Sintering behavior,structure and dielectric properties of CuO-ZnO-MgO-B2O3 glass-ceramics for ULTCC applications

High performance ultra-low temperature co-fired ceramic (ULTCC) materials were prepared from CuO- MgO- ZnO- Al₂O₃- B₂O₃- Li₂O glass-ceramics. The sintering behaviors, crystalline phase evolution, microstructure and dielectric properties, as well as their compatibility with Ag and Al electrodes, were investigated. With the suitable substitution of MgO for ZnO, the dielectric properties of glass-ceramics were improved. It is mainly associated with the fine microstructure, highly crystallinity, and decrease in tetrahedral distortion in the crystal lattice. All the glasses completed the densification at 575–600 °C, and ZnB₄O₇ is the only crystalline phase precipitated from the glasses. Moreover, the glass-ceramic with 1 wt% MgO sintered at 575 °C for 5 h, exhibited low relative permittivity ~ 7.1 and low dielectric loss ~ 6.40 × 10^?4. And the glass-ceramic with 4 wt% MgO sintered at 600 °C for 5 h, also displayed low relative permittivity ~ 7.1 and low dielectric loss ~ 5.77 × 10^?4. Both two glasses have good sintering compatibility with silver and aluminum electrodes, which provided high potential for ULTCC application.     

Influence of barium oxide on the crystallization, microstructure and mechanical properties of potassium fluorophlogopite glass-ceramics

The influence of barium oxide, heat treatment time and temperature on the crystallization, microstructure and mechanical behavior of the system Ba_x·K_1−2x·Mg₃·Al·Si₃O₁₀·F₂ (where x = 0.0, 0.3 and 0.5) was investigated in order to develop novel, high strength and machinable glass-ceramics. Three glasses were prepared and characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscope (SEM) techniques and some mechanical testing methods.The crystallization kinetics of glass-ceramics was also studied. Activation energy and Avrami exponent calculated for the crystallization peak temperature (T_p) of three different glass batches. The Vickers hardness decreased slightly on formation of the potassium fluorophlogopite and barium fluorophlogopite phases, but decreased significantly on formation of an interconnected ‘house of cards’ microstructure.     

Correlations between microstructure and dielectric properties of hexagonal boron nitride

Hexagonal boron nitride is a material with a unique combination of mechanical, chemical, and electrical properties and therefore of considerable technical and commercial interest. Nevertheless, there exists only very limited knowledge concerning the correlation of microstructure and electrical and dielectrical properties of such materials. In this work, the microstructure, dielectric breakdown resistance (dielectric strength), and low permittivity of different BN ceramics and composites were investigated. Besides exhibiting a very high specific electrical resistivity of 10¹³–10¹⁵ Ω cm, the materials had excellent dielectric strengths (up to 53 kV/mm) and low electrical permeability (4.1). The dielectric strength depended strongly on the porosity and to a lesser extent on the content of secondary phases, whereas the permittivity was influenced by the secondary phases. The aging of the materials in humid air did not significantly alter these values. The permittivity was found to be independent of frequency between 0.1 MHz and 10 MHz and temperature up to 300 °C.     

Microstructural study of dielectric breakdown in glass-ceramics insulators

Novel glass-ceramic materials based on Na and Ca-rich feldspar crystallizations with a hierarchical micro-nanostructure shown the largest dielectric strength, >57 kV/mm, reported at room temperature in ceramic insulators, due to a large amount of interfaces that favor scattering processes of charge carriers. Dielectric breakdown tests with temperature indicated they withstood up to 200 °C, with dielectric strengths of 30 kV/mm and 44 kV/mm for anorthite and albite-based glass-ceramics, respectively. These values are even larger than the ones obtained at room temperature for most of the current ceramic insulators. Microstructural characterization and micro-Raman spectroscopy carried out after breakdown allow determining the dielectric breakdown mechanisms. Glass phases in the surroundings of the crater because of local melting and fast cooling are identified. These results make feldspar based glass-ceramics suitable for electrical insulator applications at room and high temperature. Moreover, dielectric breakdown mechanism may allow tailoring new high insulating application in the future.     

