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Sintering of LTCC
(2008)
Low Temperature Co-fired Ceramic materials (LTCC) have attracted growing interest in recent years since they are promising candidates for highly integrated ceramic packaging. This paper surveys materials concepts of related glass ceramic composites (GCC) and discusses unsolved problems and challenges. The densification of GCC based on "viscous sintering" may be affected by steric effects and partial dissolution of dispersed crystal particles, crystallization and phase boundary reactions.
The manufacture of sintered glasses and glass-ceramics, glass matrix composites, and glass-bounded ceramics or pastes is often affected by gas bubble formation. Against this background, we studied sintering and foaming of barium silicate glass powders used as SOFC sealants using different powder milling procedures. Sintering was measured by means of heating microscopy backed up by XPD, differential thermal analysis, vacuum hot extraction (VHE), and optical and electron microscopy. Foaming increased significantly as milling progressed. For moderately milled glass powders, subsequent storage in air could also promote foaming. Although the powder compacts were uniaxially pressed and sintered in air, the milling atmosphere significantly affected foaming. The strength of this effect increased in the order Ar ≈ N2 < air < CO2. Conformingly, VHE studies revealed that the pores of foamed samples predominantly encapsulated CO2, even for powders milled in Ar and N2. Results of this study thus indicate that foaming is caused by carbonaceous species trapped on the glass powder surface. Foaming could be substantially reduced by milling in water and 10 wt% HCl.
For the first time evidence is provided that a nanocrystalline and stacking-disordered, chemically stabilized β-cristobalite form of AlPO4 occurs in a sewage sludge ash (SSA). This proof is based on a combined X-ray powder diffraction and X-ray fluorescence investigation of an SSA produced at a large-scale fluidized bed incineration facility serving a catching area with a population of 2 million. The structural and chemical characterization was carried out on 'as received' SSA samples as well as on solid residues remaining after leaching this SSA in sodium hydroxide solution. Thus, it was ascertained that the observed nanocrystalline and stacking-disordered cristobalite-like component belongs to the aluminum phosphate component of this SSA, rather than to its silicon dioxide component. In addition, a direct proof is presented that the chemically stabilized β-cristobalite form of AlPO4 does crystallize from X-ray amorphous precursors under conditions that mimic the huge heating rate and short retention time (just seconds at T ≈ 850°C), typical for fluidized bed incinerators.
The hydrogen tightness of high-pressure hydrogen storage is a Basic criterion for long-term storage. The H2 permeation coefficients of epoxy resin and a glass lacquer were determined to enable the geometric optimization of a glass capillary storage. It was found that the curing conditions have no significant influence on the H2 permeation coefficient of resin.
The H2 permeation coefficient of epoxy resin is only about three orders of Magnitude greater than that of borosilicate glass. This suggests that the initial pressure of 700 bar takes about 2.5 years to be halved in capillary array storage. Therefore, a high-pressure hydrogen storage tank based on glass capillaries is ideally suited for long-term storage in mobile applications.
A novel method to generalize kinetic data of viscous crack healing in glasses is proposed. The method assumes that crack healing progress is proportional to the healing time, t, and indirect proportional to viscosity, n. This way, crack length and crack width data, normalized to the initial crack length and plotted versus t/n, allow to compare crack healing progress for different cracks and healing temperatures in a master curve. Crack healing experiments conducted in this study demonstrate the applicability of this method for a commercial microscope slide glass.
A glass of the composition 37BaO·16CaO·47SiO2 wt% produced on an industrial scale is crystallized at 970 °C for times ranging from 15 min to 2 h. The crystallization at the immediate surface as well as the crystal growth into the bulk are analyzed using scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDXS) and electron backscatter diffraction (EBSD) as well as X-ray diffraction in the Θ–2Θ setup (XRD). The immediate surface shows the oriented nucleation of walstromite as well as the formation of wollastonite and an unknown phase of the composition BaCaSi3O8. All three phases also grow into the bulk where walstromite ultimately dominates the kinetic selection and grows throughout the bulk due to a lack of bulk nucleation. Walstromite shows systematic orientation changes as well as twinning during growth. A critical analysis of the XRD-patterns acquired from various crystallized samples indicates that their evaluation is problematic and that phases detected by XRD in this system should be verified by another method such as EDXS.
