Carbon burnout and densification of self-constrained LTCC for fabrication of embedded structures in a multi-layer platform

Carbon burnout and densification of self-constrained low temperature co-fired ceramic (LTCC) are investigated using thermal analysis techniques. Slow heating rates and holding at a temperature higher than initial crystallization temperature of the glass component show evidence of retarding the densification of the self-constrained LTCC. Based on these results, it is proposed that the fabrication of embedded structures in a multi-layer self-constrained LTCC platform could be achieved by controlling carbon burnout with a multi-step co-firing profile, which can ensure complete carbon burnout without affecting the densification of LTCC structures. Using this approach, fabrication of an embedded cavity with dimensions of 10 mm × 10 mm × 0.50 mm in a self-constrained LTCC platform is demonstrated.     

Zero Shrinkage of LTCC by Self-Constrained Sintering

Low shrinkage in x and y direction and low tolerances of shrinkage are an indispensable precondition for high-density component configuration. Therefore, zero shrinkage sintering technologies as pressure-assisted sintering and sacrificial tapes have been introduced in the low-temperature co-fired ceramics (LTCC) production by different manufacturers. Disadvantages of these methods are high costs of sintering equipment and an additional process step to remove the sacrificial tapes. In this article, newly developed self-constrained sintering methods are presented. The new technology, HeraLock^®, delivers LTCC modules with a sintering shrinkage in x and y direction of less than 0.2% and with a shrinkage tolerance of ±0.02% without sacrificial layers and external pressure. Each tape is self-constrained by integration of a layer showing no shrinkage in the sintering temperature range of the LTCC. Large area metallization, integration of channels, cavities and passive electronic components are possible without waviness and camber. Self-constrained laminates are an alternative way to produce zero shrinkage LTCC. They consist of tapes sintering at different temperature intervals. Precondition for a successful production of a self-constrained LTCC laminate is the development of well-adapted material and tapes, respectively. This task is very challenging, because sintering range, high-temperature reactivity and thermal expansion coefficient have to be matched and each tape has to fulfill specific functions in the final component, which requires the tailoring of many properties as permittivity, dielectric loss, mechanical strength, and roughness. A self-constrained laminate is introduced in this article. It consists of inner tapes sintering at especially low-temperature range between 650°C and 720°C and outer tapes with an as-fired surface suitable for thin-film processes.     

Recycling of waste fly ash for production of porous mullite ceramic membrane supports with increased porosity

Low-cost porous mullite ceramic membrane supports were fabricated from recycling coal fly ash with addition of natural bauxite. V₂O₅ and AlF₃ were used as additives to cause the growth of mullite crystals with various morphologies via an in situ reaction sintering. Dynamic sintering, microstructure and phase evolution of the membrane supports were characterized in detail and open porosity, pore size, gas permeation and mechanical properties were determined. It showed the membrane support with 3 wt.% V₂O₅ and 4 wt.% AlF₃ addition exhibits an open porosity of ∼50%, mechanical strength of 69.8 ± 7.2 MPa, an interlocking microstructure composed of anisotropically grown mullite whiskers with an aspect ratio of 18.2 ± 3.6 at 1300 °C. Addition of more V₂O₅ lowered the secondary mullitization temperature, resulting in more mullite formation at lower temperatures. The fabricated membrane supports feature high porosity without mechanical strength degradation, possible strengthening mechanism of the mullite whiskers was further discussed.     

