The Effect of Ca Dopant on the Electrical and Dielectric Properties of BaTi4O9 Sintered Ceramics

The current research examines the impact of Ca²⁺ substitution on the phase and electrical properties of (Ba_1−xCa_x)Ti₄O₉, (x = 0.0, 0.3, 0.6, and 0.9) sintered pellets synthesized by solid-state reaction method. The as-synthesized samples were analyzed using X-ray diffraction (XRD) and impedance spectroscopy. The emergence of orthorhombic phase fit into space group Pnmm was revealed by XRD, and the addition of Ca resulted in a considerable shift in grain size. Dielectric properties were determined using an impedance spectroscopy in a wide frequency range from 1MHz to 3 GHz. The dielectric properties i.e., dielectric constant (ε_r) and dielectric loss (tanσ), were measured at 3 GHz frequency. The frequency-dependent parameters such as conductivity, dielectric constant, and dielectric loss indicated that the relaxation process is a Maxwell–Wagner type of interfacial polarization. The improved dielectric properties and low energy loss have made (Ba_1−xCa_x)Ti₄O₉ a prominent energy storage material. This study provides the possibility to improve its dielectric properties and reduce energy loss, making it an excellent energy storage material.     

Characterization of Al2O3 Samples and NiAl–Al2O3 Composite Consolidated by Pulse Plasma Sintering

The paper describes an investigation of Al₂O₃ samples and NiAl–Al₂O₃ composites consolidated by pulse plasma sintering (PPS). In the experiment, several methods were used to determine the properties and microstructure of the raw Al₂O₃ powder, NiAl–Al₂O₃ powder after mechanical alloying, and samples obtained via the PPS. The microstructural investigation of the alumina and composite properties involves scanning electron microscopy (SEM) analysis and X-ray diffraction (XRD). The relative densities were investigated with helium pycnometer and Archimedes method measurements. Microhardness analysis with fracture toughness (K_IC) measures was applied to estimate the mechanical properties of the investigated materials. Using the PPS technique allows the production of bulk Al₂O₃ samples and intermetallic ceramic composites from the NiAl–Al₂O₃ system. To produce by PPS method the NiAl–Al₂O₃ bulk materials initially, the composite powder NiAl–Al₂O₃ was obtained by mechanical alloying. As initial powders, Ni, Al, and Al₂O₃ were used. After the PPS process, the final composite materials consist of two phases: Al₂O₃ located within the NiAl matrix. The intermetallic ceramic composites have relative densities: for composites with 10 wt.% Al₂O₃ 97.9% and samples containing 20 wt.% Al₂O₃ close to 100%. The hardness of both composites is equal to 5.8 GPa. Moreover, after PPS consolidation, NiAl–Al₂O₃ composites were characterized by high plasticity. The presented results are promising for the subsequent study of consolidation composite NiAl–Al₂O₃ powder with various initial contributions of ceramics (Al₂O₃) and a mixture of intermetallic–ceramic composite powders with the addition of ceramics to fabricate composites with complex microstructures and properties. In composites with complex microstructures that belong to the new class of composites, in particular, the synergistic effect of various mechanisms of improving the fracture toughness will be operated.     

The Effect of Temperature and Sputtered Particles on the Wettability of Al/Al2O3

Two kinds of Al₂O₃ ceramic samples with and without Al film deposited were designed respectively. The influences of temperature and high kinetic energy sputtering particles on the wettability and interface strength of Al/Al₂O₃ were studied by comparing the wetting behavior of molten aluminum on two samples. The results show that molten aluminum does not wet the Al₂O₃ sample without Al film deposited at 700 °C, the contact angle is 165°, and the interfacial shear strength is 28 MPa. With the increase of temperature, the contact angle decreases continuously, and the interface shear strength gradually increases. The fracture of the brazed joint is transferred from the interface to the brazing seam. In comparison, the sample deposited with Al film is wetted by molten aluminum at 700 °C, and the contact angle is only 12°. The interface shear strength is about 120 MPa and is less affected by temperature. The shear fracture of the joint occurs in the brazed seam of Al metal. Therefore, the high energy generated by either the temperature increase or the particle sputtering enable the Al atoms to overcome the energy barrier to form Al–O bonds with the O atoms on the Al₂O₃ ceramic surface, thereby improving the wettability of Al/Al₂O₃.     

