Sintering and crystallization of cordierite glass ceramics for high frequency multilayer chip inductors

Cordierite-based ceramics were developed by sintering a glass selected from MgO-Al₂O₃-SiO₂ system with an aim to use the material as high frequency chip inductors. A small amount of B₂O₃ and P₂O₅ were added to optimize the preparation conditions. The glass powder and sintered bodies were characterized by different analytical techniques such as TG-DTA analysis, X-ray diffraction, transmission and scanning electron microscopy. Pellets uniaxially pressed from the glass power could be sintered well at 950 °C having a density of above 99% theoretically, dielectric constant of 5.5, dielectric loss of 0.001 and thermal expansion coefficient of 26.7×10⁻⁷ °C⁻¹ (20-400 °C). Crystalline phases in this sintered sample are predominantly α-cordierite (hexagonal high cordierite) and trace amount of μ-cordierite. SEM depicted a uniformly dense microstructure with crystals of granular habit in the sintered sample.     

Synthesis, characterization, and sintering of sol–gel derived cordierite ceramics for high-frequency MLCIs

The synthesis, characterization, and sintering of sol–gel derived cordierite ceramics are investigated in the present paper. Synthesis was carried out by optimizing two main preparation parameters. The effect of the heat-treatment schedule on crystallization and the properties of crystalline phases were analyzed. The additives B₂O₃ and P₂O₅ were utilized to promote the crystallization or transformation to -cordierite and sintering. This material has a low dielectric constant and a low dissipation factor and can be co-fired with high conductivity metals such as Au, Ag/Pd, Cu paste at low temperature (below 1000 °C), suggesting that it would be a promising material for high-frequency MLCIs.     

Effect of cerium addition on phase transformation and microstructure of cordierite ceramics prepared by sol-gel method

Low-temperature sintering of cordierite ceramic depends on the phase transformation into cordierite and the properties depend on its microstructure. In the present work, the effect of cerium on the phase transformation and microstructure of cordierite ceramics prepared by sol-gel method is studied by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) in order to lower the sintering temperature and improve the properties of cordierite ceramic with the addition of cerium. It is observed that the cerium addition obviously lowers the crystallization temperature of -cordierite while slightly raises that of -cordierite. The lowest temperature for cordierite transformation, which approaches the crystallization temperature of -cordierite, is achieved in the sample containing 4 wt% of cerium, implying a possibility to lower the sintering temperature of cordierite ceramics. The Ce-contained ceramics show a biphasic microstructure that is dependent on sintering temperature. Sintered below 1300°C, a cordierite-CeO₂ microstructure is present; while sintered at the temperature above 1300°C, appears a cordierite-glass microstructure, of which the amount of glass phase is limited to a small extent. Since the addition of 4 wt% cerium to this MgO-Al₂O₃-SiO₂ system substantially enhances the densification of cordierite ceramics and lowers the sintering temperature to the level of around 1000°C, it makes the ceramics suitable for such applications, where the low-temperature sintering is required, as the substrates for electronic circuit and the catalytic supports (with oxygen storage capacity) for cleaning of automotive exhaust emissions.     

溶胶-凝胶法制备堇青石粉体的固相反应过程和烧结性的研究

将TEOS,镁铝硝酸盐以及一定量的尿素溶于乙醇-水溶液。100℃水浴加热形成凝胶。凝胶干燥后,高温煅烧可得到一种镁铝硅复合氧化物粉体。对在不同温度下煅烧的粉体和1350℃烧结的样品进行X射线衍射分析,发现在煅烧过程中,基体形态经历了从无定形态向晶态α-堇青石的转化过程,并伴随有μ-堇青石相,镁铝尖晶石相和β-石英固溶体的出现和消失。1350℃烧结的样品全部为α-堇青石相,烧结致密度最高达99.5％。以扫描电镜观察该样品的表面形貌,可看出其晶粒呈薄片状。并对纯堇青石的致密化烧结过程进行了探讨。     

Seeding effects on crystallization temperatures of cordierite glass powder

The role of solid state epitaxy in the crystallization of nanocomposite cordierite glass to glass ceramic was investigated. The use of isostructural (-cordierite) seeds in cordierite glass led to a lowering in the crystallization temperature to form glass ceramic by about 50 °C compared to the unseeded glass. The use of non-isostructural seeds such as ZrO₂ and TiO₂ did not lower the crystallization temperature of cordierite glass to glass ceramic, and in the case of the TiO₂-seeded glass the crystallization temperature increased by about 50 °C compared to the unseeded-cordierite glass. The lowering in crystallization temperature by-cordierite seeding can be attributed to the nucleation and epitaxial growth mechanism.     

