Introducing dislocations in ceramics by mechanical rolling: A first demonstration using SrTiO3 crystal

It has long been known that dislocations can be used to tune the functional and mechanical properties of ceramics. However, introducing dislocations with controllable networks and densities into ceramics is difficult. In this study, a mechanical rolling technique was proposed to introduce dislocations into ceramics. Using a hard SiC ball with a diameter of 5 mm as a roller, plastic zones and dislocations were successfully produced in a SrTiO₃ (STO) single crystal. The plastic zone area and dislocation densities were determined by the applied force (F) and number of rolling cycles. A force of 10 N produced a scalable plastic zone with an area of 140 µm × 5000 µm without crack formation after 100 rolling cycles. The dislocation density at the center of the plastic deformation zone can reach ∼10¹⁴ m², which is an order of magnitude higher than that achieved previously by others. Increasing the applied force increased the density of the introduced dislocations, for example, ∼2 × 10¹⁴ m⁻² under F = 30 and 35 N, however, lead to crack nucleation in the sample. The dislocations introduced significantly enhanced the mechanical properties of the STO crystal. The measured Vickers’ hardness and fracture toughness increased by 55%–60% and 23%–24%, respectively, compared to the crystal before rolling. This method can serve as a robust technique for engineering dislocations in ceramics, fulfilling the requirements of dislocation-tuned mechanical and functional investigations.     

Mechanical tailoring of dislocation densities in SrTiO3 at room temperature

Dislocation-tuned functional properties such as electrical conductivity, thermal conductivity, and ferroelectric properties in oxides are attracting increasing research interest. A prerequisite for harvesting these functional properties in oxides requires successful introduction and control of dislocation density and arrangement without forming cracks, which is a great challenge due to their brittle nature. Here, we report a simple method to mechanically tailor the dislocation densities in single-crystal perovskite SrTiO₃. By using a millimeter-sized Brinell indenter, dislocation densities from ∼10¹⁰ to ∼10¹³ m⁻² are achieved by increasing the number of indenting cycles. Depending on tip radius and indenting load, large and crack-free plastic zones over hundreds of micrometers are created. The dislocation multiplication mechanisms are discussed, and the work hardening in the plastic zone is evaluated by micro-hardness measurement as a function of dislocation density. This simple approach opens many new opportunities in the area of dislocation-tuned functional and mechanical studies.     

Combustion synthesis of high-performance high-entropy dielectric ceramics for energy storage applications

High-entropy dielectric ceramics have demonstrated a promising prospect for applications in energy storage recently. However, most high-entropy dielectrics synthesized by conventional solid-state reaction (SSR) method demonstrated unsatisfactory performance for energy storage. Therefore, it is meaningful to develop a feasible way to fabricate high-performance high-entropy dielectric ceramics. Herein, high-entropy (Sr_0.6Bi_0.2Na_0.2)(Ti_1-xZr_x/2Al_x/4Nb_x/4)O₃ ceramics are prepared by a solution combustion synthesis (SCS) method. The SCS fabricated ceramics (x = 0.25) demonstrate a high recoverable energy density of ∼4.46 J/cm³ at a high critical electric field of 520 kV/cm, a high energy efficiency ∼88.52%, a large power density of ∼176.65 MW/cm³ (at 400 kV/cm), an ultrafast discharge time of ∼48 ns, and a high Vikers hardness of ∼7.09 GPa. The key energy storage parameters are much better than those of the samples prepared by the SSR method owing to the absence of unexpected impurity phases, and the refined grain size at the submicrometer scale in our SCS fabricated high-entropy ceramics. The study provides a facile way to fabricate high-performance high-entropy dielectric ceramics for energy storage, indicating that the SCS routine is notably advantageous for preparing high-entropy dielectric energy ceramics.     

Enhanced optical transmittance and energy-storage performance in NaNbO3-modified Bi0.5Na0.5TiO3 ceramics

Dielectric ceramics with both excellent energy storage and optical transmittance have attracted much attention in recent years. However, the transparent Pb-free energy-storage ceramics were rare reported. In this work, we prepared transparent relaxor ferroelectric ceramics (1 − x)Bi_0.5Na_0.5TiO₃–xNaNbO₃ (BNT–xNN) by conventional solid-state reaction method. We find the NN-doping can enhance the polarization and breakdown strength of BNT by suppressing the grain growth and restrained the reduction of Ti⁴⁺ to Ti³⁺. As a result, a high recoverable energy-storage density of 5.14 J/cm³ and its energy efficiency of 79.65% are achieved in BNT–0.5NN ceramic at 286 kV/cm. Furthermore, NN-doping can promote the densification to improve the optical transmittance of BNT, rising from ∼26% (x = 0.2) to ∼32% (x = 0.5) in the visible light region. These characteristics demonstrate the potential application of BNT–xNN as transparent energy-storage dielectric ceramics.     