Electrical and Structural Properties of Li1.3Al0.3Ti1.7(PO4)3—Based Ceramics Prepared with the Addition of Li4SiO4

The currently studied materials considered as potential candidates to be solid electrolytes for Li-ion batteries usually suffer from low total ionic conductivity. One of them, the NASICON-type ceramic of the chemical formula Li_1.3Al_0.3Ti_1.7(PO₄)₃, seems to be an appropriate material for the modification of its electrical properties due to its high bulk ionic conductivity of the order of 10⁻³ S∙cm⁻¹. For this purpose, we propose an approach concerning modifying the grain boundary composition towards the higher conducting one. To achieve this goal, Li₄SiO₄ was selected and added to the LATP base matrix to support Li⁺ diffusion between the grains. The properties of the Li_1.3Al_0.3Ti_1.7(PO₄)₃−xLi₄SiO₄ (0.02 ≤ x ≤ 0.1) system were studied by means of high-temperature X-ray diffractometry (HTXRD); ⁶Li, ²⁷Al, ²⁹Si, and ³¹P magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR); thermogravimetry (TG); scanning electron microscopy (SEM); and impedance spectroscopy (IS) techniques. Referring to the experimental results, the Li₄SiO₄ additive material leads to the improvement of the electrical properties and the value of the total ionic conductivity exceeds 10⁻⁴ S∙cm⁻¹ in most studied cases. The factors affecting the enhancement of the total ionic conductivity are discussed. The highest value of σ_tot = 1.4 × 10⁻⁴ S∙cm⁻¹ has been obtained for LATP–0.1LSO material sintered at 1000 °C for 6 h.     

Transport and Electrochemical Properties of Li4Ti5O12-Li2TiO3 and Li4Ti5O12-TiO2 Composites

The study demonstrates that the introduction of the electrochemically inactive dielectric additive Li₂TiO₃ to LTO results in a strong decrease in the grain boundary resistance of LTO-Li₂TiO₃ (LTC) composites at a low concentration of Li₂TiO₃. With the increase in the concentration of Li₂TiO₃ in LTC composites, the grain boundary resistance goes through a minimum and increases again due to the growth of the insulation layer of small Li₂TiO₃ particles around LTO grains. For LTO-TiO₂ (LTT) composites, a similar effect was observed, albeit not as strong. It was found that LTC composites at low concentration of Li₂TiO₃ have unusually high charge–discharge capacity exceeding the theoretical value for pure LTO. This effect is likely to be caused by the occurrence of the electrochemical activity of Li₂TiO₃ in the vicinity of the interfaces between LTO and Li₂TiO₃. The increase in the capacity may be qualitatively described in terms of the model of two-phase composite in which there is the interface layer with a high capacity. Contrasting with LTC composites, in LTT composites, no capacity enhancement was observed, which was likely due to a noticeable difference in crystal structures of LTO and TiO₂ preventing the formation of coherent interfaces.     

The Dielectric Constant of Ba6−3x(Sm1−yNdy)8+2xTi18O54 (x = 2/3) Ceramics for Microwave Communication by Linear Regression Analysis

The electronics related to the fifth generation mobile communication technology (5G) are projected to possess significant market potential. High dielectric constant microwave ceramics used as filters and resonators in 5G have thus attracted great attention. The Ba_6−3x(Sm_1−yNd_y)_8+2xTi₁₈O₅₄ (x = 2/3) ceramic system has aroused people’s interest due to its underlying excellent microwave dielectric properties. In this paper, the relationships between the dielectric constant, Nd-doped content, sintering temperature and the density of Ba_6−3x(Sm_1−yNd_y)_8+2xTi₁₈O₅₄ (x = 2/3) ceramics were studied. The linear regression equation was established by statistical product and service solution (SPSS) data analysis software, and the factors affecting the dielectric constant have been analyzed by using the enter and stepwise methods, respectively. It is found that the model established by the stepwise method is practically significant with Y = −71.168 + 6.946x₁ + 25.799x₃, where Y, x₁ and x₃ represent the dielectric constant, Nd content and the density, respectively. According to this model, the influence of density on the dielectric constant is greater than that of Nd doping concentration. We bring the linear regression analysis method into the research field of microwave dielectric ceramics, hoping to provide an instructive for the optimization of ceramic technology.     