Effect of boron nitride nanosheets addition on the mechanical and dielectric properties of magnesium oxide ceramics

Boron nitride nanosheets (BNNSs)/magnesium oxide (MgO) composites were prepared via hot pressing. Mechanical properties of MgO ceramics were improved obviously in virtue of adding BNNSs. The bending strength of the 1 wt% BNNSs/MgO composite increased by about 85% than that of the monolithic MgO. The fracture toughness increased by 34% with the addition of 1.5 wt% BNNSs. Microstructural analyzes have shown that the toughening mechanisms are combinations of the pull-out and bridging of BNNSs, crack deflection, and crack bypassing mechanisms. The addition of a small amount of BNNSs don't destroy the excellent dielectric properties of composites. The dielectric constant of the sample doped with 1 wt% BNNSs was about 9.5 in the whole X-band and the vast majority of P-band, and the loss tangent was less than 5 × 10⁻³ in 10–15.8 GHz.     

Effect of ZnO/K2O ratio on the crystallization sequence and microstructure of lithium disilicate glass-ceramics

The effect of ZnO/K₂O (Z/K) ratio on the crystallization sequence and microstructure of lithium disilicate (Li₂Si₂O₅: LS2) glass-ceramics was carefully investigated for the SiO₂-Li₂O-K₂O-ZnO-P₂O₅ system. The Z/K ratios of precursor glasses were varied from 0 to 3.5 while the nucleating agent of P₂O₅ and glass modifiers of ZnO plus K₂O were fixed to have 1.5 and 4.5 mol% relative to LS2, respectively. For the samples prepared by two-stage heat treatments of 500 °C for 1 h and 800 °C for 2 h in air, the LS2 nucleation rate was increased with increasing the Z/K ratio due to the variation in crystallization sequence from type II (Li₂SiO₃: LS) to type I (LS + LS2) in addition to an amorphous phase separation in base glass. Consequently, with increasing the Z/K ratio, the LS2 crystalline phase within the glass matrix continuously changed from larger acicular ones to smaller equiaxed ones.     

Low temperature preparation of CaCu3Ti4O12 ceramics with high permittivity and low dielectric loss

Low temperature preparation of CaCu₃Ti₄O₁₂ ceramics with large permittivity is of practical interest for cofired multilayer ceramic capacitors. Although CaCu₃Ti₄O₁₂ ceramics have been prepared at low temperatures as previously reported, they have rather low permittivity. This work demonstrates that CaCu₃Ti₄O₁₂ ceramics can not only be prepared at low temperatures, but they also have large permittivity. Herein, CaCu₃Ti₄O₁₂ ceramics were prepared by the solid state reaction method using B₂O₃ as the doping substance. It has been shown that B₂O₃ dopant can considerably lower the calcination and sintering temperatures to 870 °C and 920 °C, respectively. The relative permittivity of the low temperature prepared CaCu₃Ti_4−xB_xO₁₂ ceramics is about 5 times larger than the previously reported results in the literature. Furthermore, the dielectric loss of the CaCu₃Ti_4−xB_xO₁₂ ceramics is found to be as low as 0.03. This work provides a beneficial base for the future commercial applications of CaCu₃Ti₄O₁₂ ceramics with large permittivity for the cofired multilayer ceramic capacitors.     

Glass forming,crystallization, and physical properties of MgO-Al2O3-SiO2-B2O3 glass-ceramics modified by ZnO replacing MgO

This study focused on the glass forming, crystallization, and physical properties of ZnO doped MgO-Al₂O₃-SiO₂-B₂O₃ glass-ceramics. The results show that the glass forming ability enhances first with ZnO increasing from 0 to 0.5 mol%, and then weakens with further addition of ZnO which acted as network modifier. No nucleating agent was used and the crystallization of studied glasses is controlled by a surface crystallization mechanism. The predominant phase in glass-ceramics changed from α-cordierite to spinel/gahnite as ZnO gradually replaced MgO. The phase type did not change; however, the crystallinity and grain size in glass-ceramics increased when the glasses were treated from 1030 °C to 1100 °C. The introduction of ZnO can improve the thermal, mechanical, and dielectric properties of the glass-ceramics. The results reveal a rational mechanism of glass formation, crystal precipitation, and evolution between structure and performance in the xZnO-(20-x)MgO-20Al₂O₃-57SiO₂-3B₂O₃ (0 ≤ x ≤ 20 mol%) system.     

