Crystallizing silicate glass seals

Non-crystallized systems

Non-crystallized sealing glasses are commercially available for many purposes usually as powdered glass.^11–13 The glass powder is given between the materials to be joined. During subsequent thermal treatment, the glass is sintered and the materials to be joined are wetted by the molten glass. ¹⁴ After cooling and during application, the seal is still glassy and is not crystallized and hence does not change its physical properties. Non-crystallizing sealing glasses are available for a wide range of CTE required for specific applications.^13,14 Besides the CTE, the required physical properties are sufficient corrosion resistance under the application conditions and sufficiently good mechanical properties for the respective purpose. For applications in electrical engineering also the electric properties, such as specific electric resistivity, dielectric constant and loss angle are of importance.

Glass seals which are not crystallized, do not reach the mechanical stability of crystallizing glass seals. If the application temperature is somewhat above T_g of the sealing glass, viscous flow may take place which affects the geometry of the composite. Otherwise, unintended crystallization may occur which changes the CTE of the seal and other physical properties in a not acceptable extend. For applications at room temperature, non-crystallizing glass seals are nevertheless the first choice, if the materials to be joined do not possess CTEs < 3·10⁻⁶ K⁻¹.

Crystallized systems

In contrast to non-crystallized glass seals, crystallized systems are especially designed for applications at high temperatures. After sintering of the glasses and wetting of the materials to be joined, during subsequent thermal treatment, the glass is crystallized. This changes the physical properties in a large extend. Then the CTE of the seal depends predominantly on the CTEs of the crystallized phases and their volume concentration, but is also affected by a residual glass phase.

In the scientific literature, the most frequently reported application of crystallizing glass seals is dedicated to Solid Oxide Fuel Cells, SOFC (see, e.g.,^15,16). SOFCs are considered as key components for hydrogen technology.^17–21 Nowadays, most SOFCs have a planar design and a seal between an ion conductive ceramic component, mostly yttrium stabilised ZrO₂ as electrolyte and a metallic component (steel or Ni-alloy) as electron conductor are required.^{17,18,20–22} The seal should be gastight and electrically insulating and stable also with respect to O₂ and H₂ at the respective application temperature which lies in between 800 to 950 °C. Furthermore, the composite must withstand thermal cycling from room temperature to the application temperature.

The materials to be joined, stabilised zirconia and the metallic component have comparatively high CTEs. The most frequently applied metal is Crofer 22 APU™ with a CTE of 11.5·10⁻⁶ K⁻¹¹⁰ while tetragonal yttrium stabilised zirconia, YSZ, (space group: P4₂/nmc (137)) has a CTE of 10.4 and 14.8·10⁻⁶ K⁻¹ for the crystallographic a- and c-axis respectively which is equivalent to a mean CTE of 11.9·10⁻⁶ K⁻¹. ²³ According to Ref., ²⁴ the cubic yttrium stabilised zirconia (space group Fm 3 ¯ m (225)) has a somewhat lower CTE of 10.9·10⁻⁶ K⁻¹. Tetragonal ZrO₂ ceramics have high strength and toughness at room temperature, which is due to transformation toughening, i.e., the transformation of the tetragonal to the monoclinic phase if tensile stresses occur. This phase transition runs parallel to a volume expansion which leads to compressive stresses at the tip of the crack. This effect, however, diminishes at high application temperatures and can hence not be utilised, but might prevent cracking during cooling. For application in SOFCs, YSZ is the most frequently used electrolyte (oxygen ion conductor). A favourable CTE for the crystallized glass seal should hence lie at around 10.8 to 12·10⁻⁶ K⁻¹. There are numerous glass compositions reported in the literature to be suitable for crystallizing glass seals in SOFCs.^{15,16,25–30} Similar requirements have to be fulfilled for water electrolysers, which are of equal or even higher technological importance than SOFCs.^31,32 Many reported compositions are based on alkaline earth silicates. There is much literature in this field and also recent reviews.^4,33,34 Applications of glass seals for SOFCs and electrolysers will hence not be further detailed.

Another important field of application are high temperature reactors, which are operating at even higher temperatures than SOFCs, i.e., at 1000 °C and even above. This requires the use of nickel alloys such as NiCrofer™, which possesses higher CTEs than Crofer™ steels. ²⁸ An example is a high temperature reactor composed of a NiCrofer™ alloy and hollow fibres of perovskites which show O²⁻ ion conductivity as well as electron conductivity. At high temperatures, air flows through the fibres under high pressure and oxygen permeates as O²⁻ ions through the walls of the hollow fibre and at the same time electrons permeate in the opposite direction, i.e., the hollow fibres are permeable for oxygen, but not for nitrogen, argon and other gases. This technology might replace the Linde process for air separation in the future ³⁵ and might also enable economically more advantageous oxyfuel technologies as well as carbon recovery.

A standard application for crystallizing glass seals is joining Al₂O₃ ceramics to other Al₂O₃ or single crystalline sapphire components, also for applications at high temperatures. Corundum has a space group of R 3 ¯ c (167) and CTEs of 7.3 and 8.3·10⁻⁶ K⁻¹ perpendicular and parallel to the crystallographic c-axis, respectively. ³⁶ This results in a mean CTE of 7.6·10⁻⁶ K⁻¹. ³⁶ A crystallizing glass seal with an appropriate CTE can be achieved by various chemical compositions. A typical device prepared by sealing is for example an alumina tube sealed to a plane and optically transparent sapphire plate which can, e.g., be used for optical measurements in high temperature furnaces. Joining of corundum to the ion-conductor β-alumina is required for sodium sulphur batteries which operate at temperatures in the range from 250 to 350 °C. ³⁷ Other materials frequently joined by crystallizing glass seals are Si₃N₄ and SiC which have high application temperatures and require lower CTEs.

