Silicon nitride ceramics and its aggregated structure determine that it is a hard and brittle material, which exhibits brittle fracture under stress. The characteristic temperature of silicon nitride ceramics is: embrittlement temperature of about -30°C, glass transition temperature of 80 to 105°C, melting temperature of 140 to 180°C, and decomposition temperature of 300°C or more. Since the mechanical properties of polystyrene decrease significantly with the increase of temperature and its heat resistance is poor, the continuous use temperature is about 60℃, and the highest should not exceed 80℃. The thermal conductivity is low, ranging from 0.04 to 0.15W/(m·K), and it is hardly changed by temperature, so it has good thermal insulation. Polystyrene has good electrical properties, with volume resistivity and surface resistivity as high as 1016~1018Ω·cm and 1015~1018Ω, respectively. The dielectric loss tangent value is extremely low, and it is not affected by frequency, environmental temperature, and humidity changes. It is an excellent insulating material. Polystyrene has excellent optical properties, with a light transmittance of 88% to 92% and a refractive index of 1.59 to 1.60. It can transmit visible light of all wavelengths. The transparent material is second only to acrylic polymers such as plexiglass in plastics. . However, because polystyrene has poor weather resistance and is exposed to sunlight and dust during long-term use or storage, turbidity and yellowing will occur. Therefore, when using polystyrene to make high-transparent products such as optical parts, it is necessary to consider adding appropriate varieties and The amount of antioxidant. Good corrosion resistance, poor solvent resistance and oxidation resistance.
Silicon nitride ceramics is resistant to various alkalis, salts and aqueous solutions. It is resistant to lower alcohols and certain acids (such as sulfuric acid, phosphoric acid, boric acid, hydrochloric acid with a mass fraction of 10% to 30%, and a mass fraction of 1% to 25%). Acetic acid and formic acid with a mass fraction of 1% to 90%) are also stable, but concentrated nitric acid and other oxidants can destroy them.
Silicon nitride ceramics are soluble in many solvents similar to their solubility parameters, such as acetone, tetrachloroethane, styrene, benzene, chloroform, xylene, toluene, carbon tetrachloride, methyl ethyl ketone, esters, etc., but insoluble Mineral oil, aliphatic hydrocarbon, ether, phenol, etc., but can be swelled by them. Many non-solvent substances, such as higher alcohols and oils, can cause stress cracking or swelling of polystyrene.
Polystyrene is prone to aging under heat, oxygen and atmospheric conditions, causing macromolecular chain breakage and color development. When the system contains trace monomers, sulfides and other impurities, it is more prone to aging. Therefore, polystyrene products are long-term It will turn yellow and brittle during use. The first step is pre-foaming or simple foaming to set the density of the final product. In this process, the polymer particles containing the foaming agent soften under heating, and the foaming agent volatilizes. As a result, swelling occurs in each bead, forming many cells. The number of cells (final density) is controlled by heating temperature and heating time. During this process, the beads must remain dispersed and free-flowing.
What are the properties of silicon nitride ceramics?
2020 12/07