Synthesis and characterization of xMgO–1.5Al2O3–5SiO2 (x = 2.6–3.0) system using mainly talc and kaolin through the glass route

Three non-stoichiometric cordierite with compositions of xMgO–1.5Al₂O₃–5SiO₂ (x = 2.6–3.0 mol) were synthesized using mainly talc and kaolin through the glass route. The densification and crystallization behaviors of these glass powders were investigated using DTA, dilatometer, and XRD. Samples were then heat treated at 900 °C for 2 h and further analyzed using XRD, CTE, FESEM, density and porosity tests, and also dielectric test to further investigate their properties. The 2.8MgO·1.5Al₂O₃·5SiO₂ glass composition fully densified and crystallized into high purity α-cordierite at the heat treatment temperature. It exhibited lower CTE and dielectric constant compared to other composition, making it suitable for high frequency applications. Other formulations which contain multi crystalline phases require much higher temperatures for densification. The CTE value depends on the type of crystalline phase which exist in the sample, while the dielectric constant decreased with increasing MgO amounts in the sample compositions.     

Compositional effect on the crystallization of the cordierite-type glasses

The determination of apparent activation energies for the crystallization of P₂O₅/B₂O₃-containing cordierite-type glasses was conducted by non-isothermal differential thermal analysis (DTA). Glass with excesses both of SiO₂ and Al₂O₃ has a higher activation energy for crystallization. In addition to the main phase of -cordierite, several other phases (farringtonite, clinoenstatite, and forsterite) were detected in the fully crystallized samples by the X-ray powder diffraction technique. The crystallization behavior for the powdered glass was governed by the surface crystallization mechanism, which would not change with composition.     

The effect of additives on the crystallization and sintering of 2MgO-2Al2O3-5SiO2 glass-ceramics

Crystallization and sintering behaviour of three cordierite (2MgO-2Al₂O₃-5SiO₂) glasses containing different amount of additives were investigated and compared by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and the Archimedes method. The stoichiometric 2MgO-2Al₂O₃-5SiO₂ (MAS) glass and the 2MgO-2Al₂O₃-5SiO₂ glass containing 3 wt% of B₂O₃ and 3 wt% of P₂O₅ (MASBP) showed two exotherms (one for -cordierite formation from a glass and the other for -cordierite formation from the -cordierite phase), whereas the 2MgO-2Al₂O₃-5SiO₂ glass containing 2 wt % of B₂O₃, 2 wt% of P₂O₅, and 2 wt % of TiO₂ (MASBPT) showed only a single exotherm representing -cordierite formation. By using Kissinger, Augis-Bennett, Ozawa, and modified Kissinger methods, the activation energy values for -cordierite formation in the MASBP and MASBPT glasses were determined as 310±6 and 326±13 kJ mol^–1, respectively, whereas that in the MAS glass was determined as 868±5 kJ mol^–1. The MASBPT glass showed the lowest peak temperature value for -cordierite formation (980 °C) amongst the three glasses. Both the MASBP and MASBPT glasses showed excellent sintering behaviour (> 99.7% of theoretical density).     

A simple synthesis route for high quality BaFe12O19 magnets

The present investigation reports the detailed analysis of the influence of B₂O₃-doping on magnetic and structural properties of hexagonal barium ferrites (BaFe₁₂O₁₉) synthesized via conventional ceramic technique. The results show that crystalline structure of barium ferrite is improved with small amounts of B₂O₃ between 0.1 and 1.0 wt%. Single phase particles has been obtained by 1.0 wt% B₂O₃-doping with calcination at temperatures as low as 850 °C and a range of boron concentration and calcination temperature has been extended in order to find the best synthesis conditions. Optimal properties have been achieved by 0.2 wt% of B₂O₃-doping and calcination at 1000 °C. The remanence and the saturation magnetizations increase considerably by about 40% in magnitude with B₂O₃.     

Affect of grinding and polishing on near-surface phase transformations in zirconia

The transformation of near-surface material on grinding and polishing has been investigated in sintered zirconia of 1 μm grain size and 99 % density containing 4.5 and 7.0 mol % Y₂O₃. Rough wet and dry grinding transformed ca 20 % cubic phase into 18 % tetragonal and 2 % monoclinic in material initially 47 % cubic and 53 % tetragonal (4.5 mol % Y₂O₃) but no change of phase in material that was fully cubic (7.0 mol % Y₂O₃). Annealing and polishing reduced lattice strain but only polishing reduced the concentration of monoclinic and tetragonal phases. Microhardness studies indicated that lattice strain and the phase transformations increased the penetration hardness to a depth of ca 4 μm.     

