Investigation of the Influence of Pulsed Radiation Generated by Functional Ceramics Based on the Principle of PTE on the Characteristics of the Cr2O3–SiO2–Fe2O3–CaO–Al2O3–MgO–CuO System

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Introduction

Modulated pulsed far-infrared radiation is observed in functional ceramics obtained by the action of photons with a wide spectrum of quantum energy in solar furnaces [1; 2]. This interesting phenomenon may open up new possibilities for the development of this field. However, further progress requires a significant expansion of the production of ceramic materials, which is difficult due to the complexity and high cost of the equipment required for high-temperature solar technology.

In this work, the possibility of obtaining similar ceramic materials using alternative methods, in addition to solar technology, is investigated. The project authors develop and study various non-equilibrium synthesis techniques that can induce phase and structural heterogeneity in ceramics, including grain boundaries. An approach is proposed aimed at the formation of local melt regions in the resulting material due to the formation of eutectics or melting of the initial components. This approach can intensify the processes of obtaining ceramic materials with specified characteristics and, thereby, contribute to the further development of this field.

This work uses a thermomechanical approach for the synthesis of powdered materials, which allows obtaining a controlled microstructure of amorphous and metastable compounds at interphase boundaries. By varying the intensity and nature of the simultaneously applied mechanical and thermal influences, it is possible to regulate the properties of the materials obtained by mechanical activation and create interphase boundaries of non-equilibrium phases with the main oxide matrix without using high-temperature solar technology.

The use of such methods opens up new possibilities for the efficient synthesis of multicomponent ceramics capable of generating pulsed infrared radiation based on the principle of pulsed tunneling effect (PTE) [3; 4]. The obtained materials can serve as an alternative to ceramics synthesized in a high-temperature regime on solar installations, such as the 1 MW solar furnace located in Uzbekistan.

The key advantage of this approach is the ability to obtain materials with controlled microstructure and properties without the use of expensive high-temperature equipment, which makes it more accessible and promising for further developments in the field of functional ceramics.

Synthesis methods

Carbonate method

In this work, high-energy thermomechanical synthesis was used to obtain photocatalytically active ceramics in the Cr₂O₃—SiO₂—Fe₂O₃—CaO—Al₂O₃—MgO—CuO system. The corresponding metal carbonates were used as the starting components.

During thermomechanical synthesis in the temperature range up to 800 °C, the decomposition of carbonates occurred. This led to a significant increase in the concentration of structural defects necessary for accelerated mass transfer in the forming ceramic system.

Differential thermal analysis and electron micro-scopy revealed three main temperature ranges for the de-composition of carbonates in the initial reaction mixture: 150–250, 400–500, and above 600 °C. The presence of these ranges is confirmed by characteristic endothermic peaks on the differential curves corresponding to the thermal effects of the decomposition processes. The experimentally determined temperatures of carbonate decomposition coincided with their tabular values.

It is shown that the formation of intermediate phases during the synthesis occurs immediately after the decomposition of carbonates, which is due to the increase in the defect structure. Exothermic peaks on the thermal effects curve are observed immediately after the endothermic decomposition peaks. In addition, the release of carbon dioxide during the decomposition of the starting components is an important factor contributing to the increased reactivity of the powdery mixture. The resulting CO₂ flows can initiate the mixing processes of the reacting components, expose new reaction surfaces, and facilitate diffusion processes at the phase boundaries.

Thus, the use of high-energy thermomechanical synthesis made it possible to obtain photocatalytically active ceramics in the studied multicomponent system due to the controlled formation of a defective structure and the activation of mass transfer at the phase boundaries.

