High Thermal Conductivity Diamond Film Microwave Attenuating Materials

Microwave attenuation materials are widely used in modern vacuum electronic devices.With rapid advances in this field,materials with a high thermal conductivity are needed.Traditionally,microwave attenuating materials are ceramic-based composites.Due to the defects such as interface in the composites,which induced scattering of phonons,and on the other hand,the thermal conductivity of the ceramic itself has a limit.This makes the thermal conductivity of the composites have a certain bottleneck.It is generally difficult to exceed 200 W/m K.In order to further increase the thermal conductivity of attenuating materials,it is necessary to use a material with a higher thermal conductivity as a matrix.At present,the highest thermal conductivity among known bulk materials is diamond,but there are little reports on the use of diamond as an attenuating material.In order to further improve the thermal conductivity of microwave attenuating materials and expand the application range of diamond,boron doping and graphitization are used to introduce loss filler components inside the diamond matrix.The influences of parameters such as boron impurity concentration and graphite fiber length on microwave loss have been studied.The paper firstly proposes a new algorithm which is used to solve the problem of large error in the process of Transmission/Reflection(T/R)due to the thin thickness of diamond films,for deducing complex permittivity.The new algorithm is based on a traditional T/R algorithm with a optimization factor and improves the original optimization factor value method.The result proves that the calculation result of the new algorithm is obvious superiority compared with the original algorithm.Secondly,we synthesized the diamond film in situ by microwave plasma chemical vapor deposition(MPCVD).The boron impurity concentration range is 1.2×1018-1.5×1020cm-3.The,T/R measurement results show that after boron doping the real and imaginary parts of complex permittivity of the diamond film increase with the increasing of boron impurity concentration.The mechanism analysis shows that the introduction of boron impurities generates bound charges and free carriers in the diamond film.The jump polarization generated by the bound charge under the microwave electric field causes the real part of permittivity.The relaxation loss caused by polarization and the conductance loss caused by free carrier are responsible for the increase in the imaginary part of permittivity.By adjusting the concentration of boron impurities in the diamond film,the value of the real and imaginary parts of permittivity can be changed.At the same time,although the thermal conductivity of the diamond film is decreased due to the incorporation of boron impurities,the thermal conductivity reaches 980 W/m K even for the sample with the highest boron impurity concentration(3.6×1020 cm-3),which is much higher than that of traditional attenuating materials.Finally,we used laser ablation to prepare continuous and short graphite fibers in diamond self-supporting films.The T/R test results show that the continuous graphite fiber has obvious anisotropy,and its complex dielectric constant is greatly affected by the angle between the incident electric field and the graphite fiber.The addition of short graphite fiber is beneficial to change the complex permittivity,and the complex permittivity of the diamond film can be flexibly and effectively adjusted by changing the length and content of the short graphite fibers.In addition,the experimental results show that changing the length of short graphite fibers is more effective than that of the content.At the same time,since the laser ablation is only on the surface of the diamond film,the damage to the overall structure is extremely limited,so the thermal conductivity of the ablated diamond film is very limited compared with the untreated.