Low‐Temperature Preparation of β‐Si3N4 Porous Ceramics with a Small Amount of Li2O–Y2O3

Porous β‐Si₃N₄ ceramics are sintered at 1600°C in N₂ and postheat treated at 1500°C under vacuum using Li₂O and Y₂O₃ as the sintering additives. The partial sintering and phase transformation are promoted at low temperature by the addition of Li₂O. The addition of Y₂O₃ is advantageous for the formation of high aspect ratio β‐Si₃N₄ grains. After postheat treatment, a large amount of intergranular glassy phase is removed, and the Li content in the samples is decreased. By this method, the β‐Si₃N₄ porous ceramic with a porosity of 54.1% and high flexural strength of 110 ± 8.1 MPa can be prepared with a small amount of sintering additives, 0.66 wt% Li₂O and 0.33 wt% Y₂O₃, and it is suitable for high‐temperature applications.     

Low‐Cost Silicon Nitride from β‐Silicon Nitride Powder and by Low‐Temperature Sintering

Aiming to manufacture low‐cost silicon nitride components, a low‐cost β powder was chosen as a raw powder and low‐temperature sintering at 1550–1600°C under atmospheric pressure nitrogen was carried out. The silicon nitride from β powder with 5 wt% Y₂O₃ and 5 wt% MgAl₂O₄ additives and sintered at 1600°C for 8 h was successfully densified, and it exhibited moderate strength and toughness of 553 MPa ± 22 and 3.5 MPa m^1/2, respectively. The results indicate that the low‐temperature sintering of the low‐cost β powder has a potential to reduce cost of components.     

Mechanical Properties and Reliability of Li2O‐Stabilized β″‐Alumina Ceramics: Effect of Synthesizing Method

Li₂O‐stabilized β″‐alumina powder has been synthesized by the solid‐state and double‐zeta processes. It was shown that by the double‐zeta process, the β″‐alumina fraction of sintered samples was 10% higher, showing around 99% of β″‐alumina fraction. Using β″‐alumina powder produced by the double‐zeta process, the sintered density improved, microstructure was more uniform and leads to improvement in hardness, strength and Weibull modulus due to more uniform microstructure and absence of abnormal grain growth. The higher fracture toughness of the solid‐state‐processed samples could be due to crack deflection mechanism.     

Low‐Temperature Sintering and Piezoelectric Properties of CuO‐Added KNbO3 Ceramics

Dielectric and piezoelectric properties of CuO‐added KNbO₃ (KN) ceramics were investigated. The CuO reacted with the Nb₂O₅, formed a CuO–Nb₂O₅‐related liquid phase during the sintering, and assisted the densification of the KN ceramics at low temperatures. Moreover, some of the Cu²⁺ ions replaced the Nb⁵⁺ ions in the matrix and behaved as a hardener. The dielectric and piezoelectric properties of the KN ceramics were considerably influenced by the relative density. The 1.0 mol% CuO‐added KN ceramic sintered at 960°C for 1.0 h, which showed a maximum relative density, exhibited a high phase angle of 86.9°, P_r of 14.8 μC/cm², and E_c of 1.8 kV/mm. This specimen also exhibited good dielectric and piezoelectric properties: ε^T₃₃/ε_o of 364, d₃₃ of 122 pC/N, k_p of 0.29, and Q_m of 611.     

Low‐Temperature Sintering of HfC/SiC Nanocomposites Using HfSi2‐C Additives

HfC/SiC nanocomposites were fabricated via the reactive spark plasma sintering (R‐SPS) of a nano‐HfC powder and HfSi₂‐C sintering additives. The densification temperature decreased to 1750°C as the additive content increased. XRD analysis indicated the formation of pure HfC–(19.3–33.8 vol%) SiC within the sintered composites without residual silicide or oxide phases or secondary nonoxide phases. Ultrafine and homogeneously distributed HfC (470 nm) and SiC (300 nm) grains were obtained in the dense composites using nano‐HfC powder through the high‐energy ball‐milling of the raw powders and R‐SPS. Grain growth was further suppressed by the low‐temperature sintering using R‐SPS. No amorphous phase was identified at the grain boundary. The maximum Vickers hardness, Young's modulus, and fracture toughness values of the HfC/SiC nanocomposites were 22 GPa, 292 GPa, and 2.44 MPa·m^1/2, respectively.     