 

Table 1

List of decomposition temperatures of carbonates of the initial mixture of components

Carbonates	T decomposition, °C
Fe2(CO3)	550
СuCO3	290
MgCO3	570
Al2(CO3)3	60
CrCO3	550
CaCO3	950

 

Mechanochemical activation

Sequential heat treatments with holding at fixed temperatures were carried out in the range from 500 to 1300 °C. Accordingly, after each heat treatment, the powder mixtures were also subjected to thermomechanical dispersion under thermal-vibrational conditions. A disperser was used for mechanical activation. The powders were placed in special trays and loaded into an apparatus that allows initiating grinding and chemical interaction of the components in a liquid medium under conditions of planetary rotation, vibration, and thermal influence.

High-energy mechanochemical activation significantly changes the mechanism of solid-state reactions, reducing the number of intermediate stages and increasing the homogeneity of the synthesis products. The main factor determining the increase in the efficiency of the reaction is the increase in the defectiveness of the particles. Mass transfer already differs from the classical case, and the system represented by the mobile powdery mass of particles can be compared to a gas, where the reaction occurs at the moment of collision of the molecules. At the same time, the plastic flow of the synthesized material, carried out due to impact loading, ensures its delivery to the site of diffusion processes and increases the contact surface. Accelerated mass transfer increases the free energy of the system and increases the diffusion coefficients of the components. Excessive free energy is a source of destruction of non-equilibrium phases, the range of temperature stability of which under normal conditions does not correspond to the considered temperature range.

Mechanochemistry, the study of chemical reactions caused by mechanical forces, plays a crucial role in this process. High-energy mechanical activation leads to the creation of structural defects, an increase in the surface area, and an increase in the mobility of atoms, which facilitates the acceleration of solid-state reactions. This approach allows the synthesis of materials with increased homogeneity and lower energy consumption compared to traditional heat treatment methods.

The impact of the activation process on ceramic powders of the Cr₂O₃–SiO₂–Fe₂O₃–CaO–Al₂O₃–MgO–CuO system

Undoubtedly, the activation process has a significant impact on the ceramic powders of the Cr₂O₃—SiO₂—Fe₂O₃—CaO—Al₂O₃—MgO—CuO system. To evaluate their ability to generate pulsed radiation, these powders were subjected to activation using pulsed infrared radiation generated by the functional ceramic MC-1, operating on the principle of the Inverse Thermoemission Effect (ITE). These pulses were characterized by a short rise time and high intensity, reaching up to 320 W/cm².

Comparison of the X-ray spectra of activated and non-activated samples revealed significant changes in the crystalline structure and phase composition of the ceramics as a result of this process (Fig. 1). The activation treatment led to a reduction in unreacted phases, indicating the completion of the main chemical processes.

 

Fig. 1. X-ray diffraction spectra for samples, before (a) and after (b) activation

 

It was found that one of the key factors in the activation was the redistribution of the phase composition of the obtained composite after synthesis. The proportion of the phase based on silicon oxide increased, while the proportion of the phase based on solid solutions with a spinel structure decreased. It is assumed that the redistribution of phases occurred at their interface boundaries and was determined by the diffusional mobility of cations in this region. Such non-equilibrium processes were accompanied by the formation of metastable compounds and solid solutions at the interface boundaries, which play a key role in the accumulation of phonons and the generation of modulated pulsed radiation.

As a result of the activation, the material was modified with an increase in the degree of crystallinity, which is clearly visible in the X-ray spectrum of the activated sample (Fig. 1, b) with insignificant fluctuations in the background line, in contrast to the non-activated sample (Fig. 1, a).

The increase in crystallinity after activation is also evidenced by the magnitude of the number of pulses, which is equal to 800, in contrast to this value for the sample before activation, which corresponds to 500 pulses (see Fig. 1, a).

The change in the parameters of the crystal structure of the ceramic powder samples after their activation is also indicated by a detailed analysis of individual sections of their X-ray spectra (Fig. 2).

 

Fig. 2. Fragments of X-ray diffraction spectra for samples before activation – the upper spectrum and after activation – the lower spectrum

 

The analysis of the X-ray diffraction patterns showed a significant shift of the reflections towards smaller diffraction angles (see Fig. 2, lower spectrum), which indicates an increase in the parameters of the crystal lattice of this phase. This effect may be due to the diffusion of cations from other crystalline phases into this phase, which leads to an expansion of its crystal lattice.