Phase formation and dielectric properties of ultralow temperature sintered Li4Mo5O17 ceramics

The Li₂MoO₄-MoO₃ ceramic system can be densely sintered at ultralow temperatures (< 500 °C). However, the phase composition of the Li₂MoO₄-MoO₃ system at a molar ratio of 1:1 has always been controversial. In this study, the traditional solid-state method was used to synthesize Li₄Mo₅O₁₇ (400 °C, 10 h) and Li₂Mo₂O₇ (350–400 °C, 4 h). Li₂Mo₂O₇ is an intermediate phase that can be converted to Li₄Mo₅O₁₇ at higher temperatures. The Li₄Mo₅O₁₇ ceramic could be sintered and densified at only 490 °C in air, exhibiting a relative density of 95.6% and excellent dielectric properties, including ε′= 12.79 and tan δ = 0.0004 at 1 GHz. Such an ultralow sintering temperature and dielectric loss indicate that the Li₄Mo₅O₁₇ ceramic is a good candidate for high-frequency capacitance applications.     

Mechanical and dielectric properties of functionalized boron nitride nanosheets/silicon nitride composites

Uniformly dispersed boron nitride nanosheets (BNNSs) reinforced silicon nitride (Si₃N₄) composites were prepared by surface modification assisted flocculation combined with SPS sintering. In order to improve the dispersibility of the BNNSs in the composites, the liquid phase stripped BNNSs are surface functionalized by a two-step covalently modification. The amino-modified BNNSs (NH₂-BNNSs) and Si₃N₄ powders have opposite surface potential, mixed evenly by electrostatic interaction during flocculation. The results showed that mechanical properties of Si₃N₄ composites were obviously enhanced by adding NH₂-BNNSs. The fracture toughness and bending strength of Si₃N₄ composites added 0.75 wt% NH₂-BNNSs were increased by 34% and 28%, respectively, compared with monolithic Si₃N₄. Toughening mechanisms are synergistic action of the torn, pull-out or bridging of BNNSs and crack deflection mechanisms with microstructural analyzes. The dielectric properties of the Si₃N₄ ceramics are also improved after the addition of NH₂-BNNSs.     

Microwave dielectric properties of Al2O3 ceramics sintered at low temperature

The sintering behavior, microstructure and microwave dielectric properties of Al₂O₃ ceramics co-doped with 3000ppmCuO₂+6000ppmTiO₂+500ppmMgO (Cu/Ti/Mg) have been investigated. The results show that 1 wt% Cu/Ti/Mg can reduce the sintering temperature of Al₂O₃ ceramics effectively. Samples with relative densities of ≥97% and uniform microstructure can be obtained when sintered at 1150 °C. Higher temperature can further increase the density of the sample, but it inevitably leads to abnormal grain growth. Meanwhile, the investigation results show that the low-firing Al₂O₃ ceramics have good microwave dielectric properties especially high Q × f value. A high Q × f value of 109616 GHz is able to be obtained for the 1150 °C sintered sample. The reason for the low temperature densification, abnormal grain growth behavior and the changing trend of the microwave dielectric properties are discussed in the paper.     

Effect of Bi-substitution on the microstructure and electromagnetic properties of YCaZrVIG with low sintering temperature

Bi substituted YCaZrVIG ferrites, Y_2.3−xBi_xCa_0.7Zr_0.3V_0.2Fe_4.42O₁₂ (x=0.1, 0.25, 0.4, 0.5, 0.75) ferrites were prepared by conventional oxide method. The addition of Bi₂O₃ promoted the sintering performance and lowered the sintering temperature from 1420–1230 °C. However, it also resulted in the formation of minor second phases and the decrease of grain size. With the increase of Bi concentration, the dielectric constant increases linearly and then remains unchanged. The dielectric loss decreased firstly and then increased. The saturation magnetization (4πM_s) almost retained unchanged as the Bi concentration increased except for the sample with 0.75. The coercivity (H_c) decreased firstly and reached the minimum of 1.32 Oe at 0.25, and then rose when x>0.25, which was related to the facility of magnetic domain wall motion and magnetic moment reverse. Moderate addition of Bi also can increase the remanence (B_r) by improving sintering process. Additionally, we got the optimum electromagnetic properties in the samples with x=0.25 at 1230 °C: RD>97%, ε_r=15.7, tan δ_e=2.48×10⁻⁴, H_c=1.32Oe, 4πM_S=1663 Gs, B_r=583.91 Gs.     

