Mechanical,morphological and dielectric properties of sintered mullite ceramics at two different heating rates prepared from alkaline monophasic salts

The sintering behavior of nanocrystalline orthorhombic mullite powders was investigated. The changes in microstructure, mechanical and dielectric properties with two different heating rates were explained. Microstructural characteristics depending on heating rate were explained at different sintering temperatures. Dielectric properties of prepared mullite nanocomposites were studied to examine the synthesized mullite ceramics as high permittivity materials in the microwave range. It was indicated that a sharp decrease in bulk density was observed at 1600 °C due to the exaggerated growth of mullite grains. Moreover, a maximum hardness value of 4.97 GPa was obtained at 1600 °C with slow heating rate (5 °min^?1). The DC electrical resistivity with a slow heating rate at 1300 °C was approximately three times the value of the mullite sample sintered with a fast heating rate (30 °min^?1). The minimum dielectric loss of about 0.017 at 1.5 GHz was achieved at a sintering temperature of 1500 °C with a slow heating rate.     

Synthesis and characterization of nanocrystalline mullite powders at low annealing temperature using a new technique

For the first time orthorhombic mullite nanoparticles (12 nm) have been synthesized via the co-precipitation technique using mixture of sodium aluminate and sodium silicate solutions as monophasic salts at low annealing temperature 1000 °C. The AC and DC electrical properties of the sintered mullite ceramics were investigated. The results indicated the precursor directly transformed to 3:2 molar ratio orthorhombic mullite. Meanwhile, the surface area of the produced o-mullite at 1000 °C was 15.1 m²/g which increased to 37.3 m²/g with addition of CTAB as a cationic surfactant. TEM images investigated that the microstructure was changed from rod-like shape to roughly spherical shape with addition of surfactant. The DC electrical resistivity of mullite sample sintered at 1600 °C was 16 times greater than that sintered at 1300 °C. The minimum dielectric loss value of 0.014 was obtained at 1600 °C sintering temperature at 1.5 MHz.     

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.     

Synthesis and characterization of LTCC compositions with middle permittivity based on CaO-B2O3-SiO2 glass/CaTiO3 system

CaTiO₃ ceramics with the addition of CaO-B₂O₃-SiO₂ (CBS) glass (45–55 wt%) composites were sintered at 830 °C, 850 °C, 875 °C and 900 °C. To illustrate influence mechanism of the different glass contents and sintering temperatures on the properties of the composites, we focused on the multiple performances of the composites by employing different qualitative and quantitative instruments. Composites with 50 wt% glass sintered at 875 °C presented fairly ideal performance: the bulk density was 3.20 g/cm³, the dielectric constant was 25.7 and the dielectric loss was 0.0009 at 7 GHz. Micro-Structure analysis of the composites showed a dense and pore-less microstructure except for few pores with size around 1 μm. In addition, the composite could meet the shrinkage requirement of Ag electrodes and could not possibly react with Ag electrodes any more. This makes them suitable for various dielectric applications at low sintering temperature.     

Low sintering of X7R ceramics based on barium titanate with SiO2–B2O3–Li2O sintering additives in reducing atmosphere

Development of a low-temperature sintered dielectric material derived from barium titanate for X7R characterized dielectric ceramics application is discussed in this paper. By addition of SiO₂–B₂O₃–Li₂O sintering additives to commercial BaTiO₃ powder, more than 95% of the theoretical density was obtained at a sintering temperature of 950 °C in H₂/N₂ atmosphere. The influence of the composition and procedures on the microstructures, lattice parameters and properties of ceramics materials were systemically studied. After explaining the reason for lower isolated resistivity (IR) in the previous experiment, several methods are tried out to improve the IR properties, which have reached the application requirement level of 10¹² Ω cm. These ceramics sintered between 900 °C and 950 °C in H₂/N₂ atmosphere are promising candidates for fabrication of Cu electrode MLCCs.     

Low temperature sintering of PZT powders coated with Pb5Ge3O11 by sol–Gel method

Low temperature sintering of PZT powders was investigated using Pb₅Ge₃O₁₁(PGO) as a sintering aid. PZT powders with 150 nm particle size were coated with PGO which was prepared from precursor solutions of Ge(OiPr)₄ and Pb(NO₃)₂ by sol–gel method. 1 wt% PGO-added PZT powders were densified at 750°C for 2 h to sintered bodies with the relative density of approximately 95%. An addition of PGO improved the sinterability of PZT powders with a reduction of sintering temperature by about 300°C. Dielectric and piezoelectric properties of PGO-added PZT ceramics sintered at ≦950°C were superior to those without PGO additives. However, a higher sintering temperature above 1000°C deteriorated the dielectric and piezoelectric properties of PGO-added PZT ceramics. This may be attributed to the change of microstructure involving the formation of solid solution between PZT and PGO. The 1 wt% PGO-added PZT bodies sintered at 750°C exhibited an electromechanical coupling factor, Kp, of about 56%.     

