Influence of firing parameters on the properties, phase composition and structure of radiotechnical ceramic materials based on the BaO—SrO—Al2O3—SiO2 system

This study is aimed to develop radio­technical ceramics with improved functional and operational properties. The base system for the synthesis of materials with slavsonite and celsian, as target phases, was justified. In the context of theoretical studies and, as a result of obtained information about ratios of compounds’ phases, the subsolidus structure of the BaO—SrO—Al2O3—SiO2 system was researched; it was found, that the system is comprised of 28 elemental tetrahedra. This work resulted in the creation of a graph showing elemental tetrahedra’s relations and calculations of the graph’s geometrical­topological characteristics. Tetrahedra, in which targeted compounds’ synthesis is possible, was determined. As a result of this theoretical study, choice was carried out about area with the most technological oxide compositions for celsian­slavsonite ceramics’ synthesis. The purpose of the work is the development of recipe­technological parameters for the creation of celsian­slavsonite ceramics and the determination of its properties and structural­phase features. Properties of obtained ceramics were determined according to relevant state standards of Ukraine, as well as international standards. The dielectric characteristics measurements were carried out for the samples at a frequency of 1 kHz with the use of the E7—8CLR unit. The thermal conductivity of materials was obtained with the help of a special unit, judging from the temperature’s value on the surface of the test sample, taking into account the temperature sensor’s thickness and thermal conductivity, as well as the value of thermal flow passing through it. The phase composition of the test samples was determined by X­ray phase analysis using a DRON—3M diffractometer with CuKα radiation and a nickel filter under standard operating conditions. The samples’ microstructure was studied on a PHENOM Pro scanning electron microscope. According to the results of experimental research, it was established, that the increase in temperature from 1350 to 1400 °С negatively impacted the dielectric properties of material; while the increase in exposure time from 2 to 4 hours positively influenced physical and mechanical properties. The study of the obtained materials’ structural­phase features indicates, that the mentioned synthesis conditions allowed for the creation of single­phased ceramics entirely comprised of Ba0.5Sr0.5Al2Si2O8 solid solution with crystals of 2—4 μm. Increased temperature resulted in the introduction of undesired strontium aluminate and high­temperature quartz phases, as well as the increase of crystalline neoplasms’ sizes to 10 μm. This explains deteriorated functional and operational properties of ceramics, sintered at 1400 °С. The used approach to the design of celsian­slavsonite ceramics’ compositions allowed the creation of radio­technical ceramic materials with the following improved operational properties: water absorption — 2.24 %, apparent density — 2.72 g/cm3, bending strength — 82.9 MPa, dielectric constant — 7.2, dielectric loss angle tangent — 0.009.