Pressureless sintering of SiC ceramics with improved specific stiffness

The effects of B₄C content on the specific stiffness and mechanical and thermal properties of pressureless-sintered SiC ceramics were investigated. SiC ceramics containing 2.5 wt% C and 0.7–20 wt% B₄C as sintering aids could be sintered to ≥ 99.4% of the theoretical density at 2150 °C for 1 h in Ar. The specific stiffness of SiC ceramics increased from 136.1 × 10⁶ to 144.4 × 10⁶ m²‧s⁻² when the B₄C content was increased from 0.7 to 20 wt%. The flexural strength and fracture toughness of the SiC ceramics were maximal with the incorporation of 10 wt% B₄C (558 MPa and 3.69 MPa‧m^1/2, respectively), while the thermal conductivity decreased from ∼154 to ∼83 W‧m⁻¹‧K⁻¹ when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 20 wt% B₄C were ∼346 MPa and ∼105 W‧m⁻¹‧K⁻¹, respectively.     

High-temperature compressive creep behavior and mechanism of Hf6Ta2O17 ceramic as a candidate for thermal barrier coatings

A novel Hf₆Ta₂O₁₇ ceramics is prepared by a solid-state reaction method. High-temperature creep behavior of Hf₆Ta₂O₁₇ and 8YSZ ceramics are investigated by compressive creep test combined with a digital image correlation (DIC) method. It is found that the creep mechanism of Hf₆Ta₂O₁₇ ceramics is controlled by grain boundary sliding associated with dislocation movement (stress exponent ∼2-3, and activation energy of 600–620 kJ/mol). Grain boundary sliding accommodated to the interface reaction is the main creep mechanism of 8YSZ ceramics (stress exponent ∼2, and activation energy of 425∼465 kJ/mol). Hf₆Ta₂O₁₇ ceramics have higher creep resistance than 8YSZ ceramics under the same conditions.     

Inelastic deformation under indentation and scratch loads in a ZrB2–SiC composite

Inelastic deformation features induced in an ultra-high temperature ceramic composite, ZrB₂–SiC, due to static indentation (rate of deformation of the order of 10^?5 s^?1), dynamic indentation (rate of deformation of the order of 10³ s^?1), and high-velocity scratch (500 mm/s) experiments are presented. It was found that this ceramic composite has up to 30% higher dynamic hardness compared to static hardness. Dynamic indentations resulted in extensive transgranular microcracking within the indented regions compared to static indentations. In addition, significant plastic deformation features in terms of slip-line formation were observed within statically and dynamically indented regions. The high-velocity scratch studies revealed extensive transgranular microcracking perpendicular to the scratch direction and slip-lines in and around the scratch path. Preliminary transmission electron microscopy (TEM) observations from regions of slip-lines surrounding the scratch grooves revealed dislocation activity in the composite.     

A novel route to produce BaTiO3 glass-ceramics with nanosized cubic BaTiO3 phase precipitating for high energy-storage applications

To meet requirements of miniaturization devices in high pulsed power technology, super dielectric energy storage performance, such as high dielectric breakdown strength (DBS), large energy storage density with high power density, is extremely important in dielectric materials. However, for BaTiO₃ based ceramics and glass ceramics, there is still a critical challenge to achieve high DBS and large energy storage density. Herein, a novel route was proposed to precipitate nanocrystals with cubic BaTiO₃ phase from glass matrix, which can elevate dielectric constant and meanwhile maintain high DBS compared to parent glass. A high recoverable energy storage density of ∼ 3.66 J cm⁻³ at 1000 kV cm⁻¹ and high discharge energy density of ∼3.57 J cm⁻³ with good thermal stability and ultra-high peak power density of ∼ 910 MW cm⁻³ can be achieved in BaTiO₃ glass ceramic, which implies this type of glass ceramics is suitable for high pulsed power technology application.     

Cold sintering process for 8 mol%Y2O3-stabilized ZrO2 ceramics

In this communication, the cold sintering process was applied to benefit the green body compaction of 8 mol%Y₂O₃-stablized ZrO₂ ceramics (8Y-YSZ). Compared to conventionally processed ceramics, an enhanced densification behavior was demonstrated in cold sintering related ones following a second step conventional sintering process. Dense ceramics up to ∼96% of theoretical density were achieved after sintering at 1200 °C. The resulted ceramics demonstrated a fine microstructure with a grain size ∼200 nm. A mechanical performance with a Vickers hardness of 13.6 GPa and a fracture toughness of 2.85 MPa m^1/2 was also reported.     

