Calcium-copper titanate (CaCu3Ti4O12/CСTO) is a highly demanded material for its colossal dielectric constant and temperature stability. CСTO has many applications as sensors, microwave absorbers, etc. However, ceramic capacitors are one of the most promising applications, but CСTO has a high loss tangent (usually tg? > 0.1 for pure CСTO). Therefore, many attempts have been aimed at reducing tg? while maintaining ?r > 10,000. This problem was solved by doping and alloying, which makes the CСTO ceramics production more difficult and expensive. In this paper, for the first time, an approach to reducing the tg? while maintaining a high ?r is demonstrated by solid-phase synthesis temperature reducing and optimizing the grinding and calcinating processes. The materials were examined using XRD analysis, the ceramics microstructure evolution was revealed using SEM; the relative density was estimated by Archimedes method. The ?r and tg? were determined by the capacitive method at 1 kHz, 100 kHz and 1 MHz. For the first time, pure CСTO ceramics with ?r = 10,821 and tg? = 0.051 at 1 kHz were obtained. The presented manufacturing approach contributes to adjustable dielectric properties of CCTO ceramics.
The effect of silicon carbide on the microstructure, phase composition, and mechanical properties of Al2O3–ZrB2 composites was investigated. It was found that in the binary Al2O3–ZrB2 composite, the aluminum borate phase Al18B4O33 (6 vol. %) forms during sintering, hindering densification. Porosity reaches 8 %, mass loss – 24 %, flexural strength – 300 MPa, fracture toughness – 4.2 MPa·m1/2. The introduction of SiC prevents aluminum borate formation due to the reaction with surface B2O3, leading to the formation of refractory SiO2 and aluminoborosilicate liquid phase. The three-component Al2O3–ZrB2–SiC composite achieves density ?98 % of theoretical, strength 450 MPa, hardness 19.1 GPa, and fracture toughness 5.5 MPa·m1/2. Crack branching on SiC and ZrB2 grains increases fracture toughness.
Previously unknown in the literature, complexes of hexamethylenetetramine (HMTA) with tungsten silicates of the composition Rb5[SiW11O39Fe(HMTA)]?20H2O, Cs5[SiW11O39Fe(HMTA)]?6H2O, Rb6[SiW11O39Zn(HMTA)]?12H2O, Cs6[SiW11O39Zn(HMTA)]?10H2O with the Keggin anion structure were synthesized and studied by electron and IR spectroscopy, nuclear magnetic resonance, thermal and X-ray phase analysis. The electronic absorption spectra of the HMTA complexes with tungsten silicatoferrates in the region of 270…600 nm indicate an octahedral environment of iron (III) atoms, and the shift of the absorption maximum of 290 nm for Rb5[SiW11O39Fe(H2O)]?10H2O to 310 nm for Rb5[SiW11O39Fe(HMTA)] indicates a change in the ligands in the immediate environment of iron atoms and the formation of HMTA complexes with tungsten silicate. This is also evidenced by the results of nuclear magnetic resonance of solutions of the synthesized zinc-containing compounds in D2O: the chemical shift of the 1H protons of the CH2 methylene groups in HMTA is 4.56 ppm, in the Rb6[SiW11O39Zn(HMTA)]?12H2O complex – 4.64 ppm; for the 13C carbon atoms in HMTA, the chemical shift is 71.1 ppm, and in the Rb6[SiW11O39Zn(HMTA)]?12H2O complex – 71.2 ppm. According to thermal analysis (TA), differential scanning calorimetry (DSC) and X-ray phase analysis, the products of thermolysis of the synthesized compounds at 650? are phases with the structure of pyrochlore and hexagonal tungsten bronzes. The catalytic properties of Rb5[SiW11O39Fe(HMTA)]?20H2O were studied in the production of carbon nanomaterials. A hybrid carbon-mineral material, nanocarbon (WOX), was obtained. It was characterized by X-ray diffraction analysis and electron microscopy. It exhibits high electrical conductivity and chemical resistance to concentrated acid solutions. The results of this study may be useful for the low-temperature synthesis of compounds with a pyrochlore structure, hexagonal tungsten bronze, and hybrid carbon-mineral nanostructures.
The study compares two types of coatings based on an alumina phosphate binder with a cobalt-containing spinel pigment: one without wollastonite and one with its addition. The coating without wollastonite exhibits a color change from blue to lilac due to the destruction of the CoAl2O4 spinel and the formation of cobalt pyrophosphate (Co2P2O7). Surface integrity damage is observed after heat treatment at 280…300 °C. X-ray diffraction (DRON-07 diffractometer) and IR spectroscopy (FT-801 Fourier spectrometer) analyses revealed that the introduction of wollastonite into the coating composition stabilizes the spinel, preserves the blue color, and maintains the acicular (needle-like) structure of wollastonite. Along with the phases of berlinite (AlPO4), wollastonite (CaSiO3), and brushite (CaHPO4·2H2O), the presence of an amorphous alumino-silico-phosphate polymer coating with a homogeneous structure was determined. The obtained composition is recommended for decorative protection of ceramics and concrete.