Impact of Li3BO3 Addition on Solid Electrode-Solid Electrolyte Interface in All-Solid-State Batteries

All-solid-state lithium-ion batteries raise the issue of high resistance at the interface between solid electrolyte and electrode materials that needs to be addressed. The article investigates the effect of a low-melting Li₃BO₃ additive introduced into LiCoO₂- and Li₄Ti₅O₁₂-based composite electrodes on the interface resistance with a Li₇La₃Zr₂O₁₂ solid electrolyte. According to DSC analysis, interaction in the studied mixtures with Li₃BO₃ begins at 768 and 725 °C for LiCoO₂ and Li₄Ti₅O₁₂, respectively. The resistance of half-cells with different contents of Li₃BO₃ additive after heating at 700 and 720 °C was studied by impedance spectroscopy in the temperature range of 25–340 °C. It was established that the introduction of 5 wt% Li₃BO₃ into LiCoO₂ and heat treatment at 720 °C led to the greatest decrease in the interface resistance from 260 to 40 Ω cm² at 300 °C in comparison with pure LiCoO₂. An SEM study demonstrated that the addition of the low-melting component to electrode mass gave better contact with ceramics. It was shown that an increase in the annealing temperature of unmodified cells with Li₄Ti₅O₁₂ led to a decrease in the interface resistance. It was found that the interface resistance between composite anodes and solid electrolyte had lower values compared to Li₄Ti₅O₁₂|Li₇La₃Zr₂O₁₂ half-cells. It was established that the resistance of cells with the Li₄Ti₅O₁₂/Li₃BO₃ composite anode annealed at 720 °C decreased from 97.2 (x = 0) to 7.0 kΩ cm² (x = 5 wt% Li₃BO₃) at 150 °C.     

Microstructure and Mechanical Properties of Alumina Composites with Addition of Structurally Modified 2D Ti3C2 (MXene) Phase

This study presents new findings related to the incorporation of MXene phases into ceramic. Aluminium oxide and synthesised Ti₃C₂ were utilised as starting materials. Knowing the tendency of MXenes to oxidation and degradation, particularly at higher temperatures, structural modifications were proposed. They consisted of creating the metallic layer on the Ti₃C₂, by sputtering the titanium or molybdenum. To prepare the composites, powder metallurgy and spark plasma sintering (SPS) techniques were adopted. In order to evaluate the effectiveness of the applied modifications, the emphasis of the research was placed on microstructural analysis. In addition, the mechanical properties of the obtained sinters were examined. Observations revealed significant changes in the MXenes degradation process, from porous areas with TiC particles (for unmodified Ti₃C₂), to in situ creation of graphitic carbon (in the case of Ti₃C₂-Ti/Mo). Moreover, the fracture changed from purely intergranular to cracking with high participation of transgranular mode, analogously. In addition, the results obtained showed an improvement in the mechanical properties for composites with Ti/Mo modifications (an increase of 10% and 15% in hardness and fracture toughness respectively, for specimens with 0.5 wt.% Ti₃C₂-Mo). For unmodified Ti₃C₂, enormously cracked areas with spatters emerged during tests, making the measurements impossible to perform.     

Microstructural Evolution,Tensile Failure,Fatigue Behavior and Wear Properties of Al2O3 Reinforced Al2014 Alloy T6 Heat Treated Metal Composites