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.     

Structural study of mullite based ceramics derived from a mica-rich kaolin waste

Mullite-based ceramics have been synthesized by reactive sintering of a mixture containing kaolin and a mica-rich kaolin waste. Samples fired in the temperature range from 1300 to 1500 °C were characterized by X-ray diffraction (XRD). The quantitative phase analysis and unit cell parameters of the mullite were determined by Rietveld refinement analysis of the XRD data. Mullite-based ceramics with 1.2 wt% quartz, 56.3 wt% glass (amorphous phase), 2.64 g/cm³ of apparent density, and 35±1.2 MPa of flexural strength were obtained after firing at 1500 °C. A liquid phase sintering mechanism activated by a total mica content of 13.3 wt% allowed to increase the mullite content to 47.6 wt% (2.3 wt% quartz and 50.1 wt% glass phase) and improve the flexural strength (70±3.9 MPa) after firing at 1400 °C.     

Effects of glass additions on the microstructure and dielectric properties of barium strontium titanate (BST) ceramics

Commercial glass frits (lead borosilicate glasses) were employed as the sintering aids to reduce the sintering temperatures of BST ceramics. The effects of the glass content and the sintering temperature on the microstructures, dielectric properties and tunabilities of BST ceramics have been investigated. Densification of BST ceramics of 5 wt% glass content becomes significant from sintering temperature of 1000 °C. The glass content shows a strong influence on the Curie temperature T_c, permittivity and the diffuse transition. X-ray results show all BST ceramics exhibit a perovskite structure and also the formation of a secondary phase, Ba₂TiSi₂O₈. The shift of BST diffraction peaks towards higher angle with increasing the glass content indicates the substitution of Pb²⁺ in Ba²⁺ site, which mainly accounts for the diffuse transition observed in these BST ceramics. BST ceramics with 10 wt% glass additives possess the highest tunability at all four sintering temperatures. A tunability of 12.2% at a bias field of 1 kV/mm was achieved for BST ceramics with 10 wt% glass content sintered at 900 °C.     

Microwave dielectric properties of BaO–4.3TiO2–0.5ZnO ceramics with BaCu(B2O5)

Effect of BaCu(B₂O₅) (BCB) addition on microwave dielectric properties and sintering behaviors of BaO–4.3TiO₂–0.5ZnO system (BTZ) ceramics were investigated to develop middle-k dielectric composition with low sintering temperatures. When a small amount of BCB was added to BTZ system, the sintering temperature can be lowered from 1100 °C to 900 °C due to the formation of BCB liquid phase. The system added with 7 wt% BCB was sintered at 900 °C for 2 h and ?_r of 31, Q × f of 18,200 GHz and τ_f of 3.8 ppm/°C were obtained. The suitability of BTZ ceramics for tape casting and cofiring with Ag electrodes was investigated, and no evidence of chemical reaction between Ag and ceramics was observed. The dielectric properties of the stacked multilayer plate without any electrodes were also measured. The result shows that the as-prepared BTZ ceramics are suitable for low-temperature co-fired ceramics applications.     

Impact of microwave processing on porcelain microstructure

Microstructural evolution on sintering of porcelain powder compacts using microwave radiation was compared with that in conventionally sintered samples. Using microwaves sintering temperature was reduced by ~ 75 °C and dwell time from 15 min to 5 min while retaining comparable physical properties i.e. apparent bulk density, water absorption to conventionally sintered porcelain. Porcelain powder absorbed microwave energy above 600 °C due to a rapid increase in its loss tangent. Mullite and glass were used as indicators of the microwave effect: mullite produced using microwaves had a nanofibre morphology with high aspect ratio (~ 32 ± 3:1) believed associated with a vapour-liquid-solid (VLS) formation mechanism not previously reported. Microwaves also produced mullite with different chemistry having ~ 63 mol% alumina content compared to ~ 60 mol% alumina in conventional sintered porcelain. This was likely due to accelerated Al⁺³ diffusion in mullite under microwave radiation. Liquid glass was observed to form at relatively low temperature (~ 900–1000 °C) using microwaves when compared to conventional sintering which promoted the porcelains ability to absorb them.     

