Study On Fabrication Of AlN-BN Composites And Its Dielectric Properties And Thermal Conductivity

The aluminum nitride (AlN), which has many excellent properties like highthermal conductivity, high electrical resistivity, low dielectric constant and lowdielectric loss, is considered to be a promising material for microwave-transparent.Hexagonal boron nitride (h-BN) has better dielectric properties, good thermalconductivity and thermal shock resistance and machinability. Therefore, it is possibleto design AlN-BN ceramic composites having excellent thermal conductivity,dielectric property, and machine property. Adding CaF₂, Y₂O₃ and CaF₂-Y₂O₃ assintering aids, AlN-BN composites were fabricated by Hot-Pressing sintering (HP)and Spark Plasma Sintering (SPS). For improving thermal conductivity, decreasingdielectric constant (ε_r) and loss tangent (tanδ), sintering aids, sintering temperatureand soaking time are discussed in this paper.AlN-BN composite ceramics with good dielectric properties were fabricated bySPS. Increasing sintering temperature promoted the densification and grains growth,increasedε_r and decreased tanδof composites. The increase in BN contentdecreased density, thermal conductivity andε_r, but increased tanδgreatly.Sintering at 1700℃for 5min, 85wt.%AlN-15wt.%BN composites were fabricatedwithε_r and tanδof 7.15 and 6.31×10^-4 separately at 1MHz. Though increasingdensity and thermal conductivity of AlN-BN composites, the adoption of CaF₂ andY₂O₃ decreased dielectric properties. Adding 3wt.% CaF₂ as sintering aids,85wt.%AlN-15wt.%BN composites were sintered at 1800℃for 5min with relativedensity of 98.15% and the thermal conductivity value of 78.7 W·m^-1·K^-1.AlN-BN composite ceramics with good dielectric properties and thermalconductivity were fabricated by HP. The increase in CaF₂ content increased density,thermal conductivity and dielectric constant, decreased tanδof AlN-BN composites.Increasing sintering temperature helps to improve densification, thermal conductivityand dielectric properties. Sintering at 1850℃for 3h, 85wt.%AlN-15wt.%BN wasfabricated with thermal conductivity value of 110 W·m^-1·K^-1,ε_r and tanδof 7.53and 6.36×10^-4(at 1MHz). The adoption of Y₂O₃ as sintering aids, increased thermalconductivity continuously, but decreased dielectric properties of AlN-BN composites. The increase in the content of Y₂O₃ in sintering aids CaF₂-Y₂O₃ promoteddensification and thermal, conductivity. Increasing soaking time improveddensification and thermal conductivity and dielectric properties. The increase in thecontent of BN decreased density, thermal conductivity and dielectric properties ofA1N-BN composites. Adding 3wt.%CaF₂-3Wt.%Y₂O₃ as sintering aids,85wt.%AlN-15wt.%BN was fabricated with thermal conductivity value of 132.7W·m^-1·K^-1,ε_r and tanδof 7.66 and 7.31×10^-4(at 1MHz).The effects of sintering aids (CaF₂, Y₂O₃ and CaF₂-Y₂O₃) on densification andmicrostructure during sintering were studied, through the analysis of XRD, TG/DSC,SEM and TEM/EDX. When adding CaF₂ as sintering aids, the liquid phase inducedby CaF₂ promoted densification during sintering and purified the grain boundariesthrough the evaporation of the Ca-Al-O compounds from the sintered bodies in acarbon-containing nitrogen atmosphere in sintering and soaking stages. When addingY₂O₃ as sintering aids, the formed Y-Al-O compounds aggraded at thegrain-boundaries in the sintered body decreased dielectric properties of AlN-BNcomposites. Adding CaF₂-Y₂O₃ as sintering aids, the liquid phase of Y-Ca-Al-O-Npromoted the densification and the purification of AlN grains at high temperaturesintering. The decomposition and evaporation of Y-Ca-Al-O compounds in acarbon-containing nitrogen atmosphere during sintering and soaking promoted theevaporation of Ca and Y elements and purified the grain-boundaries, which improvedthe dielectric properties, thermal conductivity and other properties of AlN-BNcomposite ceramics.