Optimum processing parameters to improve sealing performance in solid oxide fuel cells

Glass–ceramic composites are among the favorable candidates as a sealing material for solid oxide fuel cells (SOFC). In order to obtain a reliable, robust and hermetic sealing, the glass–ceramics must chemically bond to both the metallic interconnector and the ceramic electrolyte. A high-bonding strength and good wetting, which strongly depend on the thermal treatment, are always preferred to ensure gas-tight sealing. The thermal treatment involves three stages: binder burnout (stage-I), sintering (stage-II), and cooling (stage-III). This study investigates effects of various parameters on the sealing quality at the sintering stage. The effects of sintering temperature, clamping pressure and sealant thickness are considered. The glass–ceramic laminates are produced employing a tape casting method. The sealing quality is evaluated by measuring leakage and final macro-structure of the sealing region. It is suggested that a 900–930 °C sintering temperature and 1.5–7.6 N cm⁻² clamping pressure ranges are better for successful sealing. The initial thickness of glass–ceramic laminates is also desired to be between 0.25–0.5 mm thickness range for both a cost-effective and reliable sealing.     

Densification and grain growth kinetics of Ce0.9Gd0.1O1.95 in tape cast layers: The influence of porosity

The sintering behavior of Ce_0.9Gd_0.1O_1.95 (CGO) tape cast layers with different porosity was investigated by an extensive characterization of densification, microstructural evolution, and applying the constitutive laws of sintering. The densification of CGO tapes associates with grain coarsening process at the initial sintering stage at T < 1150 °C, which is mainly influenced by small pores and intrinsic characteristics of the starting powders. At the intermediate sintering stage, densification is remarkably influenced by large porosity. Moreover, the sintering constitutive laws indicate that increasing the initial porosity from 0.38 to 0.60, the densification at the late stage is thermally activated with typical activation energy values increasing from 367 to 578 kJ mol⁻¹. Similar effect of the porosity is observed for the thermally activated phenomena leading to grain growth in the CGO tapes. The analysis of sintering mechanisms reveals that the grain growth behavior at different porosity can be described using an identical master curve.     

Thick-film PTC thermistors and LTCC structures: The dependence of the electrical and microstructural characteristics on the firing temperature

The electrical and microstructural characteristics of 1 kΩ/sq thick-film thermistors with high positive temperature coefficients of resistivity, i.e., PTC 5093 (Du Pont) fired either on “green” LTCC (low-temperature co-fired ceramics) substrates or buried within LTCC structures, were evaluated. The thermistors were fired at different temperatures to study the influence of firing temperature on the electrical characteristics. The noise indices of the surface resistors fired at temperatures between 850 °C and 950 °C were very low, around −30 dB. The TCRs of the evaluated PTC thermistors were over 3000 × 10⁻⁶/K. The dependence of the resistivity on the temperature between −25 °C and 125 °C was linear, with the values of R² being better than 0.9999, regardless of the processing conditions. These results show that PTC thermistors co-fired on LTCC substrates can be used for temperature sensors in MCM-Cs as well as in MEMS structures. However, when the thermistors were buried in the LTCC substrates, the LTCC structures delaminated during firing and blisters formed, leading to high sheet resistivities and high noise indices. This delamination is attributed to the different sintering rates of the PTC and LTCC materials as well as to the expansion of the air bubbles captured in the viscous glass of the PTC material.     

The porosification of fired LTCC substrates by applying a wet chemical etching procedure

In this study, a novel process is presented to generate a defined and homogeneous degree of porosity in fired low temperature co-fired ceramics (LTCC) substrates. For this purpose, a phosphoric-based acid is used which is a standard wet chemical etchant in the MEMS and microelectronic industry for the patterning of aluminium-based conductors and strip lines. Varying the bath temperature between 90 and 130 °C within a time frame of up to 8 h, a maximum penetration depth of 40 μm is achieved. At short etch times up to 5 h, the porosification process is reaction controlled, while at longer exposure times, diffusion-related effects dominate verified by the determination of the corresponding activation energies. In combination with morphological investigations using scanning electron microscopy and micro-X-ray diffraction techniques, it is demonstrated that the anorthite-phase crystallizing during liquid sintering in the vicinity of the Al₂O₃ grains shows a high dissolvability in phosphoric acid and is very important to enable its penetration into the LTCC body. This surface-near process is very attractive for the realization of selected areas on conventional LTCC substrates having modified dielectric properties, especially for high frequency applications.     

