Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

Yb3+ doped Lu2O3 transparent ceramics by spark plasma sintering

Transparent ceramics of 10% Yb doped Lu₂O₃ was fabricated by spark plasma sintering. The operating vital parameters in yielding transparency and mutual effects of sintering, pressure, dwell time, heating rate and annealing temperature on microstructure have been investigated. Fully compacted specimens were obtained at 1250 °C and the average grain size increased from few nm up to 5 μm until 1700 °C, above which abnormal grain growth was witnessed. The post-annealing of sintered ceramics at 1200 °C removes discoloration and improves transparency. The ceramics prepared at 1700 °C with dwell time of 5 min and heating rate at 50 °C/min shows the maximum transmittance with a thickness of 2 mm of 55% at a wavelength of 2 μm.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

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.     

Annealing effect on dielectric property of AlN ceramics

Effects of slow-cooling at high temperatures and annealing at intermediate temperatures on dielectric loss tangent of AlN ceramics were explored. Y₂O₃ was added as a sintering additive to AlN powders, and the powders were pressureless-sintered at 1900 °C for 2 h in a nitrogen flow atmosphere. In succession to the sintering, AlN samples were slow-cooled at a rate of 1 °C/min from 1900 to 1750 °C and/or annealed at 970 °C for 4 h. Al₅Y₃O₁₂ was detected in the AlN ceramics obtained by the slow-cooling and AlYO₃ was found in the ceramics cooled at a rate of 30 °C/min. AlN ceramics with a relative density of 0.986 were obtained by the slow-cooling method. On the other hand, very low tan δ values between 2.6 and 4.6 × 10⁻⁴ were obtained when the AlN ceramics were annealed at 970 °C for 4 h.     

Influence of sintering conditions on piezoelectric properties of KNbO3 ceramics

A homogeneous KNbO₃ (KN) phase was formed in specimens that were sintered at 1020 °C and 1040 °C, without formation of the K₂O-deficient secondary phase, indicating that the amount of evaporation of K₂O during sintering was very small. However, the KN liquid phase was formed during sintering and assisted the densification of the KN ceramics. A dense microstructure was developed in the specimen sintered at 1020 °C for 6 h and abnormal grain growth occurred in this specimen. A similar microstructure was observed in the specimens sintered at 1040 °C for 1.0 h. The dielectric and piezoelectric properties of the KN ceramics were considerably influenced by the relative density. The KN ceramics sintered at 1020 °C for 6 h, which showed a large relative density that was 95% of the theoretical density, exhibited promising electrical properties: ɛ^T₃₃/ɛ_o of 540, d₃₃ of 109 pC/N, k_p of 0.29, and Q_m of 197.     

Sintering of zirconia ceramics by intense high-energy electron beam

A comparative analysis of the efficiency of zirconia ceramics sintering by thermal method and high-energy electron beam sintering was performed for compacts prepared from commercial TZ-3Y-E grade powder. The electron energy was 1.4 MeV. The samples were sintered in the temperature range of 1200–1400 °C. Sintering of zirconia ceramics by high-energy accelerated electron beam is shown to reduce the firing temperature by about 200 °C compared to that in conventional heating technique. Ceramics sintered by accelerated electron beam at 1200 °C is of high density, microhardness and smaller grain size compared to that produced by thermal firing at 1400 °C. Electron beam sintering at higher temperature causes deterioration of ceramics properties due to radiation-induced acceleration of high-temperature recrystallization at higher temperatures.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

Sintering behaviour and translucency of dense Eu2O3 ceramics

Eu₂O₃ ceramics have been obtained at sintering temperatures of between 1000 °C and 1550 °C. X-ray diffraction and scanning electron microscopy, in combination with dilatometry experiments, allowed understanding the sintering behaviour. Moderate grain growth followed an efficient densification process between 1400 °C and 1550 °C, which yielded high-density ceramics with an average grain size of 4 μm. The ceramics had Young modulus of 125 GPa, in agreement with the previously published data. The dense Eu₂O₃ ceramics were translucent (35.1% transmittance at 800 nm of 0.8 mm thick discs), showing in addition a slightly pink colour. We propose that the combination of high density and an average grain size of 4 μm is responsible for this novel functionality.     