There are numerous applications where metals are joined to ceramics or other metals of high CTEs, many of them at room temperature for electronic applications. Since numerous metals, such as copper (16.5·10⁻⁶K⁻¹), iron (11.8·10⁻⁶K⁻¹), molybdenum (4.8·10⁻⁶K⁻¹), nickel (13.4·10⁻⁶K⁻¹) ³⁸ and various steels are of importance, which have quite different CTEs, besides some standard crystallizing glass seals, also various seals with compositions tailored to the respective metal and the respective application are required.

Other applications of crystallized glasses

Other applications of crystallized glasses are first of all glass-ceramics. The most frequently industrially produced glass-ceramics possess a CTE close to zero, i.e., they do not expand during heating. The main applications of such glass-ceramics are high temperature (furnace) windows, cook top panels and telescope mirrors and^39–42 as well as numerous devices for applications in optics and photonics. The CTE close to zero is required for telescope mirrors blanks and photonic applications to minimise shape changes with temperature and for cook top panels and furnace windows to achieve a high resistance against thermal shock.

Another important application is that of dental materials. Here, besides mechanical properties and the CTE, also optical properties such as a certain translucency are required for aesthetic reasons. There are very different materials available for various purposes, such as veneers, inlays, dental crowns or bridges. ⁴³

Glass-ceramics may also have interesting mechanical properties, such as high hardness (up to 14 GPa ⁴⁴ ), high Young's modulus (>140 GPa ⁴⁵ ), high mechanical strength (>1 GPa^46,47) and high toughness (>4 MPa·m^1/2). ⁴⁸ Applications are, e.g., found as armour materials.

Another field of interest are machinable glass-ceramics. They contain, e.g., mica crystals, which have cleavage planes and enable comparatively rapid machining by conventional hard metal tools. Such materials find, e.g., applications in rapid prototyping. ⁴⁹

Additional applications for glass-ceramics are found in a wide range of electronic devices, such as piezoelectric^50,51 or dielectric^52,53 components. Here, especially surface crystallized polar glass-ceramics based on fresnoite are to be mentioned. ⁵³

Glass crystallization is also of importance, if crystalline ceramic powders are sintered using a glass phase as sintering aid. Then, it might be favourable to crystallize the glass phase after sintering is complete. This has two advantages: the first is that the mechanical properties should improve because mechanical strength and toughness of the crystalline phase should be better than of an amorphous phase. The second is that the high temperature performance should be better, because softening of a glassy phase does not occur.

Stresses between the layers

The stresses in the seal and the joined materials are formed during cooling and are due to the CTEs as well as to possible phase transitions. With respect to the CTEs, the mismatch of the mean CTEs of the seal and the respective joined materials leads to an overall stress between these materials. For a simple geometry (planar layers), stresses formed at the boundaries between two materials with different CTEs, can be calculated using equation (1). ⁵⁴

σ = ( C T E A − C T E B ) × E A × E B × ( T m i n − 25 ∘ C ) E A + E B

(1)

With σ: formed stress, CTE_A, CTE_B and E_A, E_B: coefficients of thermal expansion and Young's moduli of the materials with the indices “A” and “B”, respectively. T_min is the minimum temperature, at which mechanical stress relaxation by viscous flow of the glass phase is possible. For a comparatively high concentration of residual glass phase, T_min is equal to the glass transition temperature, T_g. If the CTE of material A is larger than that of material B, σ is, according to equation (1), positive, which means that the stresses in material A are tensile, while they are compressive in material B.

In a further approximation, T_g depends on the cooling rate according to Bartenew Equation ⁵⁵ and decreases with decreasing cooling rate:

q = A e x p ( E q R T g )

(2)

With q = cooling rate, E_q = activation energy of viscous flow, R = Avogadro's constant. A is a constant depending on the chemical composition.

The stresses between two materials with different CTEs are high, if T_g of the glass is high, the Young's moduli are high and the difference in the CTEs of both materials is large. For more complex geometries, finite element calculations, FEM, are necessary. Figure 1 shows a schematic of a three-layered material composed of a material B with the smallest CTE and a material A with the highest CTE, while the CTE of the glass seal lies in between. In the material A, tensile stresses and in the material B only compressive stresses occur. In the seal, in contact to material A, the stresses are compressive while they are tensile in contact with material B.

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Figure 1.

Stresses formed in layers composed of a material A, a glass seal and a material B for the case, the CTE of material B is smallest and that of material A largest, while that of the seal lies in between.

Stresses within the crystallized seal

In addition to the stresses between the layers, internal stresses in the crystallized seal occur, which are attributed to the mismatch in the CTEs of different crystalline phases with each other and also with the residual glass phase. In the case of crystalline phases which do not possess a cubic space group, the CTEs are different in different crystallographic directions which also notably affects the internal stresses. These internal stresses strongly depend on the respective microstructure and can only be calculated for simple geometries, such as low concentrations of spherical isotropic crystals in an amorphous matrix. The stresses around a spherical and a needle-shaped inclusion, which are stress-free at high temperature and which are afterwards cooled down to room temperature are shown in Figure 2(a). The stresses in a spherical inclusion are isostatic and if the CTE of the inclusion is larger than that of the matrix, are tensile. In the matrix, the stresses in axial direction are also tensile, while they are compressive in radial direction. ⁵⁴

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Figure 2.

Stresses around inclusions a: stresses around a spherical inclusion and a needle-shaped inclusion for the case, the CTE of the matrix is smaller than that of the inclusion. b: stresses around a needle-shaped tetragonal inclusion which CTE in the direction of the crystallographic c-axis is larger than the CTE of the a-axis, while the CTE of the matrix lies in between.

For a spherical inclusion, stresses can be calculated from the CTEs, the Young's moduli and the Poisson ratios according to Selsing. ⁵⁴ It should be noted that this equation is only valid for inclusions in an infinitely large matrix and for a realistic microstructure can only be a first approximation.