The paper focused on an experimental study on the microstructural, mechanical, and wear characteristics of 15 wt.% alumina (Al₂O₃) particulates with an average particle size of 20 µm, reinforced in Al2014 alloy matrix composite as-cast and heat-treated samples. The metal matrix composite (MMC)samples were produced via a novel two-stage stir-casting technique. The fabricated composite samples were subjected to evaluate hardness, tensile strength, fatigue behavior and wear properties for both as cast and T6 heat-treated test samples. The Al2014 alloy and Al2014-15 wt.% Al₂O₃ MMCs were in solution for 1 h at a temperature of 525 °C, quenched instantly in cold water, and then artificially aged for 10 h at a temperature of 175 °C. SEM and X-ray diffraction analyses were used to investigate the microstructure and dispersion of the reinforced Al₂O₃ particles in the composite and the base alloy Al2014. The obtained results indicated that the hardness, tensile and fatigue strength and wear resistance increased when an amount of Al₂O₃ particles was added, compared to the as-cast Al2014 alloy and it was observed that after subjecting the same composite samples to heat treatment, there was further enhancement in the mechanical and wear properties in the Al2014 matrix alloy and Al2014-15 wt.% Al₂O₃ composite samples.     

Sr and Zr Co-Doped CaCu3Ti4O12 Ceramics with Improved Dielectric Properties

The dielectric constant of CCTO materials can be as high as 10⁴, which makes it suitable for use in electronic devices but the high dielectric loss limits its application. In this paper, a series of Sr and Zr co-doped CCTO ceramics having the formula Ca_0.8Sr_0.2Cu₃Ti_4−xZr_xO₁₂ (x = 0.1, 0.2, 0.3, 0.4) were obtained via a solid-state reaction technique. We force the effect of the Zr content on the phase composition, microstructure, cationic valence states, impedance, and dielectric properties of the as-prepared ceramics to reduce dielectric loss. The results demonstrate that Sr and Zr co-doping increases dielectric constant and reduces dielectric loss simultaneously, and the maximum dielectric constant (1.87 × 10⁵, 1 Hz) and minimum dielectric loss (0.43, 10² Hz) are obtained when x = 0.3. Mixed Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ valence states are observed to coexist in the co-doped material lattices, which promote dipole polarization, and thereby increase the dielectric constant of the ceramics. The dielectric properties of the materials are analyzed according to the internal barrier layer capacitance model, which elucidates the contributions of the grains and grain boundaries to dielectric performance. The maximum grain boundary resistance (3.7 × 10⁵ Ω) is obtained for x = 0.3, which contributes toward the minimum dielectric loss (0.43) obtained for this ceramic at a frequency less than 1 kHz. The average grain sizes of the samples decrease with increasing Zr content, which is the primary factor increasing the grain boundary resistance of the co-doped ceramics.     

Microstructure and Properties of TiB2 Composites Produced by Spark Plasma Sintering with the Addition of Ti5Si3

Titanium diboride (TiB₂) is a hard, refractory material, attractive for a number of applications, including wear-resistant machine parts and tools, but it is difficult to densify. The spark plasma sintering (SPS) method allows producing TiB₂-based composites of high density with different sintering aids, among them titanium silicides. In this paper, Ti₅Si₃ is used as a sintering aid for the sintering of TiB₂/10 wt % Ti₅Si₃ and TiB₂/20 wt % Ti₅Si₃ composites at 1600 °C and 1700 °C for 10 min. The phase composition of the initial powders and produced composites was analyzed by the X-ray diffraction method using CuK_α radiation. The microstructure was examined using scanning electron microscopy, accompanied by energy-dispersive spectroscopy (EDS). The hardness was determined using a diamond indenter of Vickers geometry loaded at 9.81 N. Friction–wear properties were tested in the dry sliding test in a ball-on-disc configuration, using WC as a counterpart material. The major phases present in the TiB₂/Ti₅Si₃ composites were TiB₂ and Ti₅Si_3. Traces of TiC were also identified. The hardness of the TiB₂/Ti₅Si₃ composites was in the range of 1860–2056 HV1 and decreased with Ti₅Si₃ content, as well as the specific wear rate W_v. The coefficient of friction for the composites was in the range of 0.5–0.54, almost the same as for TiB₂ sinters. The main mechanism of wear was abrasive.     