Fracture toughness of concentric Si3N4-based laminated structures

A new concentric rectangular laminated structure was designed and fabricated by slip-casting method and densified by pressureless sintering process. One class of laminates consists of layers of Si₃N₄ with 7 wt.% Y₂O₃ and 3 wt.% Al₂O₃ as sintering aids, and of interlayers consisting of 50 wt.% BN and 50 wt.% Al₂O₃ designated as SN-(BN + Al₂O₃). The other class of laminates has the same Si₃N₄ layer composition but different interlayer composition of 90 wt.% BN and 10 wt.% Si₃N₄ designated as SN-(BN + SN). The objective of this paper is to investigate the effects of the number of layers and their thickness on apparent fracture toughness of these laminates. The interfacial layer composition was discussed in terms of its role in toughening of the laminates. For the SN-(BN + Al₂O₃) laminates the highest apparent fracture toughness of 22 MPa m^1/2 was found in the samples with 7 Si₃N₄ layers and for the SN-(BN + SN) laminates the highest apparent fracture toughness of 19.5 MPa m^1/2 was found in the samples with 4 Si₃N₄ layers.     

Constrained sintering of alumina stripe patterns on rigid substrates: Effect of stripe geometry

The sintering behaviour of alumina stripes deposited on sapphire substrates by micromolding in capillaries – a soft lithographic method – with lateral dimensions from 10 to 500 μm and thicknesses between 7 μm and 32 μm was studied. Unlike in continuous films, the lateral sintering strain is not negligible, thus reducing the constraint imposed by the substrate. Lateral shrinkage depends on the stripe width and thickness. The degree of constraint exerted on alumina stripes by a rigid sapphire substrate was investigated by comparing the lateral and vertical strains and is found to be dependent on stripe geometry. The formation of a delaminated, highly dense edge zone was observed at the free boundaries. Its influence on overall densification and local density distribution depends on its extension compared to the total film width. A gradient in local density was found that varied both with stripe thickness and width as predicted by finite element and discrete element simulations.     

Alumina porous nanomaterials obtained by colloidal processing using d-fructose as dispersant and porosity promoter

Recently, great effort has been devoted to obtain porous materials with customized pore size distribution, high surface area and submicrometer sized microstructures or nanostructures. In this work, the viability of colloidal processing routes to obtain porous bulk ceramics using alumina nanopowders and d-fructose as a dispersant and a porosity former has been explored.The rheological behaviour of nanosuspensions was studied in order to assure their stability and to analyse the influence of different parameters (solids loading, fructose content, pH, sonication time). Mesoporous green bodies were obtained by slip casting with d-fructose in concentrations ranging from 5 to 50 wt%. The drying and burning-out conditions were determined by DTA-TG measurements and the sintering cycles were selected from the dynamic sintering curve. Sintered alumina materials with high porosity (>60%), open microstructures, submicrometer sized porosity (d_p = 140–210 nm) and grain size lower than 500 nm, were obtained for pieces sintered at temperatures of 1300 and 1400 °C. The influence of different processing parameters on the porosity and the microstructure of the sintered materials is discussed.     

Nitrogen pressure effects on non-isothermal alumina sintering

The densification of a fine grained pure alumina powder was studied during gas pressure sintering. Different nitrogen pressures were applied during non-isothermal sintering runs up to final temperatures between 1150 °C and 1650 °C. Densification, porosity and microstructures have been investigated. A fine alumina powder presents a densification delay during nitrogen pressure sintering mainly due to the gas pressure effect at the beginning of the sintering. The main results of this work concern the influence of nitrogen pressure on non-densifying mechanisms and microstructural evolution, which only depends on densification rate.     