P = Δ C T E × ( T g − 25 ∘ C ) ( 1 + μ m 2 E m + 1 − 2 μ i E i )

(3)

With: P = stress inside the inclusion, E_m and E_i = Young's moduli, and μ_m and μ_i: Poisson's ratios. Index m = matrix and i = inclusion. ΔCTE = CTE_m − CTE_i. The equation is valid for room temperature (25 °C) and T_g is in °C and for small concentrations of the crystalline phase. For higher concentrations (f = volume concentration of the inclusion), another equation was proposed by Hsueh and Becher ⁵⁶ (see also ⁵⁷ ):

P = Δ C T E × ( T g − 25 ∘ C ) ( 1 3 K i + 1 4 ( 1 − f ) G m + f 3 ( 1 − f ) K m )

(4)

With: K and G are the compressive modulus and the shear modulus, respectively. The indices, “m” and “i” are again for the matrix and the inclusion, respectively.

The stresses in the inclusion do not depend on its size. In the matrix, tangential stresses (σ_t) and the radial stresses (σ_r) decrease with R³/d³ (with: d = distance from the centre of the particle and R = radius of the inclusion) as described by equation (5) ⁵⁴ :

σ r = − 2 σ t = − P R 3 d 3

(5)

Please note that the signs for the radial and tangential stresses always oppose each other.

In a needle shaped tetragonal inclusion with a CTE larger than that of the matrix, the stresses inside the inclusion are tensile (see Figure 2(a)). In the direction pointing away from the crystal, the stresses are also tensile, while they are compressive parallel to the surface of the crystal. The stresses inside the crystal are not isostatic, but depend on the locus, even if the CTEs in different crystallographic directions are the same. That is generally valid for non-spherical inclusions.

A somewhat more complicated stress field occurs in a needle shaped crystal with anisotropic thermal expansion, e.g., which CTE is larger in the direction of the crystallographic c-axis than in the direction of the a-axis, while the CTE of the glass matrix lies in between. Inside the crystal (see Figure 2(b)), parallel to the c-axis, tensile stresses occur, while the stresses are compressive perpendicular to the c-axis. In the matrix compressive stresses occur along the c-axis as well as in radial direction. Parallel to the (001)-plane, tensile stresses occur and also perpendicular to this direction.

An even greater effect on the formation of stresses may have transitions from one crystalline phase to another one below T_g, which are accompanied by a volume effect. Such crystalline phases are, e.g., β-quartz, which transforms to α-quartz^58,59 or the β-cristobalite/α-cristobalite^59,60 transition both occurring during heating and cooling. It should be noted that this volume effect is not isotropic, i.e., the relative changes in the axes of the unit cell are not the same and hence also in the case of phase transitions, around and in the crystals tensile and compressive stresses are formed.

FEM enables the calculation of stresses in more complex microstructures. Elaborated microstructures, such as high concentrations of different crystalline anisotropic phases with statistic orientations in a comparatively low concentration of glass phase up to now cannot be calculated properly. It should be noted that during crystallization also the composition of the glass melt changes and hence diffusion gradients are formed which leads to a local dependence on the CTE of the glass.

Another effect, which leads to local stresses and anisotropic CTEs is the orientation of crystals as a result of nucleation and crystallization. Nucleation of the sintered glass may happen in a different manner as in the case of an as cast bulk glass. The first is homogeneous nucleation in the glass, while the second is nucleation in residual pores caused by incomplete sintering or by nucleation during earlier states of sintering. In these two possibilities, the arrangement of the crystals with respect to their orientation should solely be statistic. If, however, nucleation takes place at the interface of the material to be joined and the sintered glass, the formed nuclei have already a preferred orientation in many cases. ⁶¹ Also in the case, the nuclei are not oriented, during the growth of crystals with non-cubic space groups and different crystal growth velocities in different crystallographic directions, the crystals are in many cases oriented in some distance (few μm) from the locus of nucleation. ⁶¹ Since these non-cubic crystals usually exhibit different CTEs in different crystallographic directions, this orientation may have a serious effect on the CTE. Hence, the CTE of a crystallized glass seal parallel to the materials to be joined is not necessarily the same as in a material the crystals are statistically oriented.

Effect of time on stresses during application

In principle, two types of applications of crystallizing glass seals can be distinguished: In the first type, the application temperature is the room temperature (±60 K). At room temperature, the crystals cannot grow and only in very rare cases can show phase transitions. An example for the latter case is the stress induced transition of tetragonal ZrO₂ to monoclinic ZrO₂. Hence, usually at room temperature, the phase composition does not change with time and hence also not the CTE. Thus, also the stresses are constant with time.

The second type are high temperature applications. Here, type and volume concentration of crystalline phase(s) as well as the microstructure may change during the application. Besides, chemical reactions between the seal and the joined materials may occur. In such applications, in many cases, thermal cycling occurs, i.e., the composite is heated up and cooled again many times.

At high temperatures, the type, quantity, and shape of the crystalline phases may be affected, i.e., may change as a function of time. For example, the crystallization of the glass phase may proceed. Here two cases can be distinguished: the first is the crystallization in an isochemical system. The crystals grow and the composition of the residual glass phase remains constant (see Figure 3(a)). During the course of the crystallization, the CTE approaches the CTE of the crystalline phase. In non-isochemical systems, diffusion gradients form around the crystals (Figure 3(c)), further phases might crystallize (Figure 3(b)) or Ostwald ripening (Figure 3(d)) takes place. Another possibility is that the shape of the crystals changes and approaches the thermodynamically most advantageous morphology (Wulff shape) ⁶² attributed to a minimum of the interfacial energy (see Figure 3(e)). For example, initially a crystal may grow in needle-like morphology for kinetic reasons, e.g., along a screw axis. After some time, the crystal, however, may minimise its interfacial energy and the length will decrease again, while the crystal gets thicker. ⁶³ Such behaviour was denoted as “crystallization pendulum” (Figure 3(e)).