High Dielectric Constant and Dielectric Relaxations in La2/3Cu3Ti4O12 Ceramics

La_2/3Cu₃Ti₄O₁₂ ceramics were prepared by the same method of solid-state reaction as CaCu₃Ti₄O₁₂ ceramics. The structure and dielectric responses for La_2/3Cu₃Ti₄O₁₂ and CaCu₃Ti₄O₁₂ ceramics were systematically investigated by X-ray diffraction, scanning electron microscope, X-ray photoelectron spectroscopy, and impedance analyzer. Compared with CaCu₃Ti₄O₁₂ ceramics, La_2/3Cu₃Ti₄O₁₂ ceramics with higher density and refined grain exhibit a high dielectric constant (ε′ ~ 10⁴) and two dielectric relaxations in a wide temperature range. The dielectric relaxation below 200 K with an activation energy of 0.087 eV in La_2/3Cu₃Ti₄O₁₂ ceramics is due to the polyvalent state of Ti³⁺/Ti⁴⁺ and Cu⁺/Cu²⁺, while the dielectric relaxation above 450 K with higher activation energy (0.596 eV) is due to grain boundary effects. These thermal activated dielectric relaxations with lower activation energy in La_2/3Cu₃Ti₄O₁₂ ceramics both move to lower temperatures, which can be associated with the enhanced polyvalent structure in La_2/3Cu₃Ti₄O₁₂ ceramics. Such high dielectric constant ceramics are also expected to be applied in capacitors and memory devices.     

High-Temperature Chemical Stability of Cr(III) Oxide Refractories in the Presence of Calcium Aluminate Cement

Al₂O₃-CaO-Cr₂O₃ castables are used in various furnaces due to excellent corrosion resistance and sufficient early strength, but toxic Cr(VI) generation during service remains a concern. Here, we investigated the relative reactivity of analogous Cr(III) phases such as Cr₂O₃, (Al_1−xCr_x)₂O₃ and in situ Cr(III) solid solution with the calcium aluminate cement under an oxidizing atmosphere at various temperatures. The aim is to comprehend the relative Cr(VI) generation in the low-cement castables (Al₂O₃-CaO-Cr₂O₃-O₂ system) and achieve an environment-friendly application. The solid-state reactions and Cr(VI) formation were investigated using powder XRD, SEM, and leaching tests. Compared to Cr₂O₃, the stability of (Al_1−xCr_x)₂O₃ against CAC was much higher, which improved gradually with the concentration of Al₂O₃ in (Al_1−xCr_x)₂O₃. The substitution of Cr₂O₃ with (Al_1−xCr_x)₂O₃ in the Al₂O₃-CaO-Cr₂O₃ castables could completely inhibit the formation of Cr(VI) compound CaCrO₄ at 500–1100 °C and could drastically suppress Ca₄Al₆CrO₁₆ generation at 900 to 1300 °C. The Cr(VI) reduction amounting up to 98.1% could be achieved by replacing Cr₂O₃ with (Al_1−xCr_x)₂O₃ solid solution. However, in situ stabilized Cr(III) phases as a mixture of (Al_1−xCr_x)₂O₃ and Ca(Al_12−xCr_x)O₁₉ solid solution hardly reveal any reoxidation. Moreover, the CA₆ was much more stable than CA and CA₂, and it did not participate in any chemical reaction with (Al_1−xCr_x)₂O₃ solid solution.     

Influence of Ti3C2Tx MXene and Surface-Modified Ti3C2Tx MXene Addition on Microstructure and Mechanical Properties of Silicon Carbide Composites Sintered via Spark Plasma Sintering Method

This article presents new findings related to the problem of the introduction of MXene phases into the silicon carbide matrix. The addition of MXene phases, as shown by the latest research, can significantly improve the mechanical properties of silicon carbide, including fracture toughness. Low fracture toughness is one of the main disadvantages that significantly limit its use. As a part of the experiment, two series of composites were produced with the addition of 2D-Ti₃C₂T_x MXene and 2D-Ti₃C₂T_x surface-modified MXene with the use of the sol-gel method with a mixture of Y₂O₃/Al₂O₃ oxides. The composites were obtained with the powder metallurgy technique and sintered with the Spark Plasma Sintering method at 1900 °C. The effect adding MXene phases had on the mechanical properties and microstructure of the produced sinters was investigated. Moreover, the influence of the performed surface modification on changes in the properties of the produced composites was determined. The analysis of the obtained results showed that during sintering, the MXene phases oxidize with the formation of carbon flakes playing the role of reinforcement. The influence of the Y₂O₃/Al₂O₃ layer on the structure of carbon flakes and the higher quality of the interface was also demonstrated. This was reflected in the higher mechanical properties of composites with the addition of modified Ti₃C₂T_x. Composites with 1 wt.% addition of Ti₃C₂T_x M are characterized with a fracture toughness of 5 MPa × m^0.5, which is over 50% higher than in the case of the reference sample and over 15% higher than for the composite with 2.5 wt.% addition of Ti₃C₂T_x, which showed the highest fracture toughness in this series.     

