Mechanical properties and bioactivity of β-Ca2SiO4 ceramics synthesized by spark plasma sintering

Bioactive beta-dicalcium silicate ceramics (β-Ca₂SiO₄) were fabricated by spark plasma sintering (SPS). The relative density of as-prepared β-Ca₂SiO₄ ceramics reached 98.1% when sintered at 1150 °C, leading to great improvement in bending strength (293 MPa), almost 10 times higher than that of the specimen prepared by conventional pressureless sintering (PLS). High fracture toughness (3.0 MPa m^1/2) and Vickers hardness (5.8 GPa) of β-Ca₂SiO₄ ceramics were also achieved by SPS at 1150 °C. The simulated body fluid (SBF) results showed that β-Ca₂SiO₄ ceramics had a good in vitro bioactivity to induce hydraxyapatite (HAp) formation on their surface, which suggests that β-Ca₂SiO₄ ceramics are promising candidates for load-bearing bone implant materials.     

Wear properties of α- and α/β-SiAlON ceramics obtained by gas pressure sintering and spark plasma sintering

In this study, α- and α/β-SiAlON materials, doped with Y₂O₃ and Nd₂O₃, were sintered using two different sintering processes: spark plasma sintering (SPS) and gas pressure sintering (GPS). The wear and mechanical properties of the samples were compared related to the composition, additives and sintering processes. The results show that the hardness was not affected by the processing type whereas the toughness values were lower for spark plasma sintered materials than gas pressure sintered materials. This can be explained by the changed microstructure of the two different types of material. Additionally, α/β-SiAlON materials, sintered using gas pressure sintering, showed a lower wear than the spark plasma sintered materials. The results of the wear test were compared with β-Si₃N₄ materials and it was observed that α/β-SiAlON, sintered by GPS, has better wear properties than the tested β-Si₃N₄ materials.     

High-temperature thermoelectric properties of Sm3+-doped Ca3Co4O9+δ fabricated by spark plasma sintering

Ca_3-xSm_xCo₄O_9+δ (0 ≤ x ≤ 0.3) samples were fabricated by the sol-gel method followed by spark plasma sintering in vacuum. The high-temperature thermoelectric properties of the Ca_3-xSm_xCo₄O_9+δ were also studied, with an emphasis placed on the partial substitution of Sm³⁺ for Ca²⁺. The sintered Ca_3-xSm_xCo₄O_9+δ formed a monoclinic Ca₃Co₄O₉ phase and exhibited fine lamellar grains and dense morphology. With increased Sm³⁺ content, the electrical and thermal conductivities decreased, whereas the Seebeck coefficient significantly increased. Of the prepared samples, Ca_2.7Sm_0.3Co₄O_9+δ had the largest dimensionless figure-of-merit (0.175) at 800 °C. The results showed that the partial substitution of Sm³⁺ for Ca²⁺ in Ca₃Co₄O_9+δ is effective for enhancing its thermoelectric properties.     

Microwave dielectric properties of 0.85CaWO4–0.15SmNbO4 ceramics with sintering additives

Low-temperature sintering and microwave dielectric properties of 0.85CaWO₄–0.15SmNbO₄ (CWSN) ceramics were investigated as a function of Li₂MoO₄ and/or Li₂WO₄ content. With an addition of Li₂MoO₄ and/or Li₂WO₄, the sintering temperature could be reduced from 1150 °C for pure CWSN ceramics to 800 °C. The dielectric constant (K) was not changed remarkably with Li₂MoO₄ and/or Li₂WO₄ content. Quality factor (Qf) of the specimens was decreased with Li₂WO₄ content, while that of the specimens was increased with Li₂MoO₄ content. Qf values of the specimens with 1.0 wt.% Li₂WO₄ showed larger value than that of the specimens with 2.5 wt.% Li₂MoO₄. The temperature coefficient of resonant frequency (TCF) was shifted to the positive value with increasing Li₂WO₄ and/or Li₂MoO₄ content. Typically, K of 12.03, Qf of 13,300 GHz and TCF of −28.6 ppm/°C were obtained for the specimens with 1.0 wt.% Li₂WO₄ sintered at 800 °C for 1 h.     