Sand erosion of solar glass: Specific energy uptake, total transmittance, and module efficiency
(2018)
Surface roughness, R Z , normal transmittance, Τ N , total transmittance, Τ T , and photovoltaic (PV) module efficiency, η S , were measured for commercial solar glass plates and PV test modules identically sandblasted with different loads of quartz sand (200 – 400 μ m), impact inclination angles, and sand particle speed. Measured data are presented versus the specific energy uptake during sand blasting, E (J/m2). Cracks, adhering particles, and scratch ‐ like textures probably caused by plastic flow phenomena could be observed after sand blasting. Their characteristic size was much smaller than that of sand particles.
After blasting and subsequent cleaning, the glass surface was still covered with adhering glass particles. These particles, cracks, and scratch ‐ like textures could not be removed by cleaning. For sand blasting with α = 30° inclination angle and E = 30 000 J/m2, normal transmittance, total transmittance, and relative module efficiency decreased by 29%, 2% and ∽ 2%, respectively. This finding indicates that diffusive transmission of light substantially contributes to PV module efficiency and that the module efficiency decrease caused by sand erosion can be better estimated from total than by normal transmittance measurements.
Borosilicate glasses (16Na2O–10B2O3–74SiO2, NBS) with water contents up to 22 mol% H2O were prepared to study the effect of water on structural relaxation using DTA, viscometry and internal friction measurements. The results show that the glass transition temperature Tg of DTA and the isokom temperature T12, of viscometry are in excellent agreement, confirming the equivalence of enthalpy and viscous relaxation for NBS glass. Combining Tg data with water speciation data demonstrates that OH groups are mainly responsible for the decrease of Tg with increasing hydration, while molecular water plays only a minor role. Internal friction spectra at 7.125 Hz confirm the decisive influence of water on mechanical relaxation. The temperature range of α-relaxation (glass transition) strongly decreases while two β-relaxation peaks (sub-Tg) progressively appear with increasing water content. A high temperature β-relaxation peak, attributed to the presence of OH groups, shifts from 670 to 450 K as total water content increases from 0.01 to 5 wt%. A low temperature β-relaxation peak, attributed to molecular water, appears at 380 K and 330 K in glasses containing 3 and 5 wt% H2O, respectively. These findings suggest that relaxation mechanism of different hydrous species at low temperature may contribute to fatigue of stressed glasses.
To explore the impact of ambient and structural water on static fatigue, the initiation and growth of 3279 Vickers induced median radial cracks were automatically recorded and analyzed. We find that humidity is more efficient in initiating cracks and promoting their growth than water, which is dissolved in the glass structure. In particular for slow crack growth (< 3x10-6 m s-1), tests in dry nitrogen showed a considerable decrease in the crack growth exponent with increasing water content of the glasses. On the other hand, if tests were performed in humid air, the crack growth exponent was independent of the water content of the hydrous glasses, while stress intensity decreased slightly. These observations indicate that water promotes the processes at the crack-tip regardless of its origin. However, ambient water is more efficient.
To understand the influence of water and alkalis on aluminosilicate glasses, three polymerized glasses with varying ratios of Na/K were synthesized [(22. 5-x)Na2O-xK2O-22.5 Al2O3-55 SiO2 with x = 0, 7.5, and 11.25]. Subsequently, these glasses were hydrated (up to 8 wt% H2O) in an internally heated gas pressure vessel. The density of hydrous glasses linearly decreased with water content above 1 wt%, consistent with the partial molar volume of H2O of 12 cm3/mol. Near-infrared spectroscopy revealed that hydroxyl groups are the dominant species at water content of <4 wt%, and molecular water becomes dominating at water content of >5 wt%. The fraction of OH is particularly high in the pure Na-bearing glass compared to the mixed alkali glasses. 27Al magic angle spinning-NMR spectroscopy shows that aluminum is exclusively fourfold coordinated with some variations in the local geometry. It appears that the local structure around Al becomes more ordered with increasing K/Na ratio. The incorporation of H2O reinforces this effect. The differential thermal analysis of hydrous glasses shows a significant mass loss in the range of glass transition already during the first upscan, implying the high mobility of water in the glasses. This observation can be explained by the open structure of the aluminosilicate network and by the low dissociation enthalpy of H2O in the glasses (≈ 8 kJ/mol). The effect of the dissolved H2O on the glass transition temperature is less pronounced than for other aluminosilicate glasses, probably because of the large fraction of Al in the glasses.
