氮化硼的介电特性及其在微波加热中的热透波性能

Dielectric Properties of Boron Nitride and Its Thermal Wave-transparent Properties in Microwave Heating

耐火结构的热透波性能对微波加热的模式和效率均具有重要影响,所以明确耐火结构在微波加热过程中的动态热透波性能变化至关重要。本文以氮化硼陶瓷为研究对象,通过实验测试了其在915 MHz、2 450 MHz两种频率下的宽温域(25~1 000℃)介电特性,采用传输线理论计算了氮化硼陶瓷的功率透过系数,并对其介电常数、介电损耗因子等参数的温度特性进行了分析。研究表明:25~1 000℃,两种频率下的介电常数变化幅度较小,介电常数仅变化了1.8%左右;两种频率下的介电损耗因子随温度呈波动性变化,900℃之后都呈指数增长的趋势。微波在氮化硼陶瓷中的波长和穿透深度均随温度的增加呈现下降趋势;当频率增大时,穿透深度随频率的增大而减小。氮化硼陶瓷的透波性能在两种极化模式下存在明显差异,水平极化(TM)明显高于垂直极化(TE)。当频率为915 MHz时,可以考虑材料厚度值为0.08 m左右作为优化材料厚度,同时以入射角度在θ_(i)≤67°范围内入射具有良好的透波性能(PTC≥0.7)。当频率为2 450 MHz时,可以考虑材料厚度值为0.06 m作为优化材料厚度,在TE极化模式中应优先考虑微波入射角度在θ_(i)≤58°范围内入射,在TM极化模式中建议入射角度θ_(i)≤77°入射具有较好的透波效果(PTC≥0.9)。本研究对微波加热设备内炉衬材料的选择、工艺参数的设定以及有效提高微波能的利用率具有重要的现实指导意义。

The thermal wave propagation characteristics within refractory structures significantly affect the efficiency and method of microwave heating.Therefore,understanding the dynamic thermal wave propagation within these structures during microwave heating is crucial.This study focuses on boron nitride ceramic(BN)to explore its behavior under microwave heating.The dielectric properties of BN in a wide temperature range(25~1000°C)at frequencies of 915 MHz and 2450 MHz are tested experimentally.The power transmission coefficient of BN is calculated by transmission line theory.The temperature characteristics of dielectric constant,dielectric loss factor and other parameters of BN are analyzed.The results show that the dielectric constant changes little at two frequencies from 25 to 1000°C,and the dielectric constant only changes by about 1.8%.However,the dielectric loss factor fluctuates with temperature,exhibiting an exponential increase beyond 900°C.Concurrently,both the wavelength and penetration depth of microwaves within BN diminish as temperature rises.Additionally,an increase in frequency leads to a re-duction in penetration depth.Notably,the study reveals significant disparities in BN’s wave transmission performance between two polarization modes,with horizontal polarization markedly outperforming vertical polarization.At the frequency of 915 MHz,a material thickness of approx-imately 0.08 meters is identified as the optimal,yielding a a transmission performance(PTC≥0.7)in the range of≤67°.At the frequency of 2450 MHz,the optimal material thickness reduces to 0.06 meters.For microwaves in the TE polarization mode at this frequency,the incident angle should ideally belimited to the range ofθ_(i)≤58°.In the TM polarization mode,it is recommended that the incident angle ofθ_(i)≤77°has a good wave transmission effect(PTC≥0.9).This study has important practical guiding significance for the selection of lining materials in microwave heating equipment,the setting of process parameters and the effective improvement of microwave energy utilization.