Study of the Influence of Sintering Atmosphere and Mechanical Activation on the Synthesis of Bulk Ti2AlN MAX Phase Obtained by Spark Plasma Sintering

The influence of the mechanical activation process and sintering atmosphere on the microstructure and mechanical properties of bulk Ti₂AlN has been investigated. The mixture of Ti and AlN powders was prepared in a 1:2 molar ratio, and a part of this powder mixture was subjected to a mechanical activation process under an argon atmosphere for 10 h using agate jars and balls as milling media. Then, the sintering and production of the Ti₂AlN MAX phase were carried out by Spark Plasma Sintering under 30 MPa with vacuum or nitrogen atmospheres and at 1200 °C for 10 min. The crystal structure and microstructure of consolidated samples were characterized by X-ray Diffraction, Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy. The X-ray diffraction patterns were fitted using the Rietveld refinement for phase quantification and determined their most critical microstructural parameters. It was determined that by using nitrogen as a sintering atmosphere, Ti₄AlN₃ MAX phase and TiN were increased at the expense of the Ti₂AlN. In the samples prepared from the activated powders, secondary phases like Ti₅Si₃ and Al₂O₃ were formed. However, the higher densification level presented in the sample produced by using both nitrogen atmosphere and MAP powder mixture is remarkable. Moreover, the high-purity Ti₂AlN zone of the MAX-1200 presented a hardness of 4.3 GPa, and the rest of the samples exhibited slightly smaller hardness values (4.1, 4.0, and 4.2 GPa, respectively) which are matched with the higher porosity observed on the SEM images.     

Mg2+ Doping Effects on the Structural and Dielectric Properties of CaCu3Ti4O12 Ceramics Obtained by Mechanochemical Synthesis

In this study, ceramic CaCu₃Ti₄O₁₂ (CCTO) and CaCu_3−xMg_xTi₄O₁₂ solid solutions in which 0.1 ≤ x ≤ 0.5 were prepared by the mechanochemical method, realized by a high-energy ball milling technique. The effects of the Mg²⁺ ion concentration and sintering time on the dielectric response in the prepared ceramics were investigated and discussed. It was demonstrated that, by the use of a sufficiently high energy of mechanochemical treatment, it is possible to produce a crystalline product after only 2 h of milling the mixture of the oxide substrates. Both the addition of magnesium ions and the longer sintering time of the mechanochemically-produced ceramics cause excessive grain growth and significantly affect the dielectric properties of the materials. The X-ray diffraction (XRD) analysis showed that all of the as-prepared solid solutions, CaCu_3−xMg_xTi₄O₁₂ (0.0 ≤ x ≤ 0.5), regardless of the sintering time, exhibit a cubic perovskite single phase. The dielectric study showed two major contributions associated with the grains and the grain boundaries. The analysis of the electric modules of these ceramics confirmed the occurrence of Maxwell–Wagner type relaxation, which is dependent on the temperature.     

