Ceramics

In this work, a transparent nanostructured ceramic magnesium aluminate spinel (MgAl₂O₄) was fabricated by Spark Plasma Sintering (SPS) from commercial spinel nano-powders at different temperatures (1300, 1350 and 1400 °C). The sintered samples were thoroughly examined to assess their microstructural, optical, and mechanical properties. Various techniques such as SEM, AFM, spectrophotometer with an integrating sphere, instrumented Vickers indenter, Pin-on-Disk tribometer, scratch tester, and sandblasting device were employed to characterize the sintered samples. The results indicated the significant impact of the sintering temperature on the properties of the spinel samples. Particularly, the samples sintered at T = 1350 °C exhibited the highest Real In-line Transmission (RIT = 72% at 550 nm and 80% at 1000 nm). These samples demonstrated the highest hardness value (HV = 16.7 GPa) compared to those sintered at 1300 °C (HV = 15.6 GPa) and 1400 °C (HV = 15.1 GPa). The measured fracture toughness of the sintered samples increased substantially with increasing sintering temperature. Similarly, the tribological study revealed that the friction coefficient of the sintered spinel samples increased with the sintering temperature, and the spinel sintered at 1350 °C exhibited the lowest wear rate. Additionally, sandblasting and scratch tests confirmed the significant influence of the sintering temperature on the mechanical properties of the fabricated spinels. Overall, the spinel sintered at 1350 °C presented the best compromise in terms of all the evaluated properties. Full article

The aim of this study was to evaluate and compare the fracture resistance of definitive zirconia dioxide restorations obtained using a computer-aided design and manufacturing (CAD/CAM) system. Methods: Two groups of ten samples were analyzed for each material (n: 20); the first group was Zolid Gen X Amann Girrbach (ZGX) and the second group was Cercon HT Dentsply Sirona (CDS). The restorations were designed with identical parameters and milled with a CAD/CAM system. Each specimen was load tested at a speed of 0.5 mm/min, with a direction parallel to the major axis of the tooth and with an initial preload of 10 N until fracture using a universal testing machine (Universal/Tensile Testing Machine, Autograph AGS-X Series) equipped with a 20 kN load cell. The results obtained were recorded in Newtons (N), using software connected to the testing machine. Results: Statistically significant differences were found, and the fracture resistance of the monolithic zirconia crowns was lower in the CDS group (1744.84 ± 172.8 N) compared to the ZGX group (2387.41 ± 516 N). Conclusions: The monolithic zirconia CAD-CAM zirconia crowns showed sufficient fracture resistance when used in posterior molar and premolar zones with either material, as they withstood fracture loads greater than the maximum masticatory force. Full article

The aim of this work is to study the mechanisms of polymorphic transformations in ZrO₂ ceramics doped with MgO with different concentrations during thermal isochronous annealing, as well as the effect of the phase composition of ceramics on the change in strength properties and resistance to mechanical stress. Solving the problem of polymorphic transformations in zirconium dioxide by doping them with MgO will increase the resistance of ceramics to external influences, as well as increase the mechanical strength of ceramics. According to the data of X-ray phase analysis, it was found that the addition of the MgO dopant to the composition of ceramics at the chosen thermal annealing temperature leads to the initialization of polymorphic transformation processes, while changing the dopant concentration leads to significant differences in the types of polymorphic transformations. In the case of an undoped ZrO₂ ceramic sample, thermal annealing at a temperature of 1500 °C leads to structural ordering due to the partial removal of deformation distortions of the crystal lattice caused by mechanochemical grinding. During the study of the effect of MgO doping and polymorphic transformations in ZrO₂ ceramics on the strength properties, it was found that the main hardening effect is due to a change in the dislocation density during the formation of a ZrO₂/MgO type structure. At the same time, polymorphic transformations of the m—ZrO₂ → t—ZrO₂ type have a greater effect on hardening at low dopant concentrations than t—ZrO₂ → c—ZrO₂ type transformations. Full article