Stable crack growth was measured for nominal dry and water-bearing (6 wt%) soda-lime silicate glasses in double cantilever beam geometry and combined with DMA studies on the effects of dissolved water on internal friction and glass transition, respectively. In vacuum, a decreased slope of logarithmic crack growth velocity versus stress intensity factor is evident for the hydrous glass in line with an increase of b-relaxation intensity indicating more energy Dissipation during fracture. Further, inert crack growth in hydrous glass is found to be divided into sections of different slope, which indicates different water related crack propagation mechanism. In ambient air, a largely extended region II is observed for the hydrous glass, which indicates that crack growth is more sensitive to ambient water.
In the present study we have investigated whether the effect of water on properties of borate glasses resembles that of alkali oxide. Soda-lime-borate glasses with nominal compositions of x Na2O, 10 CaO, (90-x) B2O3 (x = 5, 15 and 25 mol%) were doped with up to 8 wt.% H2O by processing glass powder + distilled water in platinum capsules in an internally heated gas pressure vessel at 1523 K and 500 MPa. The water content of hydrous glasses was determined by Karl-Fischer titration and near-infrared spectroscopy. The glass transition temperature T-g. was derived from DTA and micropenetration experiments for which the effect of water loss at the surface of the hydrous glasses was studied. Heating glass samples at 10 K min(-1) in the DTA resulted in T-g values which are close to T-12 isokom temperatures confirming the equivalence of enthalpy relaxation and viscous relaxation for borate glasses. For all three glass series it is shown that T-g strongly decreases whereas the liquid fragility strongly increases upon the addition of water. These findings reveal that H2O primarily causes breaking of B-O-B bonds rather than supporting 4-fold coordinated boron as it is well-known for alkali oxides in this concentration range. (C) 2015 Elsevier B.V. All rights reserved.
Hydrogen gas diffusivity of fourteen glasses of the Na2O-Al2O3-SiO2 system are studied along the joins quartzalbite-jadeite-nepheline (Qz-Ab-Jd-Np, fully polymerized) and albite-sodium disilicate (Ab-Ds, depolymerized).
Density measurements show that ionic porosity decreases from 54.4% (Qz) to 51.5% (Np) and from 52.4% (Ab) to 50.2% (Ds). Hydrogen diffusivity D follows similar trends but at another scale. D at 523 K decreases from 4×10−12 to 3×10−14m2 s−1 (Qz-Np) and from 4×10−13 to 3×10−15m2 s−1 (Ab-Ds). Charge compensating Na+ acting as a filling agent in fully polymerized network structures leads to up to one order of Magnitude higher diffusivities as depolymerized glass structures of the same SiO2 content where Na+ takes the role of a
modifier ion. Temperature dependence of the diffusivity indicates that both the activation energy involved with the moving H2 molecule as well as the accessible volume in the structure contribute to this compositional trend.
Due to the stochastic nature of crack nucleation by Vickers indentation, a statistical analysis of propagation rates of 185 radial cracks was performed. Crack growth was observed directly using a video camera with high Image acquisition rate. It is found that propagation rates are controlled by the environmental reactions at the crack-tip shortly after their initiation (< 1 s). Calibration of the stress intensity KI showed that the residual stress factor χ and the exponent n of the equation KI==χPc−n (with P==load and c==crack length) are broadly distributed among the 185 analyzed cracks, ranging from 10−16 to 104 and from 0.1 to 5, respectively. For the most frequent crack, the equation KI==0.052Pc−1.47 holds. The results show that correlations of indentation-induced crack length to stress intensity necessitate the use of statistical significant data that are calibrated by the environmental reactions at the crack-tip.