Low-Temperature Rapid Sintering of Dense Cubic Phase ZrW2−xMoxO8 Ceramics by Spark Plasma Sintering and Evaluation of Its Thermal Properties

In this study, we report a low-temperature approach involving a combination of a sol–gel hydrothermal method and spark plasma sintering (SPS) for the fabrication of cubic phase ZrW_2−xMo_xO₈ (0.00 ≤ x ≤ 2.00) bulk ceramics. The cubic-ZrW_2−xMo_xO₈ (0.00 ≤ x ≤ 1.50) bulk ceramics were successfully synthesized within a temperature range of 623–923 K in a very short amount of time (6–7 min), which is several hundred degrees lower than the typical solid-state approach. Meanwhile, scanning electron microscopy and density measurements revealed that the cubic-ZrW_2−xMo_xO₈ (0.00 ≤ x ≤ 1.50) bulk ceramics were densified to more than 90%. X-ray diffraction (XRD) results revealed that the cubic phase ZrW_2−xMo_xO₈ (0.00 ≤ x ≤ 1.5) bulk ceramics, as well as the sol–gel-hydrothermally synthesized ZrW_2−xMo_xO₇(OH)₂·2H₂O precursors correspond to their respective pure single phases. The bulk ceramics demonstrated negative thermal expansion characteristics, and the coefficients of negative thermal expansion were shown to be tunable in cubic-ZrW_2−xMo_xO₈ bulk ceramics with respect to x value and sintering temperature. The cubic-ZrW_2−xMo_xO₈ solid solution can thus have potential applications in electronic devices such as heat sinks that require regulation of thermal expansion.     

Elaboration of Alumina-Zirconia Composites: Role of the Zirconia Content on the Microstructure and Mechanical Properties

Alumina-zirconia (AZ) composites are attractive structural materials, which combine the high hardness and Young’s modulus of the alumina matrix with additional toughening effects, due to the zirconia dispersion. In this study, AZ composites containing different amounts of zirconia (in the range 5–20 vol %) were prepared by a wet chemical method, consisting on the surface coating of alumina powders by mixing them with zirconium salt aqueous solutions. After spray-drying, powders were calcined at 600 °C for 1 h. Green bodies were then prepared by two methods: uniaxial pressing of spray-dried granules and slip casting of slurries, obtained by re-dispersing the spray dried granulates. After pressureless sintering at 1500 °C for 1 h, the slip cast samples gave rise to fully dense materials, characterized by a quite homogeneous distribution of ZrO₂ grains in the alumina matrix. The microstructure, phase composition, tetragonal to monoclinic transformation behavior and mechanical properties were investigated and are here discussed as a function of the ZrO₂ content. The material containing 10 vol % ZrO₂ presented a relevant hardness and exhibited the maximum value of K_I0, mainly imputable to the t → m transformation at the crack tip.     

Effect of Current Density on the Microstructure and Mechanical Properties of 3YSZ/Al2O3 Composites by Flash Sintering

The 3YSZ/40 wt% Al₂O₃ composites were prepared by flash sintering at a low furnace temperature (700 °C). The effects of the current density on the relative density and Vickers hardness of the composites were systematically investigated. The results showed that the relative densities and Vickers hardness of the samples increased gradually with the increasing of the current densities, and the relative density was as high as 94.2%. The Vickers hardness of 11.3 GPa was obtained under a current density of 102 mA/mm². Joule heating and defects generation are suggested to be the main causes of rapid densification in flash sintering. The microstructure of the molten zone showed the formation of eutectic structures in the composite, suggesting that grain boundary overheating may have contributed to the formation of the molten zone.     

Tribo-oxide Competition and Oxide Layer Formation of Ti3SiC2/CaF2 Self-Lubricating Composites during the Friction Process in a Wide Temperature Range

Ti₃SiC₂/CaF₂ composites were prepared by the spark plasma sintering (SPS) process. Both the microstructure of Ti₃SiC₂/CaF₂ and the influence of test temperature on the tribological behavior of the Ti₃SiC₂/CaF₂ composites were investigated. The synergistic effect of friction and oxidation was evaluated by analyzing the worn surface morphology. The results showed that Ti₃SiC₂/CaF₂ were still brittle materials after adding CaF₂, which was in agreement with Ti₃SiC₂. The hardness, relative density, flexural strength and compressive strength of the Ti₃SiC₂/CaF₂ composites were slightly lower than those of Ti₃SiC₂, and the addition of CaF₂ decreased the decomposition temperature of Ti₃SiC₂ from 1350 to 1300 °C. Simultaneously, as the temperature of the test increased, the friction coefficient of Ti₃SiC₂/CaF₂ showed a downward trend (from 0.81 to 0.34), and its the wear rate was insensitive.