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Figure 3.

Possible effects during high temperature treatment: a: crystal growth in isochemical systems of spherical particles and of needle like particles (possibly along screw dislocations). b: epitaxial growth of a second crystalline phase, c: formation of diffusion gradients around the crystals in non-isochemical systems, d: Ostwald ripening to minimise the interfacial energy, e: Decrease in aspect ratio to minimise the interfacial energy.

Furthermore, also the internal stresses caused by mismatches in the CTEs combined with changes in the phase composition (including phase transitions during cooling), the microstructure (e.g., also the aspect ratio of the crystals) and the composition of the glass phase are affected.

Sintering of glass without crystallization

Sintering of glass is achieved by viscous flow. A requirement is hence, that at the temperature, the glass is sintered, crystallization is negligible and does not notably hinder the densification process, i.e., the volume concentration of crystals is still low. Since the glass used for this purpose is usually made by melting and subsequently powdered by milling, the particles are polydisperse and have a certain particle size distribution. Furthermore, particles prepared by milling are not spherical. The shape, size and size distribution of the particles are decisive for the densification process. With decreasing particle size, the driving force for sintering (minimisation of surface energy) increases.

Figure 4 shows a schematic for densification of two amorphous spheres by viscous flow in an initial state of sintering. The spheres have a radius R, which is somewhat larger than the initial radius R₀. The radius R is much larger than the curvature r. ⁶⁴ Hence, the latter leads to a transport of material to the neck by viscous flow, according to Frenkel's theory, ⁶⁵ later further developed by G. Kuczynski^66,67 and J. Mackenzie and R. Shuttleworth ⁶⁸ as well as by I. B. Cutler.^69,70

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Figure 4.

Two amorphous spheres with the radius R in an initial stage of sintering by viscous flow. Redrawn after Ref. ⁶⁴

According to, ⁶⁴ the rate of initial neck growth is given by equation (6):

x R = ( 3 γ 2 η ( T ) r ) 1 2 t 1 2

(6)

With γ = surface tension, η(T) = temperature dependent viscosity, t = time, x, r and R: see Figure 4.

The (macroscopic) volume shrinkage ΔV of particles with the radius R is given by equation (7):

Δ V V 0 = 3 Δ L L 0 = 9 γ 4 η ( T ) R t

(7)

With ΔL = linear shrinkage. A full coalescence of the glass spheres with the initial radius R₀ is reached after a time τ:

τ ≈ h γ R 0

(8)

With x ≈ R₀ and h = R(1-cosφ), h and φ: see Figure 4. If an initially dense package of glass spheres is assumed, a large specimen with closed pores forms with the densification rate according to equation (9):

γ n 1 3 ( t − t 0 ) η = 2 3 ( 3 4 π ) 1 3 ∫ 0 ρ d ρ ( 1 − ρ ′ ) 2 3 ρ ′ 1 3

(9)

With n = number of pores per volume unit, ρ’ = relative density and t₀ is the time the experiment was started. The term γn^1/3(t-t₀)/η is denoted as reduced time. The relative density as a function of the reduced time is shown in Figure 5.

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Figure 5.

Reduced density of a compact composed by glass spheres of unit size plotted against the reduced time for a constant temperature. Redrawn after. ⁶⁴

The relative density at the beginning is around 0.55 (the typical value, a powder can be densified by pressing) and during thermal treatment at a constant temperature approaches with time a value of 1, which is equivalent to a full densification. It should be noted that in a good approximation, the surface energy does not depend on temperature, while the viscosity decreases with increasing temperature according to Vogel-Fulcher-Tammann equation, η = A·exp(B/(T−T₀)) with A, B and T₀: constants. Hence, the time required for densification is proportional to the viscosity at the respective temperature during isothermal treatment. The curve displayed in Figure 5 is a master curve which in principle applies for all viscosities, i.e., all temperatures.

Sintering of glass with concurrent crystallization

The curve shown in Figure 5 does not take into account a superimposed crystallization process, which depends on the respective supplied temperature/time schedule. A more detailed description of the densification process is given in the following papers, where theoretical work taking into account the concurrent crystallization process was published by M. O. Prado and E. D. Zanotto^71–73 as well as by T. E. Clark and S. S. Reed. ⁷⁴

In principle, concurrent crystallization hinders the densification process of the glass. Nucleation as a first step of the crystallization process might take place in the bulk or, however, at the surface of the glass particles. The maximum nucleation rate is usually observed at temperatures around T_g, whereas the maximum crystal growth velocity is observed at much higher temperature.^75,76 As long as the nuclei do not grow much, they do not affect the densification process drastically. Especially, if simultaneously surface crystallization at the particles’ surface takes place, the volume concentration of crystals increases and the densification process gets more difficult. That means, in contrast to solid state sintering of crystalline powders, sintering of crystallizing glass seals is not always facilitated by smaller particle sizes.

Theoretical descriptions of glass sintering with concurrent crystallization suffer from the fact, that the glass powder used in experiments is not composed of spherical particles which all have the same size and hence are only a first approximation.

For an isothermal sintering process, the time until the maximum sintering rate, t_s, is reached is given by equation (10) ⁷¹ :

t s = 1 − ρ 0 ρ ′ 3 8 η r 3 γ ≈ 1.7 η r γ

(10)

With: ρ₀ = relative initial green density, r = initial radius of the glass particle. The particles are assumed to possess all a spherical shape and the same size. The factor 1.7 in equation (10) results for ρ₀ = 0.6 (a common value for the relative green density) and ρ’ = 0.8 (an estimate for the relative density at the maximum sintering rate).