Densification mechanism,microstructure and mechanical properties of ZrC ceramics prepared by high-pressure spark plasma sintering

The traditional way of densifying high-melting-point ceramics at high temperatures with long soaking time leads to severe grain coarsening, which degrades the mechanical properties of ceramics. Here, highly dense (∼98%) zirconium carbide (ZrC) ceramics with limited grain growth were obtained by spark plasma sintering (SPS) at relatively low temperatures, 1900 ℃, with a high pressure up to 200 MPa in a reliable carbon-fiber-reinforced carbon composite (C_f/C) mold. Subgrains and high-density dislocations formed in the high-pressure sintered ceramics. The hardness and fracture toughness of the prepared highly dense ZrC ceramics reached 20.53 GPa and 2.70 MPa·m^1/2, respectively. The densification mechanism was mainly plastic deformation under high pressure. In addition, ZrC ceramics sintered at high pressure possessed a high dislocation density of 7.30 × 10¹² m⁻², which was suggested to contribute to the high hardness.     

SiCN ceramics with controllable carbon nanomaterials for electromagnetic absorption performance

Different kinds of carbon nanomaterials, free carbon (C_free), graphene, and N-containing graphene (NG), in single-source-precursors-derived SiCN ceramics, were in situ generated by modifying polysilazane with divinylbenzene, dopamine hydrochloride and melamine, respectively. Adjusting the carbon source brings phase structure and electromagnetic wave absorption (EMA) properties differences of SiCN/C ceramics. In situ C_free enhances the EMA capacity of SiCN ceramics by improving their electrical conductivity of 9.2 × 10⁻⁴ S/cm. The electrical conductivity of SiCN ceramics with 2D graphene sheets balloons to 2.5 × 10⁻³ S/cm, causing poor impedance match thus leading to a worse EMA performance. In situ NG in SiCN ceramics has a low electrical conductivity of 5.6 × 10⁻⁸ S/cm, making for excellent impedance match. The corrugated NG boosts dielectric loss, interfacial, and dipole polarization. NG-SiCN nanocomposites possess an outstanding EMA performance with RL_min of −61.08 dB and effective absorption bandwidth of 4.05 GHz, which are ∼2.4 times lower and ∼4 times higher than those of SiCN, respectively.     

Fabrication of high-efficiency Yb:Y2O3 laser ceramics without photodarkening

High-efficiency Yb:Y₂O₃ laser ceramics were fabricated using the vacuum-sintering plus hot isostatic pressing (HIP) without sintering additives. High-purity well-dispersed nanocrystalline Yb:Y₂O₃ powder was synthesized using a modified co-precipitation method in-house. The green bodies were first vacuum sintered at a temperature as low as 1430°C and then HIPed at 1450°C. Finally, the samples were air annealed at 800°C for 10 h. Although no sintering aids were used, full density of the samples with excellent optical homogeneity and an inline transmission of 80% at 400 nm could be obtained. Moreover, photodarkening phenomenon was not detected in the ceramics. Preliminary laser experiment with the fabricated ceramics in a two-mirror cavity has demonstrated 32 W continuous-wave (CW) output at ∼1077 nm with an optical-to-optical conversion efficiency of 58.2%. To the best of our knowledge, this is so far the highest CW output power and optical-to-optical conversion efficiency achieved with the Yb³⁺-doped sesquioxide ceramics in a simple two-mirror cavity.     

Effects of sintering aids on microstructure,mechanical, and optical properties of AlON ceramics synthesized by SPS

Aluminum oxynitride (AlON) ceramics doped with different sintering aids were synthesized by spark plasma sintering process. The microstructures, mechanical, and optical properties of the ceramics were investigated. The results indicate that the optimal amount of sintering aids is 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO. The addition of La³⁺ and Mg²⁺ decreases the rate of grain boundary migration in ceramics, promotes pore elimination, and inhibits grain growth. The addition of Y³⁺ facilitates liquid-phase sintering of AlON ceramics. Moreover, the addition of Mg²⁺ effectively promotes twin formation in the ceramics, which hinders crack propagation and dislocation motion when the ceramics are loaded. Hence, the AlON ceramic doped with 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO exhibits a relative density of 99.95%, an average grain size of 9.42 μm, and a twin boundary content of 10.3%, which contributes to its excellent mechanical and optical properties.     