Microwave Processing for Improved Ionic Conductivity in Li2O–Al2O3–TiO2–P2O5 Glass‐Ceramics

Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP) was synthesized using a glass‐ceramics approach through crystallization in a conventional box furnace and a modified microwave furnace. The microstructure of samples that were microwave processed at 1000°C showed a larger average grain size (0.87 μm) when compared with the grain size of conventionally processed samples (0.30 μm) at the same temperature. Microwave processing led to significant enhancement of the conductivity when compared with conventional processing for all crystallization temperatures investigated. The highest total conductivity achieved was of glass microwave processed at 1000°C, with a conductivity of 5.33 × 10^?4 S/cm. This conductivity was five times higher than that of LATP crystallized conventionally at the same temperature.     

Preparation and Microwave Dielectric Properties of Ultra‐low Temperature Sintering Ceramics in K2O–MoO3 Binary System

A series of microwave dielectric ceramics in the compositions of K₂Mo₂O₇, K₂Mo₃O₁₀, and K₂Mo₄O₁₃ in K₂O–MoO₃ binary system with ultra low sintering temperatures were prepared using the solid‐state reaction method. Their synthesis, phase composition, compatibility with metal electrodes, microstructures, and microwave dielectric properties were investigated. The K₂Mo₂O₇ ceramic sintered at 460°C with a triclinic structure has a relative permittivity of 7.5, a Q × f value of 22 000 GHz, and a τ_f value of ?63 ppm/°C. The X‐ray diffraction patterns indicate that K₂Mo₂O₇ does not react with Ag and Al electrodes at the co‐fired temperatures. The K₂Mo₃O₁₀ ceramic can be sintered well at 520°C with a relative permittivity of 5.6, a Q × f value of 35 830 GHz, and a τ_f value of ?92 ppm/°C. It has compatibility with Ag electrode. The K₂Mo₄O₁₃ ceramic sintered at 540°C possesses good microwave dielectric properties with a relative permittivity of 6.8, a Q × f value of 39 290 GHz and a τ_f value of ?67 ppm/°C and it is compatible with Al electrode. For K₂Mo₂O₇ and K₂Mo₄O₁₃, it is found that the grain sizes and the number of grain boundaries play an important role in the dielectric loss. From this study, it can be seen that the three ceramics in K₂O–MoO₃ system have good microwave dielectric properties, ultra‐low sintering temperatures, non‐toxic, and low‐cost characteristics. So they can be potentially applied to ultra‐LTCC devices.     

Low‐Temperature Sintering Microwave Characteristics of B2O3‐doped CaLa4Ti4O15 Dielectric Ceramics

Preparation and microwave dielectric properties of B₂O₃‐doped CaLa₄Ti₄O₁₅ ceramics have been investigated. X‐ray diffraction data show that CaLa₄Ti₄O₁₅ ceramic has a trigonal structure coupled with a second phase of CaLa₄Ti₅O₁₇. The CaLa₄Ti₄O₁₅ ceramic with addition of 0.5 wt% B₂O₃, sintered at 1220°C for 4 h, exhibits microwave dielectric properties with a dielectric constant of 45.8, Q × f value of 24,000 GHz, and temperature coefficient of resonant frequency (τ_f) of ?19 ppm/^°C. B₂O₃‐doped CaLa₄Ti₄O₁₅ ceramics, which have better sintering behavior (decrease in sintering temperature ~ 330°C) and dielectric properties than pure CaLa₄Ti₄O₁₅ ceramics, are candidates for applications in microwave devices.     

Ultra‐low Sintering Temperature Microwave Dielectric Ceramics Based on Na2O‐MoO3 Binary System