Microstructure and dielectric properties of low temperature sintered ZnNb2O6 microwave ceramics

Low sintering temperature ZnNb₂O₆ microwave ceramics were prepared by doping with mixed oxides of V₂O₅–Bi₂O₃ and V₂O₅–Bi₂O₃–CuO. The effects of additives on the microstructure and dielectric properties of the ceramics were investigated. The results show that doping with V₂O₅–Bi₂O₃ can reduce the sintering temperature of ZnNb₂O₆ from 1150 °C to 1000 °C due to the formation of V₂O₅ and Bi₂O₃ based eutectic phases. The combined influence of V₂O₅ and Bi₂O₃ resulted in rod-like grains. Co-doping CuO with 1 wt.% V₂O₅–1 wt.% Bi₂O₃ further lowered the sintering temperature to 880 °C, because eutectic phases could be formed between the CuO, V₂O₅ and Bi₂O₃. A second phase of (Cu₂Zn)Nb₂O₈ also forms when the content of CuO is greater than 2.5 wt.%. A pure ZnNb₂O₆ phase can be obtained when the amount of CuO was 1.0–2.5 wt.%. The Q × f values of ZnNb₂O₆ ceramics doped with V₂O₅–Bi₂O₃–CuO were all higher than 25,000 GHz. The dielectric constants were 22.8–23.8 at microwave frequencies. In addition, theτ_f values decreased towards negative as the content of CuO increased. The ceramic with composition of ZnNb₂O₆ + 1 wt.%V₂O₅ + 1 wt.% Bi₂O₃ + 2.5 wt.% CuO sintered at 880 °C exhibited the optimum microwave dielectric properties, is 23.4, Q × f is 46,975 GHz, and τ_f is −44.89 ppm/°C, which makes it a promising material for low-temperature co-fired ceramics (LTCCs).     

Influence of fuel on phase formation,morphology, electric and dielectric properties of iron oxides obtained by SCS method

Nanopowders of iron oxides were obtained by Solution Combustion Synthesis (SCS) method from sol-gel compositions containing iron nitrates and soluble organic reducing agents (glycine, urea, citric acid). The synthesis processes and their intensity depending on the type of fuel and fuel/oxidizer ratio (?) were investigated. It was established that the combustion regime affects the phase formation of the obtained powders, their morphology, the color of the final Fe₂O₃ powders, dielectric properties and etc. It was shown that iron oxides with a preferred morphology and high dielectric properties (ε = 44.5 at ? = 0.6 vs ε = 4.0 at ? = 1.4 using urea as fuel) could be produced by the SCS method.     

Low temperature synthesis and spark plasma sintering of a boron carbide with a low residual carbon content

Using spark plasma sintering (SPS), >98.5 % dense boron carbide (B₄C) samples were made from commercially available and lab-synthesised powders made via a low temperature synthesis (LTS) process. The work showed that the LTS powder can be produced in batches of tens to hundreds of grams whilst maintaining a high purity material with lower levels of residual free carbon (20.6–21.3 wt.% C) than commercially available samples (22.4 wt.% C). The LTS material was seen to exhibit higher hardness values (37.8 GPa) than the commercial grade material (32.5 GPa) despite featuring a coarser average grain size (10.8 μm and 2.4 μm respectively). This is largely thought to be due to the influence of ZrO₂ and AlB₂ impurities introduced to the material during micronising milling of the powder after synthesis, as opposed to the influence of the materials lower carbon content.     

Effects of raw material ratio and post-treatment on properties of soda lime glass-ceramics fabricated by selective laser sintering

A reliable method for fabricating soda lime glass-ceramics by selective laser sintering was demonstrated. The effect of the ratio of glass powder to epoxy resin and sintering process on the properties and microstructure of glass-ceramics was investigated. Research shows that: with the improvement of glass powder proportion, sintering shrinkage rate declined and mechanical strength could be improved gradually. When the mass ratio of glass powder and epoxy resin powder was 18:1 and heat-holding at 740 °C for 3 h, the shrinkage rate of the sample was 21.11% and the bending strength reached 95.45 MPa. Therefore, this research laid a foundation for 3D printing to fabricate high performance and complex structure glass-ceramics.     

Effect of metastable phase on the crystallization and mechanical properties of MgO-Al2O3-SiO2 glass-ceramics without nucleating agents