For an isothermal sintering process with concurrent crystallization, the time until the maximum crystallization rate at the temperature T is reached, t_c, is given by equation (11). ⁷¹

t c = 1 2 π N s U ( T )

(11)

With: N_s = density of the nucleation sites at the surface of the glass particles, U(T) = crystal growth velocity at the temperature T.

The time until the maximum sintering rate is reached must be shorter than the time, the maximum crystallization rate is reached. For this purpose, in Ref., ⁷¹ a sinterability parameter S was defined:

S = γ N s U ( T ) η r

(12)

It was empirically shown in Ref., ⁷¹ that the parameter S should be >50 to achieve a relative density >0.99 for glass forming systems, which should be sufficiently high for most sealing purposes.

Equation (12) shows that the densification of crystallizing glasses can be facilitated by (i): smaller particles (r) and lower number of nucleation sites (N_s) at the surface of the glass particles. The latter can be decreased by reducing the time, the sample is held at temperatures around T_g, where the nucleation rate is maximum, i.e., by a larger heating rate in this temperature range. U(T) and η(T) solely depend on the glass composition and can only be affected by the temperature.

Furthermore, numerous experimental papers on concurrent sintering and crystallization have been published (see, e.g.,^77,78).

Another general problem during sintering of glass with concurrent crystallization is foaming, if closed porosity is reached.^79–81 The reason is the formation of volatiles, such as water during crystallization. Nearly all crystals formed during glass crystallization do not incorporate water and hence water (as OH groups) is enriched in the residual glass melt. If now the concentration of water exceeds the solubility, bubbles are formed. This problem can be overcome by the preparation of “dry” glasses, e.g., by bubbling the melt with dry argon and/or by the incorporation of small fluoride concentrations. Fluoride has a similar ionic radius as OH-groups and occupies similar sites in the glass structure. Hence, fluoride “pushes” the water out of the glass structure and hence helps to minimise the water concentration. Both techniques are well-known from the preparation of rare earth doped glasses for fluorescence applications. ⁸² In the case of alkaline earth containing glasses, especially those, which contain BaO, foaming might also be a result of the release of carbonaceous species that appear at the surface of the respective glass particles.^83–85

Optimisation of the sealing technique and the supplied temperature/time schedule might minimise undesirable effects of concurrent crystallization.

The effect of the interface of materials to be sealed and crystallizing glass are discussed in Section 8.

Crystallizing glass seals with CTEs <5·10⁻⁶ K⁻¹

Crystalline phases which may act as main components of a crystallized glass seal and possess CTEs close to zero or even negative are mainly β-quartz, β-cristobalite, keatite, β-spodumene, β-eucryptite and keatite. Most of these compositions are based on lithium alumosilicates. Li⁺ can partially be replaced by Na⁺ or divalent cations, such as Zn²⁺ or Mg²⁺. Besides these compositions, also BaAl₂B₂O₇, and solid solutions based on Ba_1−xSr_xZn₂Si₂O₇ are suitable phases to achieve low CTEs.

Such crystallizing glass seals are required if, e.g., pure SiO₂ glass or glass-ceramics with CTEs close to zero are to be sealed. It should be noted that for this purpose, non-crystallizing glass seals are not suitable, because the only glass compositions with such low CTE are pure SiO₂ or SiO₂/TiO₂ glasses^173,174 which, however, possess too high softening temperatures (for SiO₂ glass: 1740 °C ¹⁷⁵ ). For the sealing of lithium alumosilicate based glass-ceramics, also glasses from the same system with somewhat lower softening point are suitable. The lower softening point can be achieved by an increase of the alkali oxides concentration and/or the addition of few mol% B₂O₃. Careful optimisation of the crystallization conditions of the seal is necessary to tailor the CTE. For applications at high temperatures, such as furnace windows, the crystallization process of the seal might proceed and the CTE may change.

Materials with CTEs between 2 and 5·10⁻⁶ K⁻¹ are for example silicon nitride (CTE: 3–4·10⁻⁶ K^−1165,176) and silicon carbide (CTE: 4–5.5·10⁻⁶ K^−1176,177). Some studied crystallizing sealing glasses are based on rare earth silicates, which are also used as oxidation protecting coatings for silicon carbide and silicon nitride.^165,176,177 Other glass seals contain crystalline CaAl₂Si₂O₆, MgSiO₃, or Mg₂Al₄Si₅O₁₈, which possess CTEs of 4.4, 7.4 and 1.8·10⁻⁶ K⁻¹ (see Table 1).

Some particular chemical compositions of crystallizing glasses and the formed crystalline phases as well as the attributed CTEs are summarised in Table 5. Most of the glasses listed were not originally developed for use as glass seals. Accordingly, they are only suitable for use as solder to a limited extent. In the LAS system in particular, the glasses were developed for the production of volume-crystallizing glass-ceramics, which may counteract good compaction due to a strong tendency to crystallization. The following compositions are therefore intended to provide an overview of possible compositions, although it has not been assessed whether these compositions are suitable for use as seals. Ultimately, this also depends on the material pairings to be joined, the allowed maximum sealing temperatures and other parameters.

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Table 5.

Crystallized glasses with CTEs <5 10⁻⁶ K⁻¹. Chemical compositions (mol%), phases crystallized, CTE after crystallization and the applied crystallization temperatures, T_cr. Solid solutions are abbreviated with ss.

