The thesis is devoted to development of microwave switching materials based on compounds, which exhibit temperature-induced phase transitions (PT). In the first chapter there is a review of the literature, which highlights the various mechanisms of absorption of electromagnetic radiation of microwave range by materials, describes methods of instrumental study of microwave transmission / reflection and presents the currently known microwave switches. The nature of PT in three classes of compounds used in this work for the development of microwave switches is also described.
In the second chapter there is a description of experimental methods towards the synthesis of coordination compounds with PT, the production of polymer composites with vanadium dioxide and the synthesis of organic-inorganic perovskites. The equipment used for carrying out instrumental measurements is described.
The third chapter of the thesis shows the spin-dependent interaction of SCO materials with microwave radiation, namely the ability of coordination compounds [Fe(NH2trz)3]Br2 and [Fe(NH2trz)3](NO3)2, which undergo cooperative SCO between low-spin (LS) and high-spin (HS) states near room temperature, to change the extent of microwave radiation absorption under the influence of temperature change. The characteristics of microwave reflection and transmission of these SCO complexes were studied at different temperatures. The evolution of the transmission and reflection spectra in the frequency range of 26–37 GHz in the temperature range of SCO showed significant differences in the interaction of microwave radiation with LS and HS forms of complexes. Microwave transmission is significantly reduced during the transition to the HS state, while the reflection can both increase and decrease at certain frequencies due to SCO. The different ability of LS and HS forms of complexes to absorb microwave radiation is associated with significant changes in the dielectric constant of SCO complexes at the microwave frequency. Variable reflection / transmission of microwave radiation correlates well with the characteristics of SCO, which were obtained in optical and magnetic measurements, as well as studies of differential scanning calorimetry.
In addition, the ability to switch microwave radiation of three complexes which exhibit high-temperature SCO was demonstrated: one complex based on 1,2,4-triazole – [Fe(trz)(Htrz)2]BF4 and two bimetallic complexes with bridging cyanide ligands – [Fe(pyrazine)[M(CN)2}2] M = Au, Ag. In the general case, triazole-based complexes have been found to be more effective switches than bimetallic complexes with bridging cyanide ligands. The obtained results expand the spectroscopic range in which SCO materials can be used as switches, and also contribute to the creation of a database on microwave absorption of SCO complexes.
The fourth chapter proposes a method for producing microwave switches using a composite material based on vanadium dioxide and polymethyl- methacrylate. Differential scanning calorimetry, SQUID magnetometry and impedance spectroscopy were used to study PT in the proposed composite. MIT in the proposed material occurs at a technologically attractive temperature of 341 K. It was shown that PT in vanadium dioxide leads to a sharp decrease in the transmission of microwave radiation by the composite. The presence of a component with MIT in the composite makes it possible to control the transparency of the material in the microwave range, while the presence of a polymer matrix provides the possibility to process the switch element mechanically.
The fifth chapter shows the ability of two HOIPs, namely CH3NH3PbI3 and (C5H11NH3)2PbI4 to serve as PT microwave switches. Microwave switching is a consequence of PT, which occurs at technologically attractive temperatures – slightly above room temperature: 330 K in CH3NH3PbI3 and 320 K in (C5H11NH3)2PbI4. It was found that at selected frequencies (C5H11NH3)2PbI4 is characterized by a very high transmittance of -0.4 dB, which varies to -4.4 dB during the transition to the high-temperature phase. The transmission of CH3NH3PbI3, on the other hand, is very low (-25 dB) and increases only slightly during the transition to the high temperature phase. Microwave absorption varies by 38% in (C5H11NH3)2PbI4 and by 0.6% in CH3NH3PbI3 at selected frequencies. Studies show that it is more appropriate to use CH3NH3PbI3 as a microwave absorber, while (C5H11NH3)2PbI4 can serve as an effective switch of microwave radiation. In addition, this section compares the microwave switching characteristics of all studied materials and shows that (C5H11NH3)2PbI4 is the most efficient switch of all the described materials.