AlMgB14–TiB2 composite materials obtained by self-propagating high-temperature synthesis and spark plasma sintering

In this work, AlMgB₁₄–TiB₂ composite materials were obtained by thermochemical-coupled self-propagating high-temperature synthesis (SHS) and subsequent spark plasma sintering. The mechanism was proposed for the formation of the composite materials in the thermochemical-coupled SHS mode. The phase composition, microstructure, and properties (density and Vickers hardness) of the dense AlMgB₁₄–TiB₂ materials were investigated. At a sintering temperature of 1470 °C, AlMgB₁₄ is decomposed into AlB₁₂ and Mg. The sample sintered at 1470 °C with a holding time of 5 min had a maximum average hardness of 32.1 GPa.     

Effect of ball milling on reactive spark plasma sintering of B4C–TiB2 composites

The influence of mechanical activation by ball milling (BM) of Ti, B and graphite powders mixture on the synthesis of dense B₄C-41% vol. TiB₂ composite by Spark Plasma Sintering (SPS) is investigated. BM treatment produces grains size refinement (50–150 nm) in the processing powders and the formation of TiB and TiB₂, when milling times are longer than 6 h.     

Sintering,microstructure and conductivity of La2NiO4+δ ceramic

Microstructure and electrical conducting properties of La₂NiO_4+δ ceramic were investigated in the sintering temperature range 1200–1400 °C. The results demonstrate that the microstructure and electrical conducting properties of La₂NiO_4+δ ceramic are sensitive to sintering temperature. Compared with a progressive densification development with sintering temperature from 1200 to 1300 °C along with an insignificant change in grain size, there is an exaggerated grain growth in the specimens sintered at higher temperatures. Increasing sintering temperature from 1200 to 1300 °C resulted in an enhancement of electrical conducting properties. Further increase of sintering temperature exceeding 1300 °C reduced the electrical conducting properties. A close relation between the microstructure and electrical conducting properties was suggested for La₂NiO_4+δ ceramic. With respect to the electrical conducting properties, the preferred sintering temperature of La₂NiO_4+δ ceramic was ascertained to be 1300 °C. The specimen sintered at 1300 °C exhibits a generally uniform microstructure together with electrical conductivities of 76–95 S cm⁻¹ at 600–800 °C.     

Preparation of CaCu3Ti4O12 ceramics with low dielectric loss and giant dielectric constant by the sol–gel technique

CaCu₃Ti₄O₁₂ precursor powders were synthesized by the sol–gel process. The optimized processing parameters for the synthesis of precursor powders were as follows: the Ti concentration was 0.60 mol/L, the pH value of the sol was 1.58, and the aging time of the sol was 6 h. After sintering at 1100 °C for 15 h, the CCTO ceramics with the highest density and fine-grained microstructure were obtained, exhibiting outstanding dielectric properties: ε′≈3.50×10⁴ and tan δ=0.014 (at 1 kHz). The low dielectric loss was attributed to the highest grain boundary resistance which significantly reduced the leakage current across grain boundaries. A broad dielectric relaxation peak was observed around 300 °C. The complex impedance spectroscopy analysis suggested that the obtained CaCu₃Ti₄O₁₂ ceramics were electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries. The calculated grain boundary resistance and grain resistance were 0.87 MΩ and 3.50 Ω, respectively.     

Densification and strengthening mechanism in spark plasma sintering of B4C–VB2 ceramics via carbide boronizing

To reduce the negative effects of the long-time and B₂O₃ phase on the traditional sintering process for B₄C-based composite ceramics, nearly fully dense B₄C–VB₂ composite ceramics were prepared by reactive spark plasma sintering (SPS) technology at 2000 °C with B and V₈C₇ powders as raw materials in this paper. The effects of the degassing time during SPS on the microstructure and the mechanical properties of the final products were investigated in detail. The results revealed that the proper degassing time was beneficial for the vent of B₂O₃ during the sintering process, which refined the grain size, promoted densification and improved the mechanical properties of the composite ceramic. However, the redundant degassing time increased the holding time at high temperature, resulting in abnormal grain growth and mechanical performance deterioration. In the present work, the optimal degassing time was 6 min, and the final product prepared under the above conditions exhibited excellent comprehensive performance with a relative density of 99.2%, Vickers hardness of 31.2 GPa, bending strength of 654 MPa and fracture toughness of 5.7 MPa m^1/2. In addition, the strengthening and toughening mechanisms of the products were mainly attributed to the residual thermal stresses and bridging structure caused by the fine B₄C and VB₂ grains distributed uniformly.     