To understand the impact of dissolved water on structure and properties, four boron-rich glasses of molar compositions 15-x Na2O x CaO 15 SiO2 70 B2O3 (with x=0, 7.5, 10) and 10 Na2O 15 SiO2 75 B2O3 were prepared and subsequently hydrated (up to 8 wt% H2O). Density measurements show a non-linear trend upon hydration implying large structural changes in particular at water contents<2 wt%. Near-infrared spectroscopy shows hydroxyl groups are the dominant species in all glasses upon the entire range of water content. Molecular H2O is detectable only at total water contents>2 wt%. 11B MAS NMR spectra show that the abundance of BO4 species is mainly controlled by ratio of (Na2O+CaO)/B2O3 while incorporation of water plays a minor role.
Compared to borate glasses, the efficiency of formation of BO4 tetrahedra is favored by crosslinking of the network by SiO4-units. The glass transition temperatures, determined by differential thermal analysis, decreases continuously with water content due to breakage of B-O-B bonds by hydrolysis. However, compared to Silicates and aluminosilicates, the effect of dissolved water is less pronounced which can be explained by weaker B-O-B bonds in comparison to Si-O-Si bonds.
The effect of structural water on density, elastic constants and microhardness of water-bearing soda-lime-silica glasses of up to 21.5 mol% total water is studied. It is found that the Poisson ratio and the water content are positively correlated, while density and the elastic moduli decrease with increasing water content. Vickers hardness decreases by approximately 27% from the dry to the most hydrous glass. For water fractions <3 mol%, the dependencies are non-linear reflecting the non-linear change in the concentrations of OH and H2O molecules dissolved, whereas for water fractions >3 mol% linear dependencies are found. To distinguish the effect of structural water and environmental water, indentations were performed in toluene, nitrogen gas and air. Timedependent softening was evident for testing dry glasses in humid atmospheres as well as for tests of hydrous glasses in dry atmospheres. This indicates that the response times of dissolved water species are effectively equal in both scenarios.
The sintering of bioactive glasses allows for the preparation of complex structures, such as three‐dimensional porous scaffolds. Such 3D constructs are particularly interesting for clinical applications of bioactive glasses in bone regeneration, as the scaffolds can act as a guide for in‐growing bone cells, allowing for good Integration with existing and newly formed tissue while the scaffold slowly degrades. Owing to the pronounced tendency of many bioactive glasses to crystallize upon heat treatment, 3D scaffolds have not been much exploited commercially. Here, we investigate the influence of crystallization on the sintering behavior of several bioactive glasses. In a series of mixed‐alkali glasses an increased CaO/alkali metal oxide Ratio improved sintering compared to Bioglass 45S5, where dense sintering was inhibited.
Addition of small amounts of calcium fluoride helped to keep melting and sintering temperatures low. Unlike glass 13‐93, these new glasses crystallized during sintering but this did not prevent densification. Variation in bioactive glass particle size allowed for fine‐tuning the microporosity resulting from the sintering process.
Physical storage of gaseous hydrogen under high-pressure in glassy micro-containers such as spheres and capillaries is a promising concept for enhancing safety and the volumetric capacity of mobile hydrogen storage systems. As very low permeation through the container wall is required for storage of compressed hydrogen, development of glasses of minimal hydrogen permeability is needed. For this purpose, one has to understand better the dependence of hydrogen permeability on glass structure. The paper points out that minimizing the accessible free volume is as one strategy to minimize hydrogen permeability. Based on previously measured and comprehensive literature data, it is shown that permeation is independently controlled by ionic porosity and network modifier content. Thus, ionic porosity in modified and fully polymerized networks can be decreased equally to the lowest hydrogen permeability among the glasses under study. Applying this concept, a drop of up to 30,000 with respect to the permeation of hydrogen molecules through silica glass is attainable.
A double cantilever beam technique in air equipped with ultrasound modulation was used to measure the crack velocity v in borate and silicate glasses. In all glasses v and the stress intensity KI followed the empirical correlation v ~ KIn. Indicated by its smallest KI at v = 1 μm s − 1, KI* = 0.27 MPa m0.5, the silicoborate glass containing 70 mol% B2O3 was found most susceptible to stress-corrosion enhanced crack growth. Contrarily, the sodium calcium magnesium silicate glass appeared least susceptible with KI* = 0.57 MPa m0.5. No clear correlation is evident between KI*, reflecting the stress-corrosion susceptibility, and the hydrolytic resistance for all glasses under study, but values of n obtained from the present study and taken from previous literature for 35 glasses tend to decrease with increasing network modifier ion fraction. Energy dissipation during stress-corrosion enhanced crack propagation is assumed to cause this trend.