The compounds in Na₂O‐MoO₃ system were prepared by the solid‐state reaction route. The phase composition, crystal structures, microstructures, and microwave dielectric properties of the compounds have been investigated. This series of compounds can be sintered well at ultra‐low temperatures of 505°C–660°C. The sintered samples exhibit good microwave dielectric properties, with the relative permittivities (ε_r) of 4.1–12.9, the Q × f values of 19900–62400 GHz, and the τ_f values of ?115 ppm/°C to ?57 ppm/°C. Among the eight compounds in this binary system, three kinds of single‐phase ceramics, namely Na₂MoO₄, Na₂Mo₂O₇ and Na₆Mo₁₁O₃₆ were formed. Furthermore, the relationship between the structure and the microwave dielectric properties in this system has been discussed. The average Na^I‐O and Mo^VI‐O bond valences have an influence on the sintering temperatures in Na₂O‐MoO₃ system. The large valence deviations of Na and Mo lead to a large temperature coefficient of resonant frequency. The X‐ray diffraction and backscattered electron image results show that Na₂MoO₄ doesn't react with Ag and Al at 660°C. Also, Na₂Mo₂O₇ has a chemical compatibility with Al at 575°C.     

Dense Iodoapatite Ceramics Consolidated by Low‐Temperature Spark Plasma Sintering

Pb_9.85(VO₄)₆I_1.7, a potential waste form for long‐lived I‐129 immobilization, experiences phase decomposition and thus iodine loss at an elevated temperature above 400°C, presenting a significant challenge for effective management of radioactive iodine. In this work, we report low‐temperature consolidation of dense iodoapatite pellets with above 95% theoretical density by spark plasma sintering (SPS) at temperatures as low as 350°C for 20 min without iodine loss. Microstructure analysis indicates a nanocrystalline ceramic with an average grain size less than 100 nm. Grain growth dominates the sintered microstructure at higher temperatures and longer durations. The dense nanoceramics have significantly‐improved fracture toughness as compared with bulk coarsened grain structures. The effects of sintering temperatures (350°C, 400°C, 500°C, and 700°C) and durations (0–20 min) on microstructure, density, fracture morphology, and mechanical properties including Young's modulus and hardness of bulk samples were investigated. Low temperature densified iodoapatites suggest immense potential of SPS as an advanced materials fabrication technology for the development of waste forms for immobilization of volatile radionuclides including radioactive iodine.     

Properties of Low‐Temperature Sintering PNN–PMW–PSN–PZT Piezoelectric Ceramics with Ba(Cu1/2W1/2)O3 Sintering Aids

Piezoelectric ceramics Pb(Ni_1/3Nb_2/3)O₃–Pb(Mg_1/2W_1/2)O₃–Pb(Sb_1/2Nb_1/2)O₃–Pb(Zr_0.39Ti_0.61)O₃ with Ba(Cu_1/2W_1/2)O₃ sintering aids were fabricated using economical industrial oxide powders and their piezoelectric, dielectric, and ferroelectric properties were investigated in order to develop low‐temperature sintering ceramics for multilayer piezoelectric actuators. A quadratic formula and the Curie–Weiss law reveal that the ceramics are typical displacive‐type ferroelectric relaxors. The ceramics sintered as low as 900°C have good piezoelectric properties of d₃₃ = 551 pC/N, k_p = 0.52, ε_r = 3583, tgδ = 0.02, and T_C = 161°C, which is much promising to manufacture multilayer piezoelectric transducers.     

Low‐temperature sintering and thermal stability of Li2GeO3‐based microwave dielectric ceramics with low permittivity

A low‐permittivity dielectric ceramic Li₂GeO₃ was prepared by the solid‐state reaction route. Single‐phase Li₂GeO₃ crystallized in an orthorhombic structure. Dense ceramics with high relative density and homogeneous microstructure were obtained as sintered at 1000‐1100°C. The optimum microwave dielectric properties were achieved in the sample sintered at 1080°C with a high relative density ~ 96%, a relative permittivity ε_r ~ 6.36, a quality factor Q × f ~ 29 000 GHz (at 14.5 GHz), and a temperature coefficient of resonance frequency τ_f ~ ?72 ppm/^°C. The sintering temperature of Li₂GeO₃ was successfully lowered via the appropriate addition of B₂O₃. Only 2 wt.% B₂O₃ addition contributed to a 21.2% decrease in sintering temperature to 850°C without deteriorating the dielectric properties. The temperature dependence of the resonance frequency was successfully suppressed by the addition of TiO₂ to form Li₂TiO₃ with a positive τ_f value. These results demonstrate potential applications of Li₂GeO₃ in low‐temperature cofiring ceramics technology.     

