Glass Ceramics, lithium-aluminum-silicate (LAS) glass-ceramics

Product Information

Glass-ceramics are polycrystalline materials produced through controlled crystallization of base glass, producing a fine uniform dispersion of crystals throughout the bulk material. Crystallization is accomplished by subjecting suitable glasses to a carefully regulated heat treatment schedule, resulting in the nucleation and growth of crystal phases. In many cases, the crystallization process can proceed to near completion, but in a small proportion of processes, the residual glass phase often remains. Glass-ceramic materials share many properties with both glasses and ceramics. Glass-ceramics have an amorphous phase and one or more crystalline phases and are produced by a so-called "controlled crystallization" in contrast to a spontaneous crystallization, which is usually not wanted in glass manufacturing. Glass-ceramics have the fabrication advantage of glass, as well as special properties of ceramics. When used for sealing, some glass-ceramics do not require brazing but can withstand brazing temperatures up to 700 °C. Glass-ceramics usually have between 30% [m/m] and 90% [m/m] crystallinity and yield an array of materials with interesting properties like zero porosity, high strength, toughness, translucency or opacity, pigmentation, opalescence, low or even negative thermal expansion, high temperature stability, fluorescence, machinability, ferromagnetism, resorbability or high chemical durability, biocompatibility, bioactivity, ion conductivity, superconductivity, isolation capabilities, low dielectric constant and loss, corrosion resistance,high resistivity and break-down voltage. These properties can be tailored by controlling the base-glass composition and by controlled heat treatment/crystallization of base glass. In manufacturing, glass-ceramics are valued for having the strength of ceramic but the hermetic sealing properties of glass. Glass-ceramics are mostly produced in two steps: First, a glass is formed by a glass-manufacturing process, after which the glass is cooled down. Second, the glass is put through a controlled heat treatment schedule. In this heat treatment the glass partly crystallizes. In most cases nucleation agents are added to the base composition of the glass-ceramic. These nucleation agents aid and control the crystallization process. Because there is usually no pressing and sintering, glass-ceramics have no pores, unlike sintered ceramics. A wide variety of glass-ceramic systems exist, e.g., the Li2O × Al2O3 × nSiO2 system (LAS system), the MgO × Al2O3 × nSiO2 system (MAS system), the ZnO × Al2O3 × nSiO2 system (ZAS system).

The outstanding feature of LAS glass-ceramics is their near-zero coefficient of thermal expansion, which makes them highly resistant to extreme temperatures, as well as thermal shock. Combining this with high homogeneity, strong chemical resistance, tailored optical properties (transparency, translucency, opaqueness), and design possibilities, results in a material that adds the esthetics of glass to a wide range of application

Synonyms
Spodumene, β-Spodumene, Beta-Spodumene, Lithium ore, lithium aluminum silicon oxide, lithium silicon aluminate, lithium-aluminum-silicon, Silicic acid aluminum lithium salt, aluminum lithium dioxido(oxo)silane, aluminum lithium oxosilanediolate (1:1:2), NIST SRM 181, beta eucryptite, CAS 66057-55-4, CAS 1302-65-4 (AlLi(SiO4)

LAS Glass-Ceramics Specification

Size：customized

Purity: customized

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Properties（Theoretical）

The outstanding feature of LAS glass-ceramics is their near-zero coefficient of thermal expansion, which makes them highly resistant to extreme temperatures, as well as thermal shock. Combining this with high homogeneity, strong chemical resistance, tailored optical properties (transparency, translucency, opaqueness), and design possibilities, results in a material that adds the esthetics of glass to a wide range of application

Nucleation and Crystal Growth

The key to engineering a glass-ceramic material is controlling the nucleation and growth of crystals in the base glass. The amount of crystallinity will vary depending on the amount of nuclei present and the time and temperature at which the material is heated. [9][4] It is important to understand the types of nucleation occurring in the material, whether it is homogeneous or heterogeneous. Homogeneous nucleation is a process resulting from the inherent thermodynamic instability of a glassy material. When enough thermal energy is applied to the system, the metastable glassy phase begins to return to the lower-energy, crystalline state. The term "homogeneous" is used here because the formation of nuclei comes from the base glass without any second phases or surfaces promoting their formation.