Composition	Crystalline phases	CTE in 10−6 K−1	Tcr in °C	Ref
73.5 SiO2·10.3 Li2O·14.3 Al2O3·1.2 ZrO2·0.6 SnO2	β-SiO2ss	−2.13	880	178
57.9 SiO2·23.9 Li2O·18.2 Al2O3	β-SiO2ss	−1.57	965	179
70.8 SiO2 10.4 Li2O 14.4 Al2O3 2.6 B2O3 1.2 ZrO2 0.6 SnO2	β-SiO2ss	−0.95	830	178
57.5 SiO2·29 Al2O3·11.4 Na2O·4.87 Li2O·2.4 TiO2	LiAlSi2O6, NaAlSiO4	−0.3	900	180
59.1 SiO2·19.3 Al2O3·9 Na2O·10.3 Li2O·2.4 TiO2	LiAlSi2O6, NaAlSiO4	−0.2	900	180
66.7 SiO2·16.7 Li2O·16.7 Al2O3	β-SiO2ss	0.09	965	179
60.4 SiO2·24.5 Li2O·15.11 Al2O3	β-SiO2ss	0.87	965	179
57.5 SiO2·20 MgO·20 Al2O3·2.5 P2O5	Mg2Al4Si5O18	1.51	1050	181
56.3 SiO2·20.8 CaO·12.6 Al2O3·10.3 Li2O	LiAlSi2O6, CaSiO3, CaAl2Si2O6	2.8	1050	182
53.9 SiO2·19.9 CaO·12.1 Al2O3·14.1 Li2O	LiAlSi2O6, CaSiO3	2.9	1050	182
64.9 SiO2·8.9 Li2O·6.6 MgO·16.5 Al2O3·0.3 Sb2O3·2.8 P2O5	β-SiO2ss	2.95	950	183
62.5 SiO2·8.7 Li2O·8.3 MgO·3.4 CaO·4.1 ZnO·8.7 Al2O3·4.4 B2O3	β-SiO2ss, CaMgSi2O6, ZrO2	3.0	800	184
57.5 SiO2·20 MgO·20 Al2O3·2.5 P2O5	Mg2Al4Si5O18	3.01	1050	181
55.1 SiO2·19.7 MgO·4.1 ZnO·19.5 Al2O3·1.6 ZrO2	Mg2Al4Si5O18	3.5	900	185
57.6 SiO2·21.6 CaO·10.1 Li2O·10.7 Al2O3	LiAlSi2O6, CaAl2Si2O8, CaSiO3	3.5	920	186
51.4 SiO2·34.8 MgO·13.8 ZnO·5.5 Al2O3·3.5 TiO2	MgSiO3	3.5	800	187
48.3 SiO2·9.6 Li2O·25.7 MgO·12.2 Al2O3·4.1 B2O3	LiAlSi3O8, MgAl2Si2O8, Mg2SiO4	3.9	1280	188
59 SiO2·22 Y2O3·19 Al2O3	y-Y2Si2O8	3.95	920	189
68.17 SiO2·19.21 Y2O3·12.62 Al2O3	y-Y2Si2O8, c-SiO2, mullite	4.08	920	189
58.9 SiO2·21.7 CaO·13.1 Al2O3·6.3 Li2O	LiAlSi2O6, CaSiO3, CaAl2Si2O6	4.1	1050	182
64.8 SiO2·26.1 CaO·9.1 Al2O3	CaAl2Si2O6	4.12	1300	190
54.9 SiO2·23.1 MgO·22 ZnO·5.5 Al2O3·4.4 TiO2	MgSiO3	4.2	800	187
52.7 SiO2·20.1 MgO·22.5 Al2O3 3.8 ZrO2·0.9 Y2O3	Mg2Al4Si5O18, β-SiO2ss	4.6	925	191
53.2 SiO2·20.3 MgO·22.7 Al2O3·3.8 ZrO2	Mg2Al4Si5O18, β-SiO2ss	4.8	925	191
51.6 SiO2·24.1 Li2O·14.8 ZnO·9.1 Al2O3·0.08 Sb2O3·0.7 P2O5	β-SiO2ss	5	900	192

Crystallizing glass seals with CTEs between 5 and 9·10⁻⁶ K⁻¹

Crystallized glass seals with CTEs between 5 and 9·10⁻⁶ K⁻¹ are frequently required, e.g., for the sealing of Al₂O₃. Table 6 shows some chemical compositions of crystallizing glass seals and the phases formed during crystallization as well as the resulting CTEs. In the seals, a large variety of different crystalline phases is formed. Since many crystalline phases which can be crystallized from glasses (see Table 1) possess CTEs in this range, a required CTE can be obtained by many different glass compositions. Hence, it is possible to choose an optimum composition by other criteria, such as corrosion behaviour, adherence to the materials to be sealed, electric conductivity or mechanical properties.

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Table 6.

Crystallized glass seals with CTEs between 5 and 9·10⁻⁶ K⁻¹. Chemical compositions (mol%), phases crystallized, CTE and the crystallization temperature, T_cr.