Microstructural development during spark plasma sintering of ZrB2–SiC–Ti composite

The present study investigates the effect of Ti addition on the microstructure development and phase evolution during spark plasma sintering of ZrB₂–SiC ceramic composite. A ZrB₂–20?vol% SiC sample with 15?wt% Ti was prepared by high-energy milling and spark plasma sintering at 2000?°C for 7?min under 50?MPa. The X-ray diffraction test, microstructural studies and thermodynamic assessments indicated the in-situ formation of several compounds due to the chemical reactions of Ti with ZrB₂ and SiC. The Ti additive was completely consumed during the sintering process and converted to the ceramic compounds of TiC, TiB and TiSi₂. In addition, another refractory phase of ZrC was also formed as a result of sidelong reaction of ZrB₂ and SiC with the Ti additive.     

Surface layer formation on Li1 + xMn2O4 − δ thin film electrodes during electrochemical cycling

A surface layer formation on positive Li_{1 + x}Mn₂O_{4 − δ} thin film model electrodes as a result of electrochemical cycling procedures has been detected and characterized by scanning electron microscopy and X-ray photoemission spectroscopy. These thin film spinel electrodes, prepared by pulsed laser deposition, were cycled in 1 M LiClO₄ in propylene carbonate between 3.5 and 4.4 V vs. Li/Li⁺ at 40 °C and stopped at defined potentials and cycle numbers. The observed surface layers show, depending on the cycling conditions, a spotty and/or layered appearance and the fraction of this layer covering the cycled electrode depends on the charge potential and the number of electrochemical cycles.     

Synthesis of SiC/SiO2 core–shell powder by rotary chemical vapor deposition and its consolidation by spark plasma sintering

SiC (core) and SiO₂ (shell) powders were synthesized via rotary chemical vapor deposition (RCVD). The SiC particles (3C, <1 μm in diameter) were coated with a layer of SiO₂ (10–15 nm in thickness). Using spark plasma sintering, the SiC/SiO₂ nanopowders were then synthesized into SiC/SiO₂ composite bodies. Although a phase transformation from 3C to 6H was observed at above 2123 K in the sintered monolithic SiC bodies, sintered SiC/SiO₂ bodies did not display such phase transformation. In addition, SiC/SiO₂ bodies did not exhibited grain growth until the sintering temperature reached 2223 K. The density and Vickers hardness of the sintered SiC/SiO₂ bodies increased with increasing sintering temperature. The highest density and hardness of SiC/SiO₂ composite bodies were 98.1% and 24.4 GPa at 2223 K, respectively, which were higher than the corresponding values of 90% and 14 GPa for monolithic SiC bodies.     

Microstructure and properties of mullite–ZrO2 ceramics with silicon nitride additive prepared by spark plasma sintering

The process of densification and development of the microstructure of mullite–ZrO₂/Y₂O₃ ceramics from mixture of Al₂O₃, SiO₂, ZrO₂ and Y₂O₃ by gradually adding of α–β Si₃N₄ nanopowder from 1 to 5 wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400 °C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si₃N₄ additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33 g/cm³, while for traditionally sintered samples, the value is 2.55 g/cm³, and the compressive strength for SPS grows with Si₃N₄ additives, reaching 600 N/mm². In the case of traditional sintering, it decreases to approximately 100 N/mm². The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO₂ grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si₃N₄ additive, are filled with mullite crystalline formations.     