Processing of porous yttria-stabilized zirconia tapes: Influence of starch content and sintering temperature

The tape-casting process was used to produce porous yttria-stabilized zirconia (YSZ) substrates with volume fractions of porosity ranging from 28.9 to 53 vol.% by using starch as a fugitive additive. Concentrated aqueous YSZ slips with different amounts of starch and an acrylic latex binder were prepared. The influence of the volume fraction of starch and sintering temperature on the sintering behavior and final microstructure were investigated. The microstructure consisted of large pores created by the starch particles with lengths between 15 and 80 μm and smaller pores in the matrix with lengths between 0.6 and 3.8 μm. The pores in the matrix reduced the sinterability of the YSZ leading to the retention of closed porosity in the sintered tapes. The porosities were above those predicted for each of the starch contents. However, larger deviations from the predicted porosity were found as more starch was added. The open to total porosity ratio in the sintered tapes could be controlled by the volume fraction of added starch as well as by the sintering temperature. As the volume fraction of starch increased from 17.6 to 37.8 vol.% there was a gradual increase in the interconnectivity of the pore structure. The sintering shrinkage of the tapes at a given temperature could be directly related to the YSZ packing density in the matrix.     

Effect of polymer concentration on porosity and pore size characteristics of alumina membrane substrates prepared by gelcasting

The effect of urea–formaldehyde (UF) polymer concentration on porosity and average pore size of alumina membrane substrates prepared by gelcasting has been studied. The soluble UF oligomers formed in the initial stages of polymerization act as steric stabilizer for alumina particles in the suspension. The porosity and average pore size of the substrate samples decreased with both the decrease of amount of polymer in the gelcast body and the increase of sintering temperature. Membrane substrates obtained by sintering of gelcast bodies containing UF polymer concentrations from 24.3 to 15.6 wt% at temperatures from 1250 to 1450 °C showed porosity and average pore size of 62.5–27 vol% and 0.43–0.20 μm, respectively. The membrane substrates prepared by the gelcasting method had narrow pore size distribution.     

Constrained sintering of YSZ/Al2O3 composite coatings on metal substrates produced from eletrophoretic deposition

Yttria stabilized zirconia/alumina (YSZ/Al₂O₃) composite coatings were prepared from electrophoretic deposition (EPD), followed by sintering. The constrained sintering of the coatings on metal substrates was characterized with microstructure examination using electron microscopy, mechanical properties examination using nanoindentation, and residual stress measurement using Cr³⁺ fluorescence spectroscopy. The microstructure close to the coating/substrate interface is more porous than that near the surface of the EPD coatings due to the deposition process and the constrained sintering of the coatings. The sintering of the YSZ/Al₂O₃ composite coating took up to 200 h at 1250 °C to achieve the highest density due to the constraint of the substrate. When the coating was sintered at 1000 °C after sintering at 1250 °C for less than 100 h, the compressive stress was generated due to thermal mismatch between the coating and metal substrate, leading to further densification at 1000 °C because of the ‘hot pressing’ effect. The relative densities estimated based on the residual stress measurements are close to the densities measured by the Archimedes method, which excludes an open porosity effect. The densities estimated from the hardness and the modulus measurements are lower than those from the residual stress measurement and the Archimedes method, because it takes account of the open porosity.     

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%).     

Preparation and properties of porous ceramic aggregates using electrical insulators waste

In this paper, porous ceramic aggregates were prepared by electrical insulators waste (EIW). Effects of sintering temperature and content of EIW on the aggregates’ properties such as bulk density, and apparent porosity, total porosity, and cold crushing strength were investigated. With increasing sintering temperature and content of EIW, bulk density and cold crushing strength of the aggregates increased, apparent porosity and total porosity decreased. Based on these results, total porosity of specimens in group B sintered at 1200 °C is 62.0%, cold crushing strength is 35.3 N, and thermal conductivity is 0.165 W/(m K) at 300 °C. Comprehensive properties of specimens can be optimized by adjusting sintering temperature. Meanwhile, strength variation resulted from the combined effects of phase transformation and matrix densification under different sintering temperatures.     