Ultra-high elevated temperature strength of TiB2-based ceramics consolidated by spark plasma sintering

Spark plasma sintering of TiB₂–boron ceramics using commercially available raw powders is reported. The B₄C phase developed during reaction-driven consolidation at 1900 °C. The newly formed grains were located at the grain junctions and the triple point of TiB₂ grains, forming a covalent and stiff skeleton of B₄C. The flexural strength of the TiB₂–10 wt.% boron ceramic composites reached 910 MPa at room temperature and 1105 MPa at 1600 °С. Which is the highest strength reported for non-oxide ceramics at 1600 °C. This was followed by a rapid decrease at 1800 °C to 480–620 MPa, which was confirmed by increased number of cavitated titanium diboride grains observed after flexural strength tests.     

Spark-plasma sintering of ZrB2 ultra-high-temperature ceramics at lower temperature via nanoscale crystal refinement

We have explored the feasibility of reducing the spark-plasma-sintering (SPS) temperature of additive-free ZrB₂ ultra-high-temperature ceramics (UHTCs) via crystal size refinement of the starting powder down to the low nanoscale. We found that under otherwise the same SPS conditions (75 MPa pressure, and 100 °C/min heating ramp) nanoscale ZrB₂ can be densified at temperatures about 450 °C lower than for the typical micrometre and submicrometre ZrB₂ powders, and at least 250 °C below the ultra-fine powder temperature. Furthermore, the nanoscale crystal refinement also promotes the production of fine-grained ZrB₂ UHTCs. We also found that elimination of the B₂O₃ impurities plays an important role in the complete densification. The unequalled sinterability of the nanoscale ZrB₂ powders highlights the need to use high-energy ball-milling for the comminution of the typical commercially available ZrB₂ powders.     

Dynamic compression behavior of reactive spark plasma sintered ultrafine grained (Hf,Zr)B2–SiC composites

This paper reports the dynamic compression behavior of ultrafine grained (Hf, Zr)B₂–SiC composites, sintered using reactive spark plasma sintering at 1600 °C for 10 min. Dynamic strength of~2.3 GPa has been measured using Split Hopkinson Pressure Bar (SHPB) tests in a reproducible manner at strain rates of 800–1300 s⁻¹. A comparison with competing boride based armor ceramics, in reference to the spectrum of properties evaluated, establishes the potential of (Hf, Zr)B₂–SiC composites for armor applications.     

Effect of particle size,heating rate and CNT addition on densification,microstructure and mechanical properties of B4C ceramics

Monolithic B₄C ceramics and B₄C–CNT composites were prepared by spark plasma sintering (SPS). The influence of particle size, heating rate, and CNT addition on sintering behavior, microstructure and mechanical properties were studied. Two different B₄C powders were used to examine the effect of particle size. The effect of heating rate on monolithic B₄C was investigated by applying three different heating rates (75, 150 and 225 °C/min). Moreover, in order to evaluate the effect of CNT addition, B₄C–CNT (0.5–3 mass%) composites were also produced. Fully dense monolithic B₄C ceramics were obtained by using heating rate of 75 °C/min. Vickers hardness value increased with increasing CNT content, and B₄C–CNT composite with 3 mass% CNTs had the highest hardness value of 32.8 GPa. Addition of CNTs and increase in heating rate had a positive effect on the fracture toughness and the highest fracture toughness value, 5.9 MPa m^1/2, was achieved in composite with 3 mass% CNTs.     

Sintering kinetics of disperse ultrafine equiaxed α-Al2O3 nanoparticles

The sintering kinetics of ceramic nanoparticles is essential for preparing dense nanocrystalline ceramics with fine grains, but the sintering kinetics of disperse ultrafine α-Al₂O₃ nanoparticles has not been systematically explored so far. In this paper, the sintering kinetics of disperse ultrafine equiaxed α-Al₂O₃ nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without any agglomeration was studied systematically. Finally, α-Al₂O₃ nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered in air by two-step sintering (heated to 1100 °C without hold and then cooled down to 950 °C with a 40 h hold). The sintering temperature is the lowest for pressureless sintering of α-Al₂O₃ and almost fully dense α-Al₂O₃ nanocrystalline ceramic obtained also has the finest grain so far.     