Electron microscopy studies with energy-dispersive analysis revealed significant changes in the morphology and size of the crystallites of various phases after activation of the sample (Fig. 3). In particular, a decrease in the size of the crystallites and an increase in their density were observed, which contributed to the formation of an extensive network of interphase boundaries. Such structural transformations are likely precursors to the nucleation of metastable compounds at the phase boundaries during the activation process.

 

Fig. 3. Micrographs of samples before (a) and after (b) activation

 

The observed significant increase in the proportion of interphase boundaries as a result of sample activation appears to have contributed to the formation of a large number of metastable inclusions in the material. This effect may be one of the key factors initiating the generation of modulated IR radiation in the studied system.

The emergence of such metastable structures at the interphase boundaries is likely due to the instability of the crystal structure under the conditions of activation and the accompanying structural transformations. Local distortions of the crystal lattice, caused by diffusion processes and the redistribution of components, can create preconditions for the formation of metastable phases, which in turn can lead to the emission of modulated IR radiation.

Thus, the relationship between microstructural changes and the optical properties of the material is an important aspect that requires further study to understand the mechanisms of modulated IR radiation generation in this system.

Results

The most important results of the study:

1. Thus, the study investigated the methods of obtaining ceramic materials based on the Cr₂O₃—SiO₂—Fe₂O₃—CaO—Al₂O₃—MgO—CuO system using thermomechanical treatment and mechanical activation of carbonate powders.
2. It is shown that the proposed alternative synthe-sis methods allow obtaining ceramics with a micro-structure necessary for generating pulsed radiation in the far IR region based on the TFEL principle.
3. It has been established that the activation of such materials by radiation generated on the basis of TFEL leads to fundamental changes in the crystal structure and phase composition, as well as the formation of metastable phases at the grain boundaries.
4. This provides properties similar to the properties of ceramics synthesized in solar furnaces under the influence of a powerful, concentrated flux of photons in a wide energy range.
5. This approach allows scaling up the production of functional ceramics, in particular, composite films for greenhouses.
6. Studies of the properties of the obtained materials in the composition of light-converting composite films open up new possibilities for creating effective solar composites for various purposes and scaling up their production in the required volumes.

Conclusions

1. Alternative methods for synthesizing ceramics, which serve as the basis for composite materials used in greenhouses, have been developed and investigated, in addition to heliotechnology. These methods include thermomechanical treatment and mechanoactivation of carbonate powders. These approaches have demonstrated the ability to obtain ceramics with the necessary microstructure for generating pulsed infrared radiation.
2. It has been established that the activation of the material by pulsed tunneling effect leads to significant changes in structure and phase composition, as well as the formation of metastable phases at grain boundaries. This provides the ability of this ceramic material to exhibit properties similar to those obtained using solar furnaces.
3. The proposed approach enables the scalable pro-duction of functional ceramics for composite green-house films, opening up prospects for the creation of efficient solar composites for various purposes.

About the authors

Rustam Kh. Rakhimov

Institute of Materials Science of the SPA “Physics-Sun” of the Academy of Science of Uzbekistan

Author for correspondence.
Email: rustam-shsul@yandex.com
ORCID iD: 0000-0001-6964-9260
SPIN-code: 3026-2619

Dr. Sci. (Eng.), Head, Laboratory No. 1

Uzbekistan, Tashkent

Vladimir V. Pankov

Belarusian State University

Email: pankovbsu@gmail.com
ORCID iD: 0000-0001-5478-0194

Dr. Sci. (Chem.), Professor

Belarus, Minsk

Temur S. Saidvaliev

Institute of Materials Science of the SPA “Physics-Sun” of the Academy of Science of Uzbekistan

Email: t.saidvaliyev@imssolar.uz
ORCID iD: 0009-0008-6473-9214

chief engineer

Uzbekistan, Tashkent

References

Supplementary files