Effect of additives on the pore evolution of zirconia based ceramic foams after sintering

A new superplasticity foaming method was used to form zirconia-based ceramic foams. Silica and alumina were chosen as additives because they facilitate the 2D superplastic deformation. The effects of these additives on macroscopic pore evolution were examined after heat treatment for up to 40 h. The addition of silica or alumina also enhanced the 3D deformation during superplasticity foaming. The total porosity of mono-foams made from 3 mol% yttria-stabilised zirconia without additives increased with heat treatment for up to 24 h, and then levelled off. The porosity of silica-dispersed foam was greater than that without additives and continued to increase for up to 40 h. Conversely, the porosity of alumina-dispersed ceramic foam reached saturation within 8 h. Consequently, the porosity of alumina-dispersed foam was greater than that without additives after heating for 8 h, while the latter exceeded the former with prolonged heating for more than 16 h. The detailed effects of alumina dispersion on the foam development behaviour were examined in connection with the microstructure.     

Tape casting and characterization of Li2.08TiO3-LiF glass free LTCC for microwave applications

Li_2.08TiO₃-LiF Glass-free Low temperature co-fired ceramic (LTCC) green tapes were prepared by tape casing technique. The rheology of the slurry was characterized using rheometer. The slurry exhibited pseudoplastic behavior. The sintering kinetics of the green tape was investigated using heating microscope. The sintering activation energy was determined to be ∼173 kJ/mol. The green tape could be densified at 900 °C/2 h. Microwave dielectric properties of the sintered tape were characterized in a split-post dielectric resonator using a network analyzer. The ceramic sheet with thickness of 0.11 mm demonstrated good microwave dielectric properties: ε_r = 22.4 and Q × f = 35,490 GHz. The cross sectional microstructure of the cofired multilayer stack was observed by scanning electron microscopy (SEM). The green tape demonstrated good chemical and shrinkage compatibilities with Ag electrode during sintering process. The thermal expansion coefficient and thermal conductivity of the ceramic is 22.4 ppm/∘C and 4.75 W m⁻¹ K ⁻¹, respectively.     

Low-sintering ceramic materials based on Bi2O3–ZnO–Nb2O5 compounds

In this work, sintering behaviour of Bi₂O₃–ZnO–Nb₂O₅ compounds was investigated in order to develop LTCC materials with suitable microwave properties. Structure, dielectric properties and sintering were studied for ceramic dielectrics based on the system: Bi₂ZnNb₂O₉ with the pyrochlore structure and ZnNb₂O₆ with a columbite one. The work was carried out over a wide range of initial components concentration. Ceramic samples of these materials were prepared by the mixed oxide technique. The effect of adding glass to the materials have been discussed. The sintering behaviour, dielectric permittivity, quality factor and crystal structures have been characterized for ceramic samples depending on compositions. Low-temperature co-firable ceramic material with ɛ ∼ 30, τ_ɛ = 0 and Q × f = 3500 GHz based on the above system was synthesized.     

Inter-diffusion between NiCuZn-ferrite and LTCC and its influence on magnetic performance

The present study investigated inter-diffusion between NiCuZn-ferrite and low-temperature-cofired ceramic (LTCC) during co-firing. The copper (Cu) ions in ferrite can diffuse into LTCC at a distance of 120 μm after sintering at 900 °C for 2 h, yielding a diffusion coefficient around 10^?9 cm²/s. The magnesium (Mg) and aluminum (Al) ions from LTCC also diffuse into ferrite for a shorter distance. Several new phases form through such inter-diffusion. For example, the inter-diffusion between alumina and ferrite induces the formation of hematite whose presence is detrimental to the saturation magnetization and permeability of ferrite. Additionally, inter-diffusion also induces changes in the lattice parameters of ferrite. We note a linear relationship between the lattice constant of ferrite and saturation magnetization, which demonstrates that magnetic properties are strongly tied to the crystalline structure of ferrite.