Composition	Crystalline phases	CTE in 10−6 K−1	Tcr in °C	Ref
49.9 SiO2·23.5 Li2O·4.4 MgO·14.4 ZnO·7.1 Al2O3·0.07·Sb2O3·0.7 P2O5	β-quartzss,	5	900	192
50 SiO2·25 Al2O3·25 Y2O3	y-Y2Si2O7	5.09	920	189
57.2 SiO2·7.2 Li2O·14.2 MgO·14.2 CaO·7.2 Al2O3	CaMgSi2O6, LiAlSi4O10, Li2SiO3	5.1	775	193
50.5 SiO2·20.6 MgO·20.6 Al2O3·8.3 TiO2	β-quartzss, MgAl2O4	5.6	1050	47
59.4 SiO2·20.1 ZnO·16.1 Li2O·1.3 K2O·1.1 P2O5	Li3Zn0.5SiO4	6.0	700	194
63.1 SiO2·2.5 Li2O·3.7 Na2O·2.4 K2O·2.2 Al2O3·19.6 B2O3·3.6 Bi2O3·2.8 TiO2	Bi2Ti2O7	6.0	850	195
65.82 SiO2·22 Al2O3·12.18 Nd2O3	Al6Si2O13, Nd2Si2O7	6.1	1200	196
49.3 SiO2·33.4 MgO·10.6 Al2O3·6.7 TiO2	TiO2, MgTi2O5, MgSiO3, Mg2Al4Si5O18	6.2	1200	197
48.5 SiO2·3.3 Li2O·4.7 Na2O·3.1 K2O·2.9 Al2O3·25.1 B2O3·8.8 Bi2O3·3.7 TiO2	Bi2Ti2O7	6.4	850	195
59.7 SiO2·29 CaO·11.2 Al2O3	CaAl2Si2O8	6.5	1000	198
62.8 SiO2·15.8 BaO·21.4 Al2O3	BaAl2Si2O8 (hexacelsian)	7.2	1250	199
57.2 SiO2·7.2 Li2O·14.2 MgO·14.2 CaO·3.2 Fe2O3·4 Al2O3	CaMgSi2O6, LiAlSi4O10, LiFe5O8, Li2SiO3	7.3	770	193
50.1 SiO2·21 CaO·1 BaO·14 MgO·10.9 Al2O3	CaAl2Si2O8, CaMgSi2O6,	7.5	1100	200
51.6 SiO2·21.1 MgO·21.1 Al2O3·6.1 ZrO2	β-quartzss, MgAl2O4	7.6	1050	47
20 SiO2·44.1 CaO·4.2 MgO·4 ZnO·26.7 Al2O3	Ca2(Al, Mg, Zn)(Al, Si)2O7, Al2O3	7.7	950	94
50.1 SiO2·23 CaO·2 BaO·14 MgO·10.9 Al2O3	CaAl2Si2O8, CaMgSi2O6,	7.8	1100	200
20 SiO2·44.1 CaO·9.2 MgO·26.7 Al2O3	Ca2(Al, Mg)(Al, Si)2O7, Al2O3	8.1	950	88
37.7 SiO2·3.8 Li2O·5.5 Na2O·3.6 K2O·3.3 Al2O3·29.3 B2O3·12.6 Bi2O3·4.3 TiO2	Bi2Ti2O7	8.1	850	195
9 SiO2·61 CaO·30 Al2O3·	Ca12Al14O33, CaAl2O4	8.3	990	87
48.6 SiO2·22.9 Li2O·8.5 MgO·14 ZnO·5.2 Al2O3·0.07 Sb2O3·0.65 P2O5	β-quartzss,	8.3	900	192
46.2 SiO2·21.7 Li2O·16.2 MgO·13.3 ZnO·1.8 Al2O3·0.07 Sb2O3·0.6 P2O5	β-quartzss	8.8	900	192
20 SiO2·48 CaO·10 MgO·22 Al2O3	Ca2(Al, Mg)(Al, Si)2O7, Al2O3	9.0	950	88

Numerous crystalline phases are found in crystallized glasses with appropriate CTEs. Among these, CaAl₂Si₂O₆, CaMgSi₂O₆, Li₂SiO₃, MgAl₂O₄, Bi₂Ti₂O₇, Al₆Si₂O₁₃ and CaAl₂O₄ are to be mentioned (see Table 6).

Many commercial sealing glasses exist for this CTE range; unfortunately, the chemical compositions and the phases formed are scarcely reported in literature.

Crystallizing glass seals with CTEs >9·10⁻⁶ K⁻¹

Crystallizing glass seals with such high CTEs are predominantly required for joining of high temperature fuel cell materials or for high temperature alloys such as CroFer™ or NiCroFer™. The high CTE phases crystallized from the seals are frequently based on alkaline earth silicates. Here, especially, the barium silicates Ba₂SiO₄, BaSiO₃, Ba₅Si₈O₂₁, Ba₂Si₃O₈ and BaSi₂O₅, as well as the strontium silicates Sr₂SiO₄ and SrSiO₃ are to be mentioned. A drawback, especially of barium containing phases used for sealing of chromium containing alloys is that under certain circumstances, BaCrO₄ is formed, a phase with a mean CTE as high as 23.6·10⁻⁶ K⁻¹, which furthermore shows very different CTEs in different crystallographic directions. If this phase is formed, at the interface metal/seal cracking of the interface may occur. Other sealing glasses with CTEs > 10·10⁻⁶ K⁻¹ contain some binary alkaline earth silicates, such as CaSiO₄, Ca₃Si₂O₇ as well as BaCa₂Si₃O₉ and BaZnSiO₄ which all possess CTEs > 10·10⁻⁶ K⁻¹. Some crystallized glasses with high CTE also contain the alkaline earth alumosilicates SrAl₂Si₂O₈ or BaAl₂Si₂O₈.

Among the ternary alkaline earth silicates, åkermanite (Ca₂MgSi₂O₇, 10.5·10⁻⁶ K⁻¹) and other members of the melilite family have also high CTEs. Åkermanite is a favourable crystalline phase for two reasons: first, although it possesses a tetragonal space group, the CTEs of the a- and c-axes are 10.7 and 10.2·10⁻⁶ K⁻¹, respectively and hence are nearly isotropic. The second reason is the formation of solid solutions; it can incorporate numerous transition metal oxides at Mg-sites as well as Al³⁺ at Si⁴⁺ sites, which may contribute to a higher volume concentration of crystalline phase. Compounds added as nucleation inhibitor which widens the glass forming range such as Al₂O₃ can then be incorporated into the crystalline phase.