Glass-ceramics without nucleating agents usually undergo surface crystallization, which deteriorates the overall performance of the products. In this paper, we evaluated the effects of the metastable MgAl₂Si₃O₁₀ crystalline phase on the crystallization behavior of a MgO–Al₂O₃–SiO₂ (MAS) glass without nucleating agents and mechanical properties of the glass-ceramics obtained. The results demonstrated that the precipitation of metastable MgAl₂Si₃O₁₀ crystallites promotes the crystallization mechanism transformed from surface crystallization into volume crystallization with two-dimensional crystal growth. Furthermore, the grain size of MgAl₂Si₃O₁₀ near the surface of the prepared glass-ceramics was larger than that of MgAl₂Si₃O₁₀ inside, which helps to generate compressive stress and improves its mechanical properties. The glass-ceramics containing metastable MgAl₂Si₃O₁₀ phase exhibited an enhanced hardness in the range of 7.6 GPa–9.5 GPa for indentation loads ranging from 2.94 N to 98 N, and indentation size effect behavior was observed in Vickers hardness tests of both MAS glass and glass-ceramics. The load-independent hardness values for MAS glass and glass-ceramics were reliably evaluated by the modified proportional specimen resistance (MPSR) model of 7.1 GPa and 7.6 GPa, respectively, with a high correlation coefficient of more than 0.9999. This work reveals the unexploited potential of the metastable phase in improving the crystallization ability and mechanical properties of glass-ceramics.     

Controlling the microstructure and properties of lithium disilicate glass-ceramics by adjusting the content of MgO

In order to obtain lithium disilicate glass-ceramics for dental restoration with both high strength and high translucency, lithium disilicate glass-ceramics with different MgO contents were prepared by melt-casting and heat treatment method. The effects of MgO content on the crystallization temperature, microstructure and flexural strength of lithium disilicate glass-ceramics were investigated. The results indicate that Mg²⁺ exists in the form of [MgO₄] in the network of lithium disilicate glass-ceramics when the MgO content is 0.56 mol% (M0.56), which is beneficial to increasing the homogeneity and thermal stability of the glass system, and short rod-like lithium disilicate crystals can be formed after heat treatment at 840°C. Thus, the obtained lithium disilicate glass-ceramics exhibit excellent comprehensive performance, with the flexural strength being 312 ± 23 MPa, and the average transmittance of visible light being 37.3% (d = 1.62 mm). Especially, the glass-ceramic sample shows better translucency than the commercially available products. The research results are of great significance for developing high performance lithium disilicate glass ceramics and promoting its broad application in the field of dental restoration.     

Improved sintering and microwave dielectric properties of Li2CaSiO4 ceramic with magnesium atom substitution

The influence of Mg²⁺-ion substitution on the sintering and microwave dielectric properties of Li₂CaSiO₄ ceramic was investigated using the traditional solid-state reaction method and density functional theory. X-ray diffraction, scanning electron microscopy, Raman spectrometry, network analysis and simultaneous thermal analysis were used to characterise the macro properties. The replacement of Ca²⁺-ion with Mg²⁺-ion led to the transformation of dodecahedron into tetrahedron. In addition, the bond length and bond population related to the calcium site decreased. The atom vibration and microstructure characteristics were modified due to the lattice distortion. The densification temperature decreased from 1000 °C to 925 °C. The relative density increased from 70% to 80.5%. The dielectric properties were improved, that is, ε_r = 5.63, tanδ = 0.0019 at 8 GHz as 4% (molar ratio) Ca²⁺-ions were replaced by Mg²⁺-ions.     

A novel approach for processing CaAlSiON glass-ceramics by spark plasma sintering: Mechanical and electrical properties

Lithium containing glassy materials can be used as solid electrolytes or electrode materials for lithium-ion batteries due to their high energy density. Conventional melt-quenched Ca₁₁Al₁₄Si₁₆O₄₉N₁₀ glass powder containing 24 e/o N, doped with Li-ions (1, 3, and 6 wt. %) and sintered by spark plasma sintering technique (SPS) was studied. The benefits of using SPS to produce glass-ceramics are rapid heating rates compared to conventional consolidation techniques and tuning of properties, adjusting the temperature, holding time (closed to Tg temperature), heating rate (solidification), and pressure (densification) profile during the heat treatment using SPS. Pure glass and glass-ceramic were obtained under identical SPS conditions and compared with pristine oxynitride and soda-lime-silicate (float) glasses. XRD and SEM analysis confirmed that increasing the amount of Li increases the crystallinity in the glass matrix. Nano-indentation analysis showed a decreased hardness and reduced elastic modulus values with the addition of Li-ions. The direct current conductivity increases with the addition of Li due to the high mobility of Li-ions. However, the float glass sample doped with 6 wt.% of Li exhibits even higher values of D.C. conductivity, than the analogously doped Ca₁₁Al₁₄Si₁₆O₄₉N₁₀ glass. The magnitude of activation energy (more than 1 eV) is typical for an ion hopping mechanism and the D.C. conduction mechanism is dominated by Li⁺ hopping.