Effect of Sr2+ and Ba2+ doping on structural stability and mechanical properties of La2NiO4+δ

Structural and mechanical Characterizations of La_1.8M_0.2NiO_4+δ (M: Sr and Ba) prepared by low frequency ultra-sound assisted synthesis technique and sintered at different temperatures were studied. HRTEM and XRD analyses showed the uniform shape of calcined nanocrystalline powders with the particle size of less than 100?nm with mixed phases, which were refined by Rietveld method using orthorhombic (Fmmm) and tetragonal (F4/mmm) structures. Sintering La_1.8Sr_0.2NiO_4+δ and La_1.8Ba_0.2NiO_4+δ compacted discs at temperatures higher than 1300?°C and 1250?°C, respectively, resulted in appearance of extra peaks close to a monoclinic phase. Doping La₂NiO_4+δ with Sr²⁺ and Ba²⁺ did not affect its sinterability and average grain size significantly, however, Ba²⁺ improved the elastic modulus and microhardness, while Sr²⁺ improved the fracture toughness.     

Low temperature sintering and microwave dielectric properties of CaSiO3–Al2O3 ceramics for LTCC applications

The effects of CuO, Li₂CO₃ and CaTiO₃ additives on the densification, microstructure and microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics for low-temperature co-fired applications were investigated. With a single addition of 1 wt% Li₂CO₃, the CaSiO₃–1 wt% Al₂O₃ ceramic required a temperature of at least 975 °C to be dense enough. Large amount addition of Li₂CO₃ into the CaSiO₃–1 wt% Al₂O₃ ceramics led to the visible presence of Li₂Ca₃Si₆O₁₆ and Li₂Ca₄Si₄O₁₃ second phases. Fixing the Li₂CO₃ content at 1 wt%, a small amount of CuO addition significantly promoted the sintering process and lowered the densification temperature to 900 °C whereas its addition deteriorated the microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics. Based on 10 wt% CaTiO₃ compensation in temperature coefficient, good microwave dielectric properties of ε_r=8.92, Q×f=19,763 GHz and τ_f=−1.22 ppm/°C could be obtained for the 0.2 wt% CuO and 1.5 wt% Li₂CO₃ doped CaSiO₃–1 wt% Al₂O₃ ceramics sintered at 900 °C. The chemical compatibility of the above ceramics with silver during the cofiring process has also been investigated, and the result showed that there was no chemical reaction between silver and ceramics, indicating that the as-prepared composite ceramics were suitable for low-temperature co-fired ceramics applications.     

Structural evolution,dielectric and energy storage properties of Na(Nb1−xTax)O3 ceramics prepared by spark plasma sintering

Phase pure NaNb_1−xTaxO₃ ceramics have been successfully prepared by spark plasma sintering (SPS) process at 950–1150 °C. The structural evolution, dielectric and energy storage properties as a function of Ta-doping level were studied by X-ray diffraction (XRD) Rietveld refinement, X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, scan electron microscopy (SEM) and ferroelectric analyzer. It was found that small level of Ta-doping induced the stabilization of ferroelectric phase from antiferroelectric NaNbO₃, while high level of doping induced further transition from ferroelectric to paraelectric phase. The doping of Ta led to the decrease in Tc and dielectric loss. The NTN4 and NTN6 composition exhibited enhanced dielectric response due to the coexistence of ferroelectric/antiferroelectric or paraelectric phases. The presence of oxygen vacancies in the sintered body were confirmed, which gave rise to the loss at high temperature. The doping of Ta decreased the concentration of oxygen vacancies. Maximum energy density of ∼0.9 J/cm³ could be obtained for x = 0.6 composition with the BDS of ∼160 kV/cm and efficiency of ∼87%.     

Colossal dielectric constant and extremely low loss in T-type La2CuO4−δ ceramics

In this paper, we present the colossal dielectric behavior of T-type La₂CuO_4-δ (LCO) ceramics synthesized in fine grained form using wet chemical “Pechini” process, followed by annealing in argon (Ar) atmosphere. The colossal dielectric constant (CDC) (10³≤ε_r′≤10⁴) displayed over a wide frequency (1 Hz ≤f ≤ 1 MHz) and temperature (−100–150 °C) ranges was equally complimented by the remarkably low dielectric losses (0.01≤ tan δ≤0.1) for LCO ceramics, which are the lowest reported dielectric losses for the T-type La₂CuO₄ system, so far. This substantial decrease in losses could be attributed to the enhanced resistivity (10⁸−10⁹ Ω.cm) of the sample. Further, the origin of CDC, in this non-ferroelectric LCO, was investigated using combined ac impedance and modulus spectroscopy. The study revealed heterogeneity in electrical microstructure of LCO, with semiconducting grains and insulating grain boundaries. This electrically heterogeneous microstructure could give rise to the Internal Barrier Layer Capacitance (IBLC) mechanism, thus leading to apparent CDC in LCO.     