A special system are solid solutions of BaZn₂Si₂O₇, in which ZnO is at least partially replaced by other transition metal oxides. Although, the CTE can easily be tailored by the chemical composition, these solid solution phases possess a strongly anisotropic CTE and hence a fine-grained microstructure is required. Other phases with high CTE are e.g., fluorides of Na, Pb, Ba, Sr or Ca. These phases can be crystallized from oxyfluoride glasses, however, usually not in concentrations higher than 10 wt% and are typically not used and studied for sealing applications but, e.g., for optical applications, such as PTR (photo-thermo-refractive) glasses. Nevertheless, such fluorides may precipitate, if blowing is to be suppressed by the addition of fluorides to the glass batch and hence may affect the CTE.

Table 7 presents some chemical compositions of sealing glasses with CTEs >9·10⁻⁶ K⁻¹, the formed crystalline phases, the resulting CTEs and the crystallization temperatures.

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Table 7.

Crystallized glass seals with CTEs >9·10⁻⁶ K⁻¹. Chemical compositions (mol%), crystallized phases, CTE and the maximum applied crystallization temperature, T_cr.

Composition	Crystalline phases	CTE in 10−6 K−1	Tcr in °C	Ref.
45 SiO2·27 SrO·18 MgO·10 B2O3	SrSiO3	9.2		201
20 SiO2·48 CaO·10 MgO·22 Al2O3	Ca2(Al, Mg)(Al, Si)2O7, Al2O3	9.3	1000	88
50 SiO2·20 BaO·63.5 CaO·7.65 B2O3·3 Al2O3·1 Y2O3	BaSi2O5	9.4		202
50 SiO2·30 BaO·20 ZnO	Ba5Si9O21, BaZnSiO6	9.5–11	800	203
40 SrO·12 Al2O3·8 B2O3·40 SiO2	Sr2Al2SiO7, SrAl2Si2O8, SrSiO3	9.6	800	204
50 SiO2·30 BaO·15 MgO·5 Al2O3	Ba5Si8O21, BaAl2Si2O8	9.6	900	205
47.5 SiO2·27 BaO·18 MgO·7.5 B2O3	BaMg2Si2O7, BaSi2O5	9.7		201
35.7 SiO2·54.5 BaO·3.3 Al2O3·6.5 B2O3	BaSiO3, BaAl2Si2O8	9.7	850	206
54 SiO2·27 BaO·9.5 NiO·9.5 ZnO	BaZn1.5Ni0.5Si2O7, BaSi2O5	10.4		8
38 SiO2·52.3 BaO·4.9 Al2O3·4.8 B2O3	BaSiO3, BaAl2Si2O8	10.5	857	206
45.97 SiO2·24.06 CaO·23.21 SrO·3.83 B2O3·1.91 TiO2	CaSiO3, Ca2SiO4, Sr2SiO4	11.1	850	207
47.73 SiO2·24.54 MgO·14.73 CaO·7.36 SrO·1.23 Al2O3· 1.79 B2O3·0.55 BaO·0.84 NiO	CaMgSi2O6 with Sr-incorporation	11.2	900	208
33.3 SiO2·33.8 BaO·2 CaO·10 Al2O3·16.7 B2O3·5 La2O3	BaAl2Si2O8, Ca2Al2SiO8	11.7	800	209
35SiO2·35 BaO·15 CaO·5 Al2O3·10 B2O3	BaAl2Si2O8, BaSiO3	11.8	800	210
55 SiO2·18 BaO·27 ZnO	BaZn2Si2O7, BaSi2O5, BaZnSi3O8	12	850–1000	211
33.3 SiO2·35 BaO·10 Al2O3·16.7 B2O3·5 La2O3	BaAl2Si2O8	13	800	209
38 SiO2·39.1 BaO·10.5 CaO·2.7 Al2O3·9.8 B2O3	Ba0.9Ca0.1SiO2	13.2–13.9	850	212
55 SiO2·27 BaO·18 ZnO	BaZn2Si2O7, BaSi2O5	13.5–13.9	850–1000	211
46.72 SiO2·24.71 CaO·21.95 SrO·3.36 B2O3·1.03 TiO2	CaSiO3, Ca2SiO4, Sr2SiO4	13.9	850	207
55 SiO2·22.5 BaO·22.5 ZnO	BaZn2Si2O7, BaSi2O5	14.4–14.9	850–1000	211
55.2 SiO2·25.7 BaO·15.7 ZnO·3.3 NiO	BaZn2−xNixSi2O7, BaSi2O5	15.0–16.0	935–1000	8

Adjustment of the main components of a crystallizing glass seal

The main components of the glass seal should be chosen with respect to:

Sufficient chemical durability under the conditions of the respective application. That might be a strongly reducing or an oxidising atmosphere, the humidity or any other corrosive atmosphere or also chemical reactions with the sealed materials.

Addition of minor components

The addition of minor components is frequently necessary in order to control the crystallization behaviour and sometimes also to improve the adherence of the seal with the materials to be sealed. The minor components of a crystallizing glass seal should be chosen with respect to the following considerations:

The glass composition should not notably crystallize before densification is reached. The crystallization behaviour can be affected by the addition of nucleation agents or nucleation inhibitors as well as by the applied temperature/time schedule. Nucleation inhibitors decrease the nucleation rate at the same viscosity and increase the induction period of nucleation.^6,218–222 For this purpose, comparatively small additions of 1–2 mol% of compounds such as ZrO₂, TiO₂, La₂O₃, Nb₂O₅ or Ta₂O₅ are sufficient. Nucleation agents, such as ZrO₂ or TiO₂ in most aluminosilicate glasses have to be added in much higher concentrations (4–9 mol%). These compounds (or compounds derived hereof) are precipitated just above T_g and then induce volume crystallization. Volume nucleation is highly advantageous for crystallizing glass seals for two reasons: the first is the statistical orientation of the crystal, which means that the seal should have the same CTE parallel and perpendicular to the sealed materials. The second reason is that if the glass grains should show high nucleation rates at the surface, sintering is much more difficult, because the glass grains are covered by the formed crystalline phase.