Sintering behavior and microwave dielectric properties of 0.67CaTiO3–0.33LaAlO3 ceramics modified by B2O3–Li2O–Al2O3 and CeO2

A low temperature sintering method was used to avoid the relatively high sintering temperatures typically required to prepare 0.67CaTiO₃–0.33LaAlO₃ (CTLA) ceramics. Additionally, CeO₂ was introduced into the CTLA ceramics as an oxygen-storage material to improve their microwave dielectric properties. 0.67CaTiO₃–0.33LaAlO₃ ceramics co-doped with B₂O₃–Li₂O–Al₂O₃ and CeO₂ were prepared by a conventional two-step solid-state reaction process. The sintering behavior, crystal structure, surface morphology, and microwave dielectric proprieties of the prepared ceramic samples were studied, and the reaction mechanism of CeO₂ was elucidated. CTLA+0.05 wt% BLA+3 wt% CeO₂ ceramics sintered at 1360 °C for 4 h exhibited the optimal microwave dielectric properties: dielectric constant (ε_r)=45.02, quality factor (Q×f)=43102 GHz, and temperature coefficient of resonant frequency (τ_f)=2.1 ppm/°C. The successful preparation of high-performance microwave dielectric ceramics provides a direction for the future development and commercialization of CTLA ceramics.     

Synthesis and characterization of high-density B2O3-added forsterite ceramics

Boria (B₂O₃)-added forsterite (Mg₂SiO₄) ceramics were synthesized and their properties were characterized. The addition of B₂O₃ was aimed to produce high density forsterite ceramics at a low sintering temperature. The raw materials were purified silica sand and commercial magnesia powders. Fosterite powder was produced from a solid reaction between the raw powders at 1100 °C prior to addition of B₂O₃, uniaxial pressing and sintering at 1200 °C. The amount of added B₂O₃ varied between 0%, 4%, and 8% by weight. Elemental analysis was performed by X-ray fluorescence (XRF) spectroscopy on the purified silica powder, whereas phase analyses were obtained from X-ray diffraction (XRD) data. Characterization of the ceramics included diameter shrinkage, density-porosity, thermal expansion, Vickers hardness, and dielectric constant. The results showed that the silica powder contained 98.7 at% Si with minor impurities, including 0.5 at% Ti, but the only identified crystalline phase was quartz. Further phase analysis of the ceramics showed that the addition of B₂O₃ reduced the amount of formed forsterite and increased the amount of cristobalite, proto- and clino-enstatite (MgSiO₃) as well as suanite (Mg₂B₂O₅). The highest forsterite content was found in B₂O₃-free ceramics, approximately 88.1 wt%. Moreover, the addition of B₂O₃ also reduced the diameter of the sample by more than 21%, resulting in a very dense ceramic with an apparent porosity of only 0.3%. The Vickers hardness significantly increased from 0.3 GPa for the B₂O₃-free ceramic to 10.9 GPa for the 4% B₂O₃ sample. The dielectric constant of the B₂O₃-added forsterite ceramics was improved by approximately 2–6 times that of the B₂O₃-free ceramic, which was primarily attributed to the loss of porosity in the samples.     

Preparation and thermal shock resistance of cordierite-spodumene composite ceramics for solar heat transmission pipeline

Cordierite-spodumene composite ceramics with 5, 10, 15 wt% spodumene used for solar heat transmission pipeline were in-situ prepared via pressureless sintering from kaolin, talc, γ-Al₂O₃ and spodumene. Effects of spodumene on densification, mechanical properties, thermal shock resistance, phase composition and microstructure of the composite ceramics were investigated. The results showed that spodumene used as flux material decreased the sintering temperature greatly by 40–80 °C, and improved densification and mechanical properties of the composite ceramics. Especially, sample A3 with 10 wt% spodumene additive sintered at 1380 °C exhibited the best bending strength and thermal shock resistance. The bending strengths of A3 before and after 30 thermal shock cycles (wind cooling from 1100 °C to room temperature) were 102.88 MPa and 96.29 MPa, respectively. XRD analysis indicated that the main phases of the samples before 30 thermal shock cycles were α-cordierite, α-quartz and MgAl₂O₄, and plenty of β-spodumene appeared after thermal shock. SEM micrographs illustrated that the submicron β-spodumene grains generated at the grain boundaries after thermal shock improved the thermal shock resistance. It is believed that the cordierite-spodumene composite ceramics can be a promising candidate material for heat transmission pipeline in the solar thermal power generation.     

