pores increased and the size of small pores were unchanged. Through the research on the influence of solid phase content on microstructure and properties, it suggested that when the solid phase content increased from 10 vol% to 40 vol%, the porosity decreased from 87.8% to 87.8%, the volume density increased from 0.39 g/cm3 to 1.9 g/cm3, shrinkage rate increased from 20.6% to 20.6%, the flexural strength increased from 0.1 MPa to 94.7 MPa, the compressive strength increased from 1.3 MPa to 314 MPa, the dielectric constant increased from 1.4 to 3.2, dielectric loss increased from 2×10-3 to 1×10-2 respectively. The results of pore size distribution showed that the quantities and size of the large pores decreased gradually with the increasing volume content. The quantities of the small pores increased and the size of small pores were unchanged. The results of SEM showed that when the solid content was more than 40 vol%, the typical microstructure of the layered structure derived from the freeze-drying method would disappear, which turned into a dense microstructure at length.Finally, with the addition of glycerol, the experiment successfully altered the microstructure of porous ceramic, which resulted from the influence to the crystallization behavior of ice. The results of pore size distribution showed that when the glycerol content was more than 3 vol%, a new kind of pore size appeared and the bimodal distribution of pore size turned into a three-peak distribution. The quantities of new pore size increased and the original pore size decreased. The results of SEM showed that with the increasing content of glycerol, the layered structure and dendritic microstructure gradually disappeared and became the inerratic cellular microstructure, which resulted in the decreasing of dielectric constant from 2.1 to 1.7.The addition of silica sol successfully altered the phase transformation of the sample at high temperature. The results of XRD showed that the phase composition of sample transferred from single phase of β-Si3N4 into multi-phases of β-Si3N4, yttrium silica phase(Y-Si-O) and O’-sialon(Si-Al-O-N) which has more excellent oxidation resistance. The results of pore size distribution showed that the quantities of small pore decreased and gradually disappeared with the increasing content of silica sol, which turned the bimodal distribution of pore size into the unimodal distribution. The results of SEM showed that rod-like Si3N4 crystals turned into thickflat crystals which consisted of Si3N4 crystals, O’- sialon grains and yttrium silica phase, which made the dielectric constant increase with the increasing content of silica sol and the frequency in the P-band.