Uranium-containing glass samples with an age of 140–145 million years were collected within the volcanic rocks of the largest volcanic-related uranium ore deposit in the world. Main features of their composition are high concentrations of silica and uranium, the largest for the rocks of this type. In contrast to this, the ages of fresh (unaltered) low-silica natural glasses of a basic composition (basalts) usually do not exceed a few million years. The volcanic low-silica glass is unstable at longer times and in older ancient rocks is transformed into a crystalline mass. The geochemistry of uranium including the behavior in solids and solutions is similar to that of long-lived transuranic actinides such as radioactive Np and Pu from high-level radioactive waste. This allows uranium to be used as a simulant of these long-lived hazardous radionuclides both at the synthesis and for the study of various nuclear wasteforms: glasses, glass crystalline materials and crystalline ceramics. The data obtained on long-term behavior of natural glasses are of importance for prognosis and validation of stability of nuclear wasteforms disposed of in geological disposal facilities (GDF). Full article

The possibility of obtaining building foamed glass-ceramic using the diamond concentration tailings of the Lomonosov deposit in Arkhangelsk Region, Russia, is demonstrated here. The effect of the tailings’ particle size distribution, feed temperature, the addition of a foaming agent, and the content of oxidizer on the feed charge foaming is established. The process conditions for obtaining foamed glass-ceramic materials are described. The specifications of the materials with the optimal composition (tailings 50 wt.%, glass waste 50 wt.%, SiC 0.5 wt.%, Fe₂O₃ 1 wt.%) foamed at 1020–1050 °C were as follows: apparent density 0.23–0.51 g/cm³, compression strength 0.58–2.40 MPa, water absorption (by volume) 8.7–19.0%. Based on the combination of the measured properties when used in dry conditions, the obtained materials can be considered heat-insulating foam materials. The thermal conductivity was 0.060–0.066 W/m·K. Full article

Nanostructured single-phase hafnium-zirconium carbonitride powders were synthesized using a simple and fast mechanochemical synthesis approach. The critical milling duration, after which a (Hf,Zr)(C,N) solid solution formation inside a jar occurred via mechanically induced self-sustained reaction (MSR), was 10 min. After 30 min of treatment, a solid-gas reaction was completed, and as a result, a homogeneous (Hf,Zr)(C,N) powder consisting of 10–500 nm submicron particles was obtained. The phase and structure evolution of the powders after different treatment durations allowed for the establishment of possible reaction mechanisms, which included the formation of Hf/Zr/C-layered composite particles, their interaction via MSR, and further grinding and nitridization. Spark plasma sintering (SPS) was used to produce bulk hafnium-zirconium carbonitride ceramics from nanostructured powder. The sample had higher values of relative density, hardness, and fracture toughness than those for binary compounds of a similar composition. Full article

A developing energy-saving approach of cold sintering in a pure aqueous medium was applied to the preparation of α-Al₂O₃ ceramics and performed on spark plasma sintering equipment. The initial γ-Al(OH)₃ and γ-AlOOH powders and the cold-sintered ceramics were studied by X-ray diffraction analysis, infrared spectroscopy, thermal analysis, and scanning electron microscopy to reveal the chemical and structural transformations they experienced during the cold sintering. At 450 °C and 70 MPa, initially γ-AlOOH transformed into a fragile α-Al₂O₃ material. Porous α-Al₂O₃ ceramics with about 60% porosity were obtained after cold sintering of γ-Al(OH)₃ in the same conditions combined with subsequent annealing at 1250 °C for 3 h. The role of water molecules in the studied processes was considered as the enhancement of structural mobility in the cold-sintered material due to its reversible hydroxylation similar to earlier investigated supercritical water actions on the precursors during α-Al₂O₃ formation. Further improvement of the cold sintering setup and regimens would open prospects in α-Al₂O₃ ceramics manufacturing by an ecologically benign route. Full article

Finely dispersed (CeO₂)_1−x(Nd₂O₃)_x (x = 0.05, 0.10, 0.15, 0.20, 0.25) powders are synthesized via liquid-phase techniques based on the co-precipitation of hydroxides and co-crystallization of nitrates. The prepared powders are used to obtain ceramic materials comprising fluorite-like solid solutions with the coherent scattering region (CSR) of about 88 nm (upon annealing at 1300 °C) and open porosity in the range of 1–15%. The effect of the synthesis procedure and sintering additives (SiO₂, ZnO) on physicochemical and electrophysical properties of the resulting ceramics is studied. The prepared materials are found to possess a predominantly ionic type of electric conductivity with ion transfer numbers t_i = 0.96–0.71 in the temperature range of 300–700 °C. The conductivity in solid solutions follows a vacancy mechanism with σ_{700 °C} = 0.48 × 10⁻² S/cm. Physicochemical properties (density, open porosity, type and mechanism of electrical conductivity) of the obtained ceramic materials make them promising as solid oxide electrolytes for medium temperature fuel cells. Full article