Synthesis,processing and properties of TaC–TaB2–C ceramics

Multi-phase ceramics in the TaC–TaB₂–C system were prepared from TaC and B₄C mixtures by reactive pressureless sintering at 1700–1900 °C. The pressureless densification was promoted by the use of nano-TaC and by the presence of active carbon in the reaction products. The presence of TaB₂ inhibited grain growth of TaC and increased the hardness compared to pure TaC. If a coarse TaC powder was used, the compositions did not densify. In contrast, pure nano-TaC was pressureless sintered at 1800 °C by the addition of 2 wt.% carbon introduced as carbon black or graphite. The introduction of carbon black resulted in fully dense TaC ceramics at temperatures as low as 1500 °C. The grain size of nominally pure TaC ceramics was a strong function of carbon stoichiometry. Enhanced grain size in sub-stoichiometric TaC, compared to stoichiometric TaC, was observed. Additional work is necessary to optimize processing parameters and evaluate the properties of ceramics in the TaC–TaB₂–C system.     

Effect of BaCu(B2O5) on the sintering and microwave dielectric properties of Ca0.4Li0.3Sm0.05Nd0.25TiO3 ceramics

The sintering behaviors and microwave dielectric properties of the Ca_0.4Li_0.3Sm_0.05Nd_0.25TiO₃ (abbreviated CLSNT) ceramics with different amounts of BaCu(B₂O₅) addition were investigated in this paper. Adding BaCu(B₂O₅) to CLSNT lowered its sintering temperature from 1300 °C to 925 °C. No secondary phase was observed in the CLSNT ceramics and complete solid solution of the complex perovskite phase was confirmed. The CLSNT ceramics with small amounts of BaCu(B₂O₅) addition could be well sintered at 925 °C without much degradation in the microwave dielectric properties. Especially, the 1.75 wt.% BaCu(B₂O₅)-doped CLSNT ceramic sample sintered at 925 °C for 3 h had optimum microwave dielectric properties of ε_r = 93.5 ± 3.2, Q × f = 6486 ± 434 GHz, and τ_f = 5 ± 1.5 ppm/°C (at 3–4 GHz), enabling it a promising candidate material for LTCC applications. Obviously, BaCu(B₂O₅) could be a suitable sintering aid to facilitate the densification and microwave dielectric properties of the CLSNT ceramics.     

Microstructure,ferroelectric and piezoelectric properties of Bi0.5K0.5TiO3-modified BiFeO3–BaTiO3 lead-free ceramics with high Curie temperature

The effects of composition, sintering temperature and dwell time on the microstructure and electrical properties of (0.75 ? x)BiFeO₃–0.25BaTiO₃–xBi_0.5K_0.5TiO₃ + 1 mol% MnO₂ ceramics were studied. The ceramics sintered at 1000 °C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and pseudocubic phases is formed at x = 0.025. The addition of Bi_0.5K_0.5TiO₃ retards the grain growth and induces two dielectric anomalies at high temperatures (T₁ ～ 450–550 °C and T₂ ～ 700 °C, respectively). After the addition of 2.5 mol% Bi_0.5K_0.5TiO₃, the ferroelectric and piezoelectric properties of the ceramics are improved and very high Curie temperature of 708 °C is obtained. Sintering temperature has an important influence on the microstructure and electrical properties of the ceramics. Critical sintering temperature is 970 °C. For the ceramic with x = 0.025 sintered at/above 970 °C, large grains, good densification, high resistivity and enhanced electrical properties are obtained. The weak dependences of microstructure and electrical properties on dwell time are observed for the ceramic with x = 0.025.