Effect of the B2O3 addition on the sintering behavior and microwave dielectric properties of Ba3(VO4)2–Zn1.87SiO3.87 composite ceramics

The effect of the B₂O₃ addition on the low-temperature sintering, microstructure and microwave dielectric properties of the Ba₃(VO₄)₂–Zn_1.87SiO_3.87 composite ceramics was investigated. The results indicate that the addition of B₂O₃ can effectively promote the densification and further improve the microwave dielectric properties of the composite. The low-temperature sintering mechanism was ascribed to the formation of the liquid phase owing to the reaction between the additive B₂O₃ and the residual SiO₂ in the composite. B₂O₃–SiO₂ liquid phases can not only lower the sintering temperature, but also speed up the grain growth of the composite ceramics. The rapid grain growth occurs as the B₂O₃ content is more than 6 wt%. The 3 wt% B₂O₃ doped 0.5Ba₃(VO₄)₂–0.5Zn_1.87SiO_3.87 ceramics can be well sintered at 925 °C and exhibit excellent microwave dielectric properties of Q×f∼40,800 GHz, ε_r∼10 and τ_f∼0.5 ppm/°C.     

Colossal permittivity and dielectric relaxations in Tl + Nb co-doped TiO2 ceramics

A series of Tl?+?Nb co-doped TiO₂ ceramics ((Tl_0.5Nb_0.5)_x%Ti_1-x%O₂ 0.5?≤?x?≤?10.0) were prepared by a solid-state reaction method under N₂ atmosphere. The evolution of their microstructures, and dielectric properties were systematically studied. The co-doped ceramics exhibited a tetragonal rutile structure wherein the Nb and Tl elements were homogeneously distributed. The cell volumes, grain size, and permittivity increased with doping x, whereas the impedance values of the grain and grain boundary decreased with an increasing x. The optimum dielectric performance (ε_r >?10⁴, tanδ?<?0.05) in the range of 10–10⁶ Hz was obtained for x?=?1.5 with a corresponding grain boundary active energy of 0.86?eV. Four types of dielectric relaxation were observed at different temperature ranges: 10–30?K, 30–200?K, 200–350?K and 350–475?K; those dielectric relaxtions were respectively caused by electron-pinned defect-dipoles, electron hopping, oxygen vacancy hopping, and Maxwell–Wagner polarization. The colossal permittivity is primarily a result of the electron-pinned defect-dipole polarization.     

Spark plasma sintering of Al-doped ZrB2–SiC composite

This research presents the influence of Al addition on microstructure and mechanical behavior of ZrB₂–SiC ultra-high temperature ceramic matrix composite (UHTCMC) fabricated by spark plasma sintering (SPS). A 2.5?wt% Al-doped ZrB₂–20?vol% SiC UHTCMC was produced by SPS method at 1900?°C under a pressure of 40?MPa for 7?min. The microstructural and phase analysis of the composite showed that aluminum-containing compounds were formed in-situ during the SPS as a result of chemical reactions between Al and surface oxide films of the raw materials (i.e. ZrO₂ and SiO₂ on the surfaces of ZrB₂ and SiC particles, respectively). The Al dopant was completely consumed and converted to the intermetallic Al₃Zr and Al₄Si compounds as well as Al₂O₃ and Al₂SiO₅. A relative density of 99.8%, a hardness (HV5) of 21.5?GPa and a fracture toughness (indentation method) of 6.3?MPa?m^1/2 were estimated for the Al-doped ZrB₂–SiC composite. Crack bridging, branching, and deflection were identified as the main toughening mechanisms.