The crystallization behavior of sodium ion conductive Na2MnP2O7 glass was examined to clarify the crystallization mechanism. The formation of thermodynamically metastable phase, layered Na2MnP2O7, at the surface of the glass occurred. Heat treatment at 430 °C for 3 h lead to surface crystals of Na2MnP2O7 oriented with the (101) direction perpendicular to the sample surface. As the heat treatment temperature increased, the glass-ceramic samples deformed, and the presence numerous micro bubbles due to dissolved water was detected.
Glass-ceramics are noted for their unusual combination of properties and manifold commercialized products for consumer and specialized markets. Evolution of novel glass and ceramic processing routes, a plethora of new compositions, and unique exotic nano- and microstructures over the past 60 years led us to review the Definition of glass-ceramics. Well-established and emerging processing methods, such as co-firing, additive manufacturing, and laser patterning are analyzed concerning the core requirements of processing glass-ceramics and the Performance of the final products. In this communication, we propose a revised, updated definition of glass-ceramics, which reads “Glass-ceramics are inorganic, non-metallic materials prepared by controlled crystallization of glasses via different processing methods. They contain at least one type of functional crystalline phase and a residual glass. The volume fraction crystallized may vary from ppm to almost 100%”.
Glasses with compositions of 21Gd2O3-63MoO3-(16-x)B2O3-xTeO2 (mol%) (x= 0, 2, 4, 8) were prepared using a conventional melt quenching technique, and the crystallization behavior of ferroelastic β′-Gd2 MoO4)3 Crystals was examined to clarify the mechanism of self-powdering phenomenon and to design bulk crystallized glasses. It was found that the self-powdering phenomenon appeared significantly during the crystallization at temperatures near the crystallization peak temperature, but the phenomenon is suppressed in the crystallization at temperatures much higher than the glass transition temperature. It was also found that the substitution of TeO2 for B2O3 in the base glasses suppresses the self-powdering phenomenon and consequently bulk crystallized glasses were obtained in the glass with x=8 mol%. The densities at room temperature of the base glasses are d =4.755–4.906 g/cm3, being much higher than the value of d=4.555 g/cm3 for β′-Gd2(MoO4)3 crystal. It is proposed that the stresses in the inside of crystals induced by large density differences (i.e., large molar volume differences) between the glassy phase and crystals might be relaxed effectively in the glasses containing TeO2 with weak TeeO bonds and fragile character.
In this review article, the impact of dissolved water on the viscous properties of soda lime silicate melts is addressed against the background of the upcoming switch from natural gas to hydrogen combustion. This change will lead to an increase in the total water content of the glasses by up to 0.4 mol%. In order to better define possible influences of water speciation, water-rich glasses were synthesised under increasing pressure up to the kbar range. It is shown that a distinction must be made between the influence of dissolved OH-groups and H2Omolecules in order to accurately reflect the dependence of isokom temperatures on water content. In addition, an increase of one order of magnitude in the tolerance to higher deformation rates was observed for the range of expected increased water contents during isothermal deformation processes, which is based on the timetemperature superposition principle, i.e. congruent flow curves were determined under isokomal conditions.
The permeation of hydrogen gas was studied in meta-aluminous (tectosilicate) glass powders of Li2O×Al2O3×SiO2 (LAS), Na2O×Al2O3×SiO2 (NAS) and MgO×Al2O3×SiO2 (MAS) systems by pressure loading and vacuum extraction in the temperatures range 210–310 °C. With this method, both the solubility S and the diffusivity D were determined, while the permeability was given by the product SD. For all glasses, S was found to decrease with temperature, while D increased. Since the activation energy of diffusion of H2 molecules exceeded that of dissolution, permeation increased slightly with temperature. When extrapolated to standard conditions (25 °C), the permeability of tectosilicate glasses was found to be only 10-22–10-24 mol H2 (m s Pa)-1, which is 8–10 magnitudes lower than most polymers. Thin glass liners of these compositions are expected to be the most effective barrier for tanks of pressurised hydrogen.