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.     

Refractory glass–ceramics based on alkaline earth aluminosilicates

Glass–ceramics that can be used at temperatures of 1200–1500 °C are found in the alkaline earth aluminosilicate field, and are generally nucleated internally with titania. These glass–ceramics have good strength (>100 MPa, abraded), can be tailored to produce high fracture toughness (2–5 MPa m^1/2), and have good dielectric properties. Coefficients of thermal expansion (CTEs) are low to moderate ((25–45) × 10^?7 °C^?1, from 25 to 1000 °C).The major crystalline phase in the glass–ceramics exhibiting the lowest CTEs is hexagonal cordierite (indialite), while important toughening accessory phases are enstatite and acicular magnesium dititanate.The most refractory glass–ceramics that are easily melted at 1650 °C, yet when crystallized do not deform at 1450 °C, are based on strontium and barium monoclinic feldspars of the celsian type. CTEs range from 35 to 45 × 10^?7 °C^?1. Acicular mullite is an important accessory phase aiding fracture toughness in these materials.Mullite glass–ceramics which contain considerable siliceous residual glass are probably the most refractory of these glass–ceramics, but they require melting above 1700 °C. Nevertheless, they can be used at temperatures near 1600 °C.Potential applications for refractory glass–ceramics include improved radomes, engine components, substrates for semiconductors and precision metallurgical molds.     

Fabrication and characterization of anorthite–mullite–corundum porous ceramics from construction waste

In this work, anorthite–mullite–corundum porous ceramics were prepared from construction waste and Al₂O₃ powders by adding AlF₃ and MoO₃ as mineralizer and crystallization catalyst, respectively. The effects of the sintering temperature and time on open porosity, mechanical properties, pore size distribution, microstructure, and phase composition were characterized in detail. The results showed that the formation of the mullite whiskers and the properties of the anorthite–mullite–corundum porous ceramics depended more on the sintering temperature than the holding time. By co-adding 12 wt% AlF₃ and 4 wt% MoO₃, mullite whiskers were successfully obtained at sintering temperatures upon 1350 °C for 1 h. Furthermore, the resultant specimens exhibited excellent properties, including open porosity of 66.1±0.7%, biaxial flexural strength of 23.8±0.9 MPa, and average pore size of 1.32 µm (the corresponding cumulative volume percent was 37.29%).     

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.     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.     

Structure and conductivity of NiO/YSZ composite prepared via modified glycine-nitrate process at varying sintering temperatures

Nickel oxide and Yttria-stabilized zirconia (NiO/YSZ) composite is one of the most promising mixed conducting electrode materials in both solid oxide electrolysis cell and solid oxide fuel cell applications. In this study, 50 wt% NiO and 50 wt% YSZ composite was synthesized via a modified glycine-nitrate combustion process (GNP) and the effect of sintering temperatures (1100 °C, 1300 °C and 1500 °C) on its microstructure and electrical properties were investigated. TG/DTA and in-situ high temperature XRD revealed the thermal property behavior and the structural changes of the as-combusted precursor material. For all the samples sintered at different temperatures, room temperature XRD patterns revealed a distinct cubic phases of both YSZ and NiO while SEM images showed a porous microstructure. The total conductivities at 700 °C are 9.87 × 10⁻³, 5.26 × 10⁻³, 4.02 × 10⁻³ S/cm for the 1100, 1300, and 1500 °C with activation energies of 0.1722, 0.3555, and 0.3768 eV, respectively. Conductivity measurements of the different sintered samples revealed that the total conductivities as well as the activation energies are greatly affected by different sintering temperatures.     

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al₂O₃·2SiO₂ (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H₂O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.     

Microstructural evolution,mechanical and thermal properties of LaB6 embedded in Si-B-C-N prepared by spark plasma sintering

Si-B-C-N monoliths with 5 wt% LaB₆ additives were prepared by spark plasma sintering at 1250–2000 °C and 50 MPa using a mechanically alloyed mixture of graphite, c-Si, h-BN and LaB₆ powders as the starting materials. Microstructural evolution, mechanical and thermal properties of the as-prepared La/Si-B-C-N monoliths were investigated. The densification of the ceramics starts at 1160° and ends at 1800 °C with the formation of La-containing compounds coupled with SiC and BN(C) phases. La-containing BN(C) grains develop into a lamellar structure at 1900 °C offering improved fracture toughness and decreased Vickers hardness, flexural strength and elastic modulus. The formation of lamellar BN(C) is also responsible for a high thermal expansion coefficient of 4.2×10⁻⁶ /°C.