Microstructure and mechanical properties of zirconia stabilized with increasing Y2O3, for use in dental restorations

The present in vitro study aims at characterizing dental zirconia ceramics, which are stabilized with a high amount of Y₂O₃. Two groups of specimens were fabricated by computer-aided design/computer-aided manufacturing technique. The specimens of each group were divided into two subgroups (SGs): SGs 1a and 2a contained a relatively low amount of Y₂O₃ (6–8 wt.%), whereas SGs 1b and 2b contained a higher amount of Y₂O₃ (8–10 wt.%). The influence of yttria content on their microstructure and mechanical properties was experimentally determined. The statistical significance of the differences in the mechanical properties between the SGs was evaluated by the t-test (p < 5% was considered statistically significant). Homogeneous and dense ceramics with fine nanostructure, comprising grains of yttria-stabilized tetragonal and cubic zirconia, sized between ∼160 and ∼800 nm, were produced. The increase of yttria content, which causes an increase in grain size, favors the formation of cubic zirconia, resulting in mechanical properties’ slight reduction; yet, the differences were not statistically significant. Consequently, the mechanical properties (HV 11.74–12.91 GPa, and K_IC 2.66–4.25 MPa m^0.5) and the good esthetics of the investigated zirconia ceramics stabilized with high yttria content qualify these zirconia materials for fabricating dental restorations, because they can approach the properties and the esthetics of dental hard tissues as well as the tooth structure.     

Ultrahigh energy storage density in Ba0.85Ca0.15Zr0.1Ti0.9O3-based lead-free ceramics by introducing a relaxor end-member

Dielectric ceramics with relaxor characteristics are promising candidates to meet the demand for capacitors in next-generation pulse devices. In this work, Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT)-based lead-free ceramics with an ultrahigh recoverable energy storage density (W_rec) were designed and fabricated by introducing the relaxor end-member of Bi(Zn_2/3Ta_1/3)O₃ (BZT). The addition of BZT disrupted the ferroelectric (FE) long-range order and triggered an FE-to-relaxor FE (RFE) phase, leading to the formation of locally polar nano-regions (PNRs) and significantly inhibiting grain growth. Meanwhile, the presence of PNRs with good thermal stability improved the temperature stability of both the dielectric constant (ε') and W_rec. More importantly, the breakdown electric field strength was significantly improved up to ∼640 kV/cm, resulting in an ultrahigh W_rec of ∼7.11 J/cm³ for the 8%BZT doped BCZT (BCZT-BZT8) ceramic. Furthermore, the BCZT-BZT8 ceramic exhibited excellent charge/discharge performances (C_D ∼ 458.4 A/cm², P_D ∼ 50.4 MW/cm³, W_D ∼ 1.354 J/cm³, t_0.9 ∼ 320 ns) with good thermal stability in the temperature range of 298–373 K. The defect chemistry of the BCZT-BZT8 was explored using electron paramagnetic resonance (EPR) spectroscopy which revealed an EPR signal (g ∼ 1.955), associated with oxygen vacancies. The above findings indicate that the novel composition of BCZT-BZT8 has great prospects in energy storage capacitor applications.     

Tb3+/Yb3+ co-doped Y2O3 upconversion transparent ceramics: Fabrication and characterization for IR excited green emission

Tb³⁺/Yb³⁺ co-doped Y₂O₃ transparent ceramics were fabricated by vacuum sintering of the pellets (prepared from nanopowders by uniaxial pressing) at 1750 °C for 5 h. Zr⁴⁺ and La³⁺ ions were incorporated in Tb³⁺/Yb³⁺ co-doped Y₂O₃ nanoparticle to reduce the formation of pores which limits the transparency of ceramic. An optical transmittance of ∼80% was achieved in ∼450 to 2000 nm range for 1 mm thick pellet which is very close to the theoretical value by taking account of Fresnel’s correction. High intensity luminescence peak at 543 nm (green) was observed in these transparent ceramics under 976 and 929 nm excitations due to Yb–Tb energy transfer upconversion.     