This paper considers the results of an examination of painted pottery from prehistoric sites of the Prmor’ye region (Southern Russian Far East) in the northwestern part of the Sea of Japan basin. Red-painted and black-painted ceramic wares occur here only in the remains of the Yankovskaya archaeological culture dated to the 1st mil. BCE. Red painting appears as a colored surface coating, and black painting is represented by very simple drawn patterns. Until recently painting decorations have not been intentionally studied. The objects of our investigation are a small series of red-painted and black-painted ceramic fragments originated from archaeological sites. The methods of optical microscopy, SEM-EDS, and Raman spectroscopy were applied to the study of research materials. As a result, the data on characteristics of texture and composition of red and black paints were obtained. Both were determined to be pre-firing paints. Red paint is a clayish substance mixed with natural ochre pigment containing the hematite coloring agent. Black paint is carbon-based. Black carbon and burnt bone are recognized as colorants. The presented materials are new evidence of pottery paint technologies in prehistoric Eurasia. Full article

As an example of the implementation of digital materials science approaches based on statistical processing of electron micrographs with the analysis of fractal parameters, the digital characteristics of microstructure of diamond–silicon carbide ceramic composite material are calculated. The lacunarity parameter characterizing the non-uniform distribution of filler particles in the matrix is found. Based on lacunarity values calculated at different scales, scale invariance parameter characterizing the dependence of lacunarity on the scale is evaluated. Voronoi entropy characterizing the structure based on the quantity of information is also calculated and used to determine the average number of neighboring particles and average distance between them. For the composites with high mechanical properties, the number of nearest neighbors approaches six, indicating an almost closest packing. Full article

The objective of this study was to investigate the variability of flexural strength for flame-sprayed ceramic components and to determine which two-parametric distribution function was best suited to represent the experimental data. Moreover, the influence of the number of tested specimens was addressed. The stochastic nature of the flame-spraying process causes a pronounced variation in the properties of potential components, making it crucial to characterise the fracture statistics. To achieve this, this study used two large data sets consisting of 1000 flame-sprayed specimens each. In addition to the standard Weibull approach, the study examined the quality of representing the experimental data using other two-parametric distribution functions (Normal, Log-Normal, and Gamma). To evaluate the accuracy of the distribution functions and their characteristic parameters, random subsamples were generated by resampling of the experimental data, and the results were assessed based on the sampling size. It was found that the experimental data were best represented by either the Weibull or Gamma distribution, and the quality of the fit was correlated with the number of positive and negative outliers. The Weibull fit was more sensitive to positive outliers, whereas the Gamma fit was more sensitive to negative outliers. Full article

Monolithic zirconia and hybrid ceramic restorations have been widely used in the last decade for both anterior and posterior dental restorations. However, their use lacks sufficient scientific evidence in most cases, as the expeditious manufacturing of these versatile ceramic materials exceeds the limits of in vitro and/or in vivo validation. This study aimed to evaluate and compare the mechanical properties (flexural strength, fracture toughness, Vickers hardness, and brittleness index) of three CAD-CAM monolithic multilayer zirconia ceramics (GNX—Ceramill Zolid^® Gen-X, ZCP—IPS e.max^® ZirCAD, and UPC—Upcera^® Esthetic Explore Prime) and one CAD-CAM monolithic multilayer polymer-infiltrated hybrid ceramic (ENM—Vita^® Enamic) with a CAD-CAM monolithic lithium disilicate ceramic as a control (EMX —IPS e.max^® CAD). A total of 160 discs (GNX = 32, ZCP = 32, UPC = 32, ENM = 32, and EMX = 32) were cut, polished, and fully sintered (except for the ENM). Half of the samples for each group were subjected to hydrothermal aging. Descriptive analysis and ANOVA tests were used to compare the groups. The zirconia groups showed significantly higher mechanical properties than the EMX group for both the non-aged and aged samples (p < 0.05). The ENM group showed the lowest brittleness index, while EMX showed the highest. The mechanical properties of monolithic multilayer zirconia ceramics were generally better than those of monolithic multilayer polymer-infiltrated hybrid ceramic and lithium disilicate ceramic. All groups showed, to some extent, a change in their mechanical properties after aging, with the ENM being the most affected. Full article