ESR study of CuO-doped cordierite

The reactivity of cordierite precursor with CuO powder was studied by means of X-ray diffraction measurements and electron spin resonance spectroscopy. Two concentrations of CuO were used: 10 and 20 mol %. An ESR signal was detected after heating the mixture of cordierite precursor-CuO at 500°C showing that CuO begins to dissolve with the cordierite precursor. The reactivity strongly increased between 700 and 800°C, in relation to the transition amorphous -cordierite. However, the crystallization of -cordierite prevented the dissolution. Cu²⁺ was located in the amorphous phase and characterized byg=2.35 andg=2.08.     

Effect of replacement of MgO by CaO on sintering,crystallization and properties of MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system glass-ceramics

The effects of replacement of MgO by CaO on the sintering and crystallization behavior of MgO–Al₂O₃–SiO₂ system glass-ceramics were investigated. The results show that with increasing CaO content, the glass transition temperature firstly increased and then decreased, the melting temperature was lowered and the crystallization temperature of the glass-ceramics shifted clearly towards higher temperatures. With the replacement of MgO by less than 3 wt.% CaO, the predominant crystalline phase in the glass-ceramics fired at 900 °C was found to be α-cordierite and the secondary crystalline phase to be μ-cordierite. When the replacement was increased to 10 wt.%, the predominant crystalline phase was found to be anorthite and the secondary phase to be α-cordierite. Both thermal expansion coefficient (TCE) and dielectric constant of samples increases with the replacement of MgO by CaO. The dielectric loss of sample with 5 wt.% CaO fired at 900 °C has the lowest value of 0.08%. Only the sample containing 5 wt.% and10 wt.% CaO (abbreviated as sample C5 and C10) can be fully sintered before 900 °C. Therefore, a dense and low dielectric loss glass-ceramic with predominant crystal phase of α-cordierite and some amount of anorthite was achieved by using fine glass powders (D₅₀ = 3 μm) fired at 875–900 °C. The as-sintered density approaches 98% theoretical density. The flexural strength of sample C5 firstly increases and then decreases with sintering temperature, which closely corresponds to its relative density. The TCE of sample C5 increases with increasing temperature. The dielectric property of sample C5 sintered at different temperatures depends on not only its relative density but also its crystalline phases. The dense and crystallized glass-ceramic C5 exhibits a low sintering temperature (≤900 °C), a fairly low dielectric constant (5.2–5.3), a low dielectric loss (≤10⁻³) at 1 MHz, a low TCE (4.0–4.25 × 10⁻⁶ K⁻¹), very close to that of Si (∼3.5 × 10⁻⁶ K⁻¹), and a higher flexural strength (≥134 MPa), suggesting that it would be a promising material in the electronic packaging field.     

Enhancement of structural stability of nanosized amorphous Fe2O3 powders by surface modification

The surface of low-dimensional solids plays a key role in their phase transition. In the present study, to enhance the structural stability of nanosized amorphous Fe₂O₃ powders their surfaces were modified by employing NaOH solution, which leads to an increase in both the crystallization temperature from 364 °C to 411 °C and the crystallization activation energy from 81.5 kJ/mol to 156.8 kJ/mol. The surface-modified amorphous Fe₂O₃ powders show an entirely different crystallization behavior as compared with the as-prepared amorphous powders. The enhanced structural stability is attributed to the increase of the amount of hydroxide groupings at the surfaces of amorphous powders, which lowers their surface energy.     

Control of the tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia in hot water

The tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia caused by annealing in hot water was investigated in the temperature range 80 to 200° C using sintered bodies in zirconia containing 2, 3 and 4 mol % Y₂O₃. Three approaches, alloying ZrO₂(Y₂O₃) with 0 to 20wt% CeO₂, dispersing 0 to 40 wt % Al₂O₃ into ZrO₂(Y₂O₃) ceramics and decreasing the grain size of zirconia, were examined to inhibit the tetragonal-to-monoclinic phase transformation. The amount of monoclinic phase formed decreased with increasing concentrations of CeO₂ alloyed and Al₂O₃ dispersed, and with decreasing grain size of zirconia.     