AbstractViscous healing of cracks induced by the Vickers indentation in a soda lime magnesium silicate, a soda borosilicate, and a soda aluminosilicate glass (NAS) was studied by laser scanning microscopy. Plots of the crack length, width, and depth normalized to the initial crack length versus time over viscosity merge into single master curves of each of these quantities for each glass. Despite glass properties do not differ strikingly from each other, however, these master curves strongly differ among the glasses. This finding was attributed to a different interplay of various crack healing phenomena. Lateral cracks were found to be responsible for the bulging of the sample surface around the Vickers imprint, which in turn promotes radial crack widening as the main cause of healing delay. The most rapid healing of lateral cracks was observed in NAS in which bulging and crack widening were least pronounced.
Silicate glass fracture surface energy calculated from crystal structure and bond-energy data
(2023)
We present a novel method to predict the fracture surface energy, γ, of isochemically crystallizing silicate glasses using readily available crystallographic structure data of their crystalline counterpart and tabled diatomic chemical bond energies, D0. The method assumes that γ equals the fracture surface energy of the most likely cleavage plane of the crystal. Calculated values were in excellent agreement with those calculated from glass density, network connectivity and D0 data in earlier work. This finding demonstrates a remarkable equivalence between crystal cleavage planes and glass fracture surfaces.
Controlled oriented crystallization of glass surfaces is desired for high precision applications, since the uppermost crystal layer significantly influences the properties of the material. In contrast to previous studies, the data presented here deal with separated crystals growing at defect-free surfaces in four atmospheres with different degrees of humidity (ambient/dry air, argon and vacuum). A glass with the composition 2 BaO–TiO2–2.75 SiO2 was heat-treated at 825 °C until fresnoite (Ba2TiSi2O8) grew to a significant size. The crystal growth rate is found to increase with increasing humidity. The morphology of the crystals changes from highly distorted dendrites in the driest atmosphere (vacuum) to circular/spear-head-shaped crystals in the wettest atmosphere (ambient air), which we attribute to a decrease in viscosity of the glass surface due to water uptake. The least distorted crystals appear in the form of depressions of up to 6 µm. This has an influence on the observed crystal orientation, as measured by electron backscatter diffraction (EBSD). The pulled-in crystals change the orientation during growth relative to the flat glass surface due to an enrichment in SiO2 at the crystal fronts. This confirms that the orientation of crystals is not fixed following nucleation.
The process of viscous flow sintering is a phenomenon that is closely linked to the surface properties of the glass particles. In this work, we studied the extreme case of acid-leaching of soda-lime-silicate glass beads of two different particle size distributions and its effects on non-isothermal viscous sintering of powder compacts. Depth profiling of the chemical composition after leaching revealed a near-surface layer depleted in alkali and alkaline earth ions, associated with concurrent hydration as mass loss was detected by thermogravimetry. Heating microscopy showed that acid treatment of glasses shifted the sinter curves to higher temperatures with increasing leaching time. Modelling of the shrinkage with the cluster model predicted a higher viscosity of the altered surface layer, while analysis of the time scales of mass transport of mobile species (Na+, Ca2+ and H2O) during isochronous sintering revealed that diffusion of Na+ can compensate for concentration gradients before sintering begins. Also, exchanged water species can diffuse out of the altered layer, but the depletion of Ca2+ in the altered surface layer persists during the sinter interval, resulting in a glass with higher viscosity, which causes sintering to slow down.
This study investigates the sintering and crystallization behavior and kinetic of the bioactive glass (BG) 13–93 with nominal composition (in mol%): 54.6 SiO2 - 1.7 P2O3 - 22.1 CaO - 6.0 Na2O - 7.9 K2O - 7.7 MgO. Sintering and crystallization were investigated non-isothermally for various particle size fractions smaller than 315 μm as well as for bulk samples. Densification was not hindered by the presence of crystalline phases across all particle size fractions. Afterwards, wollastonite was found as the dominant crystal phase at higher temperature which resorb primary surface precipitation-like quartz crystallites. The growth direction shifts into volume when the sample surface is nearly covered. The crystal growth rate of wollastonite was calculated from the crystalline surface layer thickness measured during heating. The findings of this study are relevant for the high temperature processing of BG 13–93.