A study into the use of the Solution Combustion Synthesis (SCS) method with glycine and citric acid to synthesize fine powders of multicomponent solid solutions of oxides of rare earth (RE) metals (Nd, Sm, Eu, Gd, Dy, and Ho) for the preparation of ceramic materials is presented. Synthesis parameters of 4-, 5-, and 6-component entropy-stabilized rare earth oxides (REOs) with a C-type cubic structure are determined. The stability of entropy-stabilized oxides (ESOs) with a C-type structure is shown to depend not only on heavy RE metal quantity, but also on the rate of heating/cooling of the samples. The temperature of the polymorphic transformation of C-type REO structures into B-type (monoclinic) or H-type (hexagonal) structural variants can be described by the equation T (°C) = 0.0214V_cr² − 62.737V_cr + 46390, where V_cr is the unit cell volume of an oxide with a C-type structure regardless of the number of cations in the solid solution. High-temperature thermal analysis up to 1250 °C revealed that dispersed powders, which contain impurities of basic carbonates along with hydroxocarbonates of RE metals and X-ray amorphous carbon formed during SCS reactions, also react with air moisture during storage. The influence of the ESO phase and cationic composition on the morphology, porosity and microhardness of ceramics was studied. Higher-entropy oxides form samples with higher density, microhardness and a smaller size of particle agglomerates. Full article

A rapidly developing area of ceramic science and technology involves research on the interaction between implanted biomaterials and the human body. Over the past half century, the use of bioceramics has revolutionized the surgical treatment of various diseases that primarily affect bone, thus contributing to significantly improving the quality of life of rehabilitated patients. Calcium phosphates, bioactive glasses and glass-ceramics are mostly used in tissue engineering applications where bone regeneration is the major goal, while stronger but almost inert biocompatible ceramics such as alumina and alumina/zirconia composites are preferable in joint prostheses. Over the last few years, non-oxide ceramics—primarily silicon nitride, silicon carbide and diamond-like coatings—have been proposed as new options in orthopaedics in order to overcome some tribological and biomechanical limitations of existing commercial products, yielding very promising results. This review is specifically addressed to these relatively less popular, non-oxide biomaterials for bone applications, highlighting their potential advantages and critical aspects deserving further research in the future. Special focus is also given to the use of non-oxide ceramics in the manufacturing of the acetabular cup, which is the most critical component of hip joint prostheses. Full article

Long-term sustainability and decreasing amount of fossil oil reserves require a partial or complete transformation of traditional lubricating oils. The use of silica nanoparticles as a lubricant additive has a huge tribological potential, which has already been discussed in numerous articles. Nanosized silica shows excellent results in reducing friction and preventing wear, but they quickly aggregate and settle after homogenization in oils. For long-term stable dispersion of lubricating oils containing nanoceramics, the surface of the particles was modified with ethyl oleate. The surface modification, the ethyl oleate applied to the surface of the nanosilica, was confirmed by Fourier-transform infrared spectroscopy. Group III based lubricating oil was prepared using the surface-modified nanosilica. The particle size of the nanoparticles in the lubricating oil dispersion was examined by dynamic light scattering. Oscillating tribometer measurements were performed with different concentrations (0.1; 0.2; 0.3 wt%) of nanolubricants. Based on the tribological results, the friction coefficient of the surface-modified nanosilica is more stable, its wear is 15% lower compared to the reference. There is no significant change in the magnitude of the friction coefficient. It can be concluded that the ethyl oleate surface modification method may be suitable for tribological investigations of the acting mechanisms of nanoparticles. Full article

The rock-salt ordered A₂CuWO₆ (A = Sr, Ba) with I4/m space group and disordered SrCu_0.5M_0.5O_3−δ (M = Ta, Nb) with Pm3m space group perovskites were successfully obtained via a solid-state reaction route. Heat treatment of Ba₂CuWO₆ over 900 °C in air leads to phase decomposition to the barium tungstate and copper oxide. Thermogravimetric measurements reveal the strong stoichiometric oxygen content and specific oxygen capacity (ΔW_o) exceeding 2.5% for Ba₂CuWO₆. At the same time, oxygen content reveals Cu³⁺ content in SrCu_0.5Ta_0.5O_3−δ. Under the following reoxidation of Ba₂CuWO₆, step-like behavior in weight changes was observed, corresponding to possible Cu⁺ ion formation at 900 °C; in contrast, no similar effect was detected for M⁵⁺ cations. The yellow color of Ba₂CuWO₆ enables to measure the band gap 2.59 eV. SrCu_0.5Ta_0.5O_3−δ due to high oxygen valance concentration has a low thermal conductivity 1.28 W·m⁻¹·K⁻¹ in the temperature range 25–400 °C. Full article

Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) solid electrolytes have been investigated as potential electrolyte materials for solid oxygen fuel cells (SOFCs), operating in the medium temperature range (600–700 °C). The design of new oxygen-conducting materials is of importance for their possible utilization in the solid oxide fuel cells. The oxygen–ion conductivity of the Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) “stuffed” pyrochlores ceramics was investigated by electrochemical impedance spectroscopy (two-probe AC) in dry and wet air. The synthesis of precursors via co-precipitation and the precipitate decomposition temperature have been shown to be of key importance for obtaining dense and highly conductive ceramics. At ~770 °C, the highest total conductivity, ~3.16 × 10⁻³ S/cm, is offered by Tm₂Ti₂O₇. The conductivity of the fluorite-like solid solution Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0.74) is an order of magnitude lower. However, for the first time a proton contribution of ~5 × 10⁻⁵ S/cm at 600 °C has been found in Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0.74) fluorite. Until now, compositions with proton conductivity were not known for the intermediate and heavy rare earth titanates Ln₂(Ti_2−xLn_x)O_7−x/2 (Ln = Ho − Lu) systems. The use of X-ray diffraction (structural analysis with Rietveld refinement), optical spectroscopy and dielectric permittivity data allowed us to follow structural disordering in the solid solution series with increasing thulium oxide content. High and low cooling rates have been shown to have different effects on the properties of the ceramics. Slow cooling initiates’ growth of fluorite nanodomains in a pyrochlore matrix. The fabrication of such nanostructured dense composites is a promising direction in the synthesis of highly conductive solid electrolytes for SOFCs. We assume that high-temperature firing of nanophase precursors helps to obtain lightly doped “stuffed” pyrochlores, which also provide the high oxygen–ion conductivity. Full article

One of the most important steps in the extrusion processing of ceramic inks is the initial drying of the ceramic parts. This study aimed to investigate the drying behaviour of an Al₂O₃-based ceramic ink optimised to be processed by extrusion processing methods, e.g., direct ink writing. Carboxymethylcellulose (CMC) was singly added to a suspension of deionised water and Al₂O₃ (50:50 wt.%) to perform as a dispersing and plasticising agent. To assess moisture loss as a function of time, the ceramic inks were extruded into two types of polymeric moulds: one with a completely closed profile producing cylindrical samples (disks) and one with an open profile producing ceramic bars. After the injection of the inks, the moulds were exposed to different controlled temperatures (20 and 40 °C) for up to 180 h; moisture loss and warpage were periodically measured, and exponential mathematical expressions (moisture loss × drying time) were obtained. The Al₂O₃-bars dried for 24 h in open moulds at 20 and 40 °C presented longitudinal warpages of 4.5% and 9%, respectively, while the Al₂O₃ disks dried in closed moulds presented warpages of 3.5% and 7% in these same temperatures (20 and 40 °C, respectively). The samples were sintered at 1610 °C for 4 h and characterised by scanning electron microscopy (SEM), relative density (Archimedes principle), and X-ray diffraction (XRD), presenting a relative density of 92.3 ± 0.5%, α-Al₂O₃ as crystalline phase and grain with equiaxed morphology varying between 1 and 5 μm. Full article

Layered silicates with a montmorillonite structure are widely used in various fields related to adsorption, gas and water treatment, catalysis, cosmetology and medicine. Under conditions of directed hydrothermal synthesis, montmorillonites with the specified characteristics can be obtained. The influence of the preparation method for montmorillonites of two compositions (Mg₃Si₄O₁₀(OH)₂ H₂O and Na_1.5Al_0.5Mg_1.5Si₄O₁₀(OH)₂ H₂O) on their sorption properties, moisture absorption, porous textural characteristics and surface properties has been studied. The nature of the initial reagents, the pH of the reaction medium and the synthesis temperature were chosen as the variable synthesis parameters. It has been established that the synthesis conditions significantly affect the properties of montmorillonite, which, in turn, determines the possibilities of using the materials obtained in specific areas. Full article