Crystallization behavior of amorphous solid solutions and phase sepration in the Cr2O3-Fe2O3 system

Fine particles of the amorphous Cr₂O₃-Fe₂O₂ solid solutions were prepared by dehydration of coprecipitated hydroxides and their crystallization behavior was studied by differential thermal analysis and X-ray diffraction. The peak temperature for crystallization attained a maximum at a composition near Fe₂O₃ content of about 60 mol % and the activation energy for crystallization attained a minimum at a composition near Fe₂O₃ content of about 50 mol % in this quasibinary system. Phase separation occurred in a range of Fe₂O₃ content from about 35 to 80 mol % in the corundum-type solid solutions heat treated at 600 °C for 2 h. Crystallization behavior was discussed briefly related with phase separation and diffusion in fine particles.     

Sintering and characterization of Al2O3-B4C composites

The effect of B₄C on the densification, microstructure and mechanical properties of pressureless sintered Al₂O₃-B₄C composites have been studied. Sintering was performed without sintering additives with varying B₄C content from 0–40 vol %. Up to 20 vol % B₄C, more than 97% theoretical density was always obtained when sintered at 1850 °C for 60 min. On increasing the sintering time from 30–120 min, there was no change in density. The result of X-ray diffraction analysis showed that no reaction occurred between Al₂O₃ and B₄C. The grain growth of Al₂O₃ was inhibited by B₄C particles pinned at the grain boundary and the grain-boundary drag effect. The critical amount of B₄C to drag the grain boundary migration effectively was believed to occur at 10 vol % B₄C sintered at 1850 °C for 60 min. The maximum three-point flexural strength was found to be 550 MPa for the specimen containing 20 vol % B₄C, and the maximum microhardness was 2100 kg mm^–2 for 30 vol % B₄C specimen.     

Preparation of cordierite ceramics by sol-gel technique

Xerogel with cordierite composition Mg₂Al₃[AlSi₅O₁₈] from aluminium and magnesium 2-ethoxyethoxides and tetraethylortosilicate have been prepared, the latter being not previously partially hydrolized. Conditions to obtain not only homogeneous but also sinterable gel have been determined. All the powders derived were characterized by DTA, TG, BET and X-ray diffraction. Bulk α-cordierite material of 97% from theoretical density have been obtained by sintering.     

Preparation of polycrystalline BaTi2O5 ferroelectric ceramics

Polycrystalline BaTi₂O₅ (BT₂) was prepared by solid state sintering in air using BaCO₃ and TiO₂ as starting materials. A rod-like BT₂ sintered bodies in a single phase were obtained above 1025 °C by adding 5 wt.% B₂O₃. The densification of BT₂ sintered bodies decreased with increasing sintering temperature. The maximum permittivity of BT₂ sintered body was 640 at the Curie temperature (T_c) of 461 °C and the frequency of 100 kHz. The addition of B₂O₃ was effective to obtain BT₂ in a single phase at low temperatures with elongated microstructure and to improve the permittivity, indicating the formation of preferred orientation at low temperature was related with liquid-phase sintering mechanism.     

Microscopic characterization of sol-gel processed cordierite

Different analytical techniques have been used to characterize green and sintered sol-gel processed cordierite on a microscopic scale. The sintered material, although identified macroscopically as -cordierite, contains many small precipitates of different chemical composition. Larger precipitates, characterized by an increased Mg concentration, result from inhomogeneities in the precursor gel. Smaller precipitates, identified as mullite and spinel, could not be related to gel heterogeneities. The amorphous phase surrounding the cordierite grains and the precipitates is Mg depleted and Si enriched. Neither -cordierite nor -cordierite were observed; slight variations of the -cordierite lattice parameters are attributed to some local perturbations due to a slight departure from stoichiometry or Si, Al ordering on tiny domains.     

Crystallization and sinterability of cordierite-based glass powders containing CeO2

Dense glass-ceramics for low firing temperature substrates were prepared by the addition of CeO₂ flux to a glass of the MgO-Al₂O₃-SiO₂ system. The glass powders were fabricated by melting at 1500°C and ball milling. Glass powder compacts prepared by dry pressing were heated at 800–1000°C for 0.5–4 h and sintered at 900–1000°C for 3 h. The crystallization behaviour and sinterability of the glass powder compacts were analysed by thermal and thermomechanical techniques. X-ray diffractometry and scanning electron microscopy. The addition of CeO₂ prevents the formation of -cordierite phase in the glass-ceramics and improves the formation of -cordierite phase. The activation energy of the glass containing CeO₂ for crystallization was lower than that of the CeO₂-free glass. Therefore, crystallization properties were enhanced. Because the crystallization onset temperature increased and the softening temperature decreased on the addition of CeO₂, the sinterability increased and dense glass-ceramics were fabricated below 1000°C. The properties of the glass-ceramics containing CeO₂ appeared to be correct for low firing temperature substrates.