Low‐Fire Processing of Microwave BNBT‐Based High‐k Dielectric with Li2O–ZnO–B2O3 Glass

The effects of Li₂O–ZnO–B₂O₃ (LZB) glass addition on densification and dielectric properties of Ba₄(Nd_0.85Bi_0.15)_9.33Ti₁₈O₅₄ (BNBT) have been investigated. At a given ratio of ZnO/B₂O₃, the glass softening point decreases, but the thermal expansion coefficient and dielectric constant increase with increasing Li₂O content in the LZB glass. With 10 vol% LZB glass, the densification temperature reduces greatly from 1300°C for pure BNBT to 875°C–900°C for BNBT + LZB dielectric, and the densification enhancement becomes more significant with increasing Li₂O content in the LZB glass. The above result is attributed to a chemical reaction taking place at the interface of LZB/BNBT during firing, which becomes less extensive with increasing Li₂O content in the LZB glass. Therefore, more residual LZB glass, which acts as a densification promoter to BNBT, is left with increasing Li₂O content. For the LZB glass with a Li₂O content in the range 10–30 mol%, the resulting 90 vol% BNBT + 10 vol% LZB microwave dielectric has a dielectric constant of 55–70, product (Q × f_r) of quality factor (Q) and resonant frequency (f_r) of 1000–3000 GHz at 5–5.79 GHz, and a temperature coefficient of resonant frequency (τ_f) of 10–60 ppm/°C in the temperature range between 25°C and 80°C.     

New Sintered Li2O–Al2O3–SiO2 Ultra‐Low Expansion Glass‐Ceramic

We developed a new Li₂O–Al₂O₃–SiO₂ (LAS) ultra‐low expansion glass‐ceramic by nonisothermal sintering with concurrent crystallization. The optimum sintering conditions were 30°C/min with a maximum temperature of 1000°C. The best sintered material reached 98% of the theoretical density of the parent glass and has an extremely low linear thermal expansion coefficient (0.02 × 10^?6/°C) in the temperature range of 40°C–500°C, which is even lower than that of the commercial glass‐ceramic Ceran^® that is produced by the traditional ceramization method. The sintered glass‐ceramic presents a four‐point bending strength of 92 ± 15 MPa, which is similar to that of Ceran^® (98 ± 6 MPa), in spite of the 2% porosity. It is white opaque and does not have significant infrared transmission. The maximum use temperature is 600°C. It could thus be used on modern inductively heated cooktops.     

Low‐Temperature Sintering and Microwave Dielectric Properties of ZnNb2O6–TiO2 Ceramics with BaCu(B2O5) Additions

The influence of BaCu(B₂O₅) (BCB) on densification, phases, microstructure and microwave dielectric properties of ZnNb₂O₆–xTiO₂ (x = 1.70–1.90) composite ceramics have been investigated. Undoped ZnNb₂O₆–1.8TiO₂ ceramics sintered at 1200°C exhibit temperature coefficient of resonant frequency (τ_f) ~9.25 ppm/°C. When BaCu(B₂O₅) was added, the sintering temperature of the ZnNb₂O₆–1.8TiO₂ composite ceramics was effectively reduced to 950°C. The results indicated that the permittivity and Q × f were dependent on the sintering temperature and the amounts of BaCu(B₂O₅). Addition of 3.0 wt% BaCu(B₂O₅) in ZnNb₂O₆–1.8TiO₂ ceramics sintered at 950°C showed excellent dielectric properties of ε_r = 40.9, Q × f = 12,200 GHz (f = 5.015 GHz) and τ_f = +0.3 ppm/°C.     

Structure,Microwave Dielectric Properties,and Novel Low‐Temperature Sintering of xSrTiO3–(1−x)LaAlO3 Ceramics with LTCC Application

xSrTiO₃–(1?x)LaAlO₃ ceramics with ZnO–B₂O₃ sintering aid were prepared by solid‐state reaction method leading to a significant decrease in sintering temperature from 1550°C to 1050°C. The structure, microwave dielectric properties, and low‐temperature sintering behavior were systematically investigated. The results revealed the relationships among ionic size, ionic polarizability and cell volume. With increasing additive, chemical ordering of B‐site cations was indicated with selected area electron diffraction (SAED) patterns, HRTEM images and Raman spectrum, which contributed to the greatly enhanced microwave dielectric properties. Particularly, the 0.7Sr_0.85 Mg_0.15TiO₃–0.3LaAlO₃ ceramics modified with 10 wt % ZnO‐B₂O₃ can further decrease the sintering temperature down to 950°C without deteriorating its performance. Thermal tests implied ceramics featured good chemical compatibility with Cu/Ag electrode. Thus, they can be cofired with internal Cu/Ag electrodes in special patterns to fulfill different electrical functions for LTCC (low‐temperature cofired ceramic) application.     