Final-stage densification kinetics of direct current–sintered ZrB2

Final-stage sintering was analyzed for nominally phase pure zirconium diboride synthesized by borothermal reduction of high-purity ZrO₂. Analysis was conducted on ZrB₂ ceramics with relative densities greater than 90% using the Nabarro–Herring stress–directed vacancy diffusion model. Temperatures of 1900°C or above and an applied uniaxial pressure of 50 MPa were required to fully densify ZrB₂ ceramics by direct current sintering. Ram travel data were collected and used to determine the relative density of the specimens during sintering. Specimens sintered between 1900 and 2100°C achieved relative densities greater than 97%, whereas specimens sintered below 1900°C failed to reach the final stage of sintering. The average grain size ranged from 1.0 to 14.7 μm. The activation energy was calculated from the slope of an Arrhenius plot that used the Kalish equation. The activation energy was 162 ± 34 kJ/mol, which is consistent with the activation energy for dislocation movement in ZrB₂. The diffusion coefficients for dislocation motion that controls densification were 5.1 × 10⁻⁶ cm²/s at 1900°C and 5.1 × 10⁻⁵ cm²/s at 2100°C, as calculated from activation energy and average grain sizes. This study provides evidence that the dominant mechanism for final-stage sintering of ZrB₂ ceramics is dislocation motion.     

Effect of glassy bonding materials on the properties of LAS/SiC porous ceramics with near zero thermal expansion

Near zero thermal expansion porous ceramics were fabricated by using SiC and LiAlSiO₄ as positive and negative thermal expansion materials, respectively, bonded by glassy material. The coefficient of thermal expansion value of a desired porous composite can be easily controlled by choosing the appropriate ratios of the different phases. It was shown that some of LiAlSiO₄ was decomposed to LiAlSi₂O₆ and LiAlO₂, some of LiAlSiO₄ reacted with SiO₂ to form LiAlSi₂O₆ during sintering. With increasing the content of glassy materials, the reaction between LiAlSiO₄ and SiO₂ was accelerated. The Young's modulus increased due to the neck growth between the SiC grains. The 52.5 vol% LiAlSiO₄ (LAS)/SiC ceramics with ∼36% porosity had a combination of near zero coefficient of thermal expansion ∼0.39 × 10⁻⁶ K⁻¹ at room temperature and relatively high Young's modulus ∼59 GPa.     

Study on the subsurface damage mechanism of optical quartz glass during single grain scratching

The single grain scratching SPH simulation model was established to study the subsurface damage of optical quartz glass. Based on the analysis of the stress, strain and scratching force during scratching, the generation and propagation of subsurface cracks were studied by combining with the scratch elastic stress field model. The simulation results show that the cracks generate firstly at the elastic-plastic deformation boundary in front of the grain (φ = 28°) due to the influence of the maximum principal tensile stress. During the scratching process, the median crack closes to form the subsurface damage by extending downward, the lateral crack promotes the brittle removal of the material by extending upward to the free surface, and microcracks remain in the elastic-plastic boundary at the bottom of the scratch after scratching. The depth of subsurface crack and plastic deformation increases with rising scratching depth. The increase of scratching speed leads to the greater dynamic fracture toughness, accompanied by a significant decrease of the maximum depth of subsurface crack and the number of subsurface cracks. The subsurface residual stress is concentrated at the bottom of the scratch, and the residual stress on both sides of the scratch surface would generate and propogate the Hertz crack. When the scratching depth is less than 1.5 μm or the scratching speed is greater than 75 m/s, the residual stress value and the depth of residual stress are relatively small. Finally, the scratching experiment was carried out. The simulation analysis is verified to be correct, as the generation and propagation of the cracks in the scratching experiment are consistent with the simulation analysis and the experimental scratching force indicates the same variation tendency with the simulation scratching force. The research results in this paper could help to restrain the subsurface damage in grinding process.     

Annealing induced discoloration of transparent YAG ceramics using divalent additives in solid-state reaction sintering

Tetraethyl orthosilicate (TEOS) was commonly served as a sintering additive to promote the densification of transparent Y₃Al₅O₁₂ (YAG) ceramics. However, Si⁴⁺ that decomposed from TEOS would restrain the conversion of dopants into a higher valence state (e.g., Cr³⁺ → Cr⁴⁺). In this study, by using divalent sintering additives (CaO and MgO), the colorless and highly transparent YAG ceramics (T = 84.6%, at 1064 nm) were obtained after vacuum sintering at 1840 °C for 8 h and without subsequent annealing in air. An absorption peak centered at ∼320 nm was observed before annealing, and it extended to ∼550 nm after annealing at 1450 °C for 10 h in air. A discoloration phenomenon occurred and more scattering centers were observed with the formation of new [Mg/Ca²⁺F⁺] color centers. Air annealing did not improve the optical quality of the as-fabricated YAG ceramics with divalent dopants as sintering additives, owing to the formation of scattering centers.