The article is devoted to the study of optimal temperature-time conditions for the formation of amorphized inorganic coatings for the internal protection of main pipelines. A glass-enamel frit composition based on the aluminoborosilicate system RO–R2O–B2O3–Al2O3–SiO2–TiO2 has been developed. The influence of the temperature and time of batch boiling (in the range of 800…1400 °C) and subsequent coating firing (in the range of 700…950 °C, holding for 0.5…3.5 min) on the structure and quality of the protective layer has been studied using X-ray phase analysis and optical microscopy. It has been established that complete transition to the amorphous state and homogenization of the melt are achieved at a boiling temperature of 1300 °C for 30 min. A continuous, pore-free coating with high adhesion to steel is formed at a firing temperature of 925 °C and a holding time of 3.0 min. The developed conditions improve the operational reliability of pipelines, reduce energy costs, and reduce the duration of the process cycle.
The article presents experience in producing milled chamotte powders with improved particle size distribution using a two-product impact-centrifugal mill. It is shown that dense particle packing is achieved by polydisperse two-fraction mixtures with a size ratio of about 10 between fractions. The technological scheme includes an open-circuit grinding with air classification followed by screening on a vibrating screen. Two fractions are obtained: fine powder (?0.06 mm) and granular fraction (0.3…2.0 mm) in a ratio of 1:2 after recycling of oversize and undersize products. The final two-fraction powder (33.3 % fine fraction, 66.7 % granular fraction) exhibits porosity below 15 % (compaction coefficient 0.85). Energy consumption is 22.8 kWh per tonne of finished product. The economic feasibility of using two-product mills for producing chamotte powders for ceramic and refractory applications is substantiated.
In this work, we investigated borophosphate glasses with additions of 0.5 to 1.5 mol. % of rare-earth fluorides LnF3 (Ln = La, Ce, Nd), considered as simulants of radioactive waste from the now developing molten salt nuclear reactors. During the synthesis of the samples, it was found that even the small additions of LnF3 (?1.25 mol. % of LaF3 or NdF3, ?1.5 mol. % of CeF3) leads to formation of LnPO4 crystalline phase, which precipitated to the bottom of the crucible with the melt. This affects the ratios of the nominal and actual compositions of the glasses. Glass samples obtained from the upper, homogeneous, region of the melt exhibit an amorphous structure and a uniform distribution of the components throughout the volume. Their glass transition temperature Tg is 430 °C and remains constant over the entire range of the considered compositions; however, the introduction of LnF3 increases thermal stability and significantly changes the nature of crystallization processes. Moreover, for the samples with CeF3, the crystallization processes from that for the samples with LaF3 or NdF3, which is apparently due to the partial transition Ce3+ ? Ce4+. Analysis of the local structure indicates the presence of the ordering in the glass at the level of 2–3-coordination spheres and the invariance of phosphate groupings upon the introduction of LnF3. At the same time, with an increase in the LnF3 content, an increase in the proportion of 6-coordinated aluminum ions is detected, which is associated with the incorporation of fluoride ions into its environment. Consequently, in the environment of Ln ions, fluorine anions are replaced by oxygen, which causes the appearance of Ce4+. Based on the results of chemical and mechanical stability tests, it is shown that the glasses meet the regulatory requirements for the matrices for immobilization of radioactive wastes and can be considered as promising matrices for the incorporation of rare-earth fluorides.
The effect of various concentrations of zirconium dioxide nanoparticles on the strength and porosity of corundum ceramics obtained from domestic raw materials has been studied. The effect of an additive of eutectic composition on these properties is considered. It was found that the introduction of 20 wt. % solid solution based on zirconium dioxide and 5 wt. %. The addition of a eutectic additive in the MnO–TiO2 system to the aluminum oxide matrix makes it possible to obtain ceramics with a bending strength of up to 600 MPa.
The results of producing an Al2O3 composite coating on titanium by electron-beam cladding in the forevacuum pressure range (3…30 Pa) are presented. The microstructure, phase composition, and mechanical properties of the resulting composite layer were investigated. Using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis, it was found that the coating consists predominantly of the ?-phase of Al2O3 (82 %), along with ?-Ti and cubic TiO. The average surface microhardness reaches 13.6 GPa, which is five times higher than that of the uncoated titanium. It was shown that preliminary outgassing of the powder by electron-beam irradiation ensures process stability and the formation of a dense composite layer. The obtained results demonstrate the promise of the developed method for creating ceramic coatings on titanium.
Monolithic samples of functionally graded ceramic materials were fabricated via reaction sintering (siliconizing) of silicon carbide and diamond powders. The resulting materials are layered composites bonded by a continuous silicon carbide matrix through a reaction-diffusion mechanism. Structural characterization and physicomechanical testing of the multilayer ceramics were performed on prototype samples. The microstructural features of reaction-bonded materials based on silicon carbide and diamond–silicon carbide composites, both as monoliths and within multilayer architectures, are experimentally described. Recommendations are provided regarding the potential applications of functionally graded silicon-carbide-based ceramics with diamond particles in specialized engineering fields.