Microwave Dielectric Properties of Novel Glass‐Free Low‐Firing Li2CeO3 Ceramics

Novel glass–free low temperature firing microwave dielectric ceramics Li₂CeO₃ with high Q prepared through a conventional solid‐state reaction method had been investigated. All the specimens in this paper have sintering temperature lower than 750°C. XRD studies revealed single cubic phase. The microwave dielectric properties were correlated with the sintering conditions. At 720°C/4 h, Li₂CeO₃ ceramics possessed the excellent microwave dielectric properties of ε_r = 15.8, Q × f = 143 700 (GHz), and τ_f  = ?123 ppm/°C. Li₂CeO₃ ceramics could be excellent candidates for glass‐free low‐temperature co‐fired ceramics substrates.     

Investigation of Sm2O3 additive on mechanical and electrical properties of Li2O‐ stabilized β″‐alumina electrolyte

Li₂O‐ stabilized β″‐alumina was synthesized by the double zeta process. The effect of Sm₂O₃ additive as the sintering aid, on microstructure, mechanical and electrical properties of Li₂O‐ stabilized β″‐alumina ceramics was studied by means of X‐ray diffraction, field emission scanning electron microscope, biaxial flexure test and ionic conductivity measurement. The results indicated that both the fracture strength and the ionic conductivity of the sample containing 0.2 wt% Sm₂O₃ improved approximately 52% and 54%, respectively, that can be attributed to its higher density, higher amount of β″‐Al₂O₃ phase and more uniform microstructure.     

Highly Dense,Transparent α‐Al2O3 Ceramics From Ultrafine Nanoparticles Via a Standard SPS Sintering

Using discrete, ultrafine alumina, highly dense transparent (71% real in‐line transmission, RIT, λ = 640 nm) ceramics were achieved with grain size as small as 260 nm using standard SPS sintering. We show that use of La³⁺ as a dopant greatly reduces sensitivity to the sintering temperatures. Transparent alumina were achieved in a large range of sintering temperatures, 1140°C < T < 1200°C, thus providing better reliability and flexibility into the fabrication of large sintered transparent ceramic bodies.     

Structure,Microwave Dielectric Properties,and Low‐Temperature Sintering of Acceptor/Donor Codoped Li2Ti1−x(Al0.5Nb0.5)xO3 Ceramics

The structure, microwave dielectric properties, and low‐temperature sintering behavior of acceptor/donor codoped Li₂TiO₃ ceramics [Li₂Ti_1?x(Al_0.5Nb_0.5)_xO₃, x = 0–0.3] were investigated systematically. The x‐ray diffraction confirmed that a single‐phase solid solution remained within 0 < x ≤ 0.2 and secondary phases started to appear as x > 0.2, accompanied by an order–disorder phase transition in the whole range. Scanning electron microscopy observation indicated that the complex substitution of Al³⁺ and Nb⁵⁺ produced a significant effect on the microstructural morphology. Both microcrack healing and grain growth contributed to the obviously enhanced Q×f values. By comparison, the decrease of ε_r and τ_f values was ascribed to the ionic polarizability and the cell volume, respectively. Excellent microwave dielectric properties of ε_r ~ 21.2, Q×f ~ 181 800 GHz and τ_f  ~ 12.8 ppm/°C were achieved in the x = 0.15 sample when sintered at 1150°C. After 1.5 mol% BaCu(B₂O₅) additive was introduced, it could be well sintered at 950°C and exhibited good microwave dielectric properties of ε_r ~ 20.4, Q×f ~ 53 290 GHz and τ_f ~ 3.6 ppm/°C as well. The cofiring test of the low‐sintering sample with Ag powder proved its good chemical stability during high temperature, which enables it to be a promising middle‐permittivity candidate material for the applications of low‐temperature cofired ceramics.