Heterogeneous nucleation is a term used when a nucleating agent is introduced into the system to aid and control the crystallization process.[4] The presence of this nucleating agent, in the form of an additional phase or surface, can act as a catalyst for nucleation and is particularly effective if there is epitaxy between the nucleus and the substrate.There are a number of metals that can act as nucleating agents in glass because they can exist in the glass in the form of particle dispersion of colloidal dimensions. Examples include copper, metallic silver, and platinum. It was suggested by Stookey in 1959 that the effectiveness of metallic nucleation catalysts relates to the similarities between the crystal structures of the metals and the phase being nucleated. The most important feature of heterogenous nucleation is that the interfacial tension between the heterogeneity and the nucleated phase is minimized. This means that the influence that the catalyzing surface has on the rate of nucleation is determined by the contact angle at the interface. Based on this, Turnbull and Vonnegut (1952) modified the equation for homogenous nucleation rate to give an expression for heterogenous nucleation rate.

In addition to nucleation, crystal growth is also required for the formation of glass ceramics. The crystal growth process is of considerable importance in determining the morphology of the produced glass ceramic composite material. Crystal growth is primarily dependent on two factors. First, it is dependent upon the rate at which the disordered structure can be re-arranged into a periodic lattice with longer-range order. Second, it is dependent upon the rate at which energy is released in the phase transformation (essentially the rate of cooling at the interface)

Applications of LAS Glass-Ceramics

The commercially most important system is the Li2O × Al2O3 × nSiO2 system (LAS system). [citation needed] The LAS system mainly refers to a mix of lithium, silicon, and aluminum oxides with additional components, e.g., glass-phase-forming agents such as Na2O, K2O and CaO and refining agents. As nucleation agents most commonly zirconium(IV) oxide in combination with titanium(IV) oxide is used. After crystallization the dominant crystal phase in this type of glass-ceramic is a high-quartz solid solution (HQ s.s.). If the glass-ceramic is subjected to a more intense heat treatment, this HQ s.s. transforms into a keatite-solid solution (K s.s., sometimes wrongly named as beta-spodumene). This transition is non-reversible and reconstructive, which means bonds in the crystal-lattice are broken and new arranged. However, these two crystal phases show a very similar structure as Li could show. An interesting property of these glass-ceramics is their thermomechanical durability. Glass-ceramic from the LAS system is a mechanically strong material and can sustain repeated and quick temperature changes up to 800–1000 °C. The dominant crystalline phase of the LAS glass-ceramics, HQ s.s., has a strong negative coefficient of thermal expansion (CTE), keatite-solid solution as still a negative CTE but much higher than HQ s.s. These negative CTEs of the crystalline phase contrasts with the positive CTE of the residual glass. Adjusting the proportion of these phases offers a wide range of possible CTEs in the finished composite. Mostly for today's applications a low or even zero CTE is desired. Also a negative CTE is possible, which means, in contrast to most materials when heated up, such a glass-ceramic contracts. At a certain point, generally between 60% [m/m] and 80% [m/m] crystallinity, the two coefficients balance such that the glass-ceramic as a whole has a thermal expansion coefficient that is very close to zero. Also, when an interface between material will be subject to thermal fatigue, glass-ceramics can be adjusted to match the coefficient of the material they will be bonded to. Originally developed for use in the mirrors and mirror mounts of astronomical telescopes, LAS glass-ceramics have become known and entered the domestic market through its use in glass-ceramic cooktops, as well as cookware and bakeware or as high-performance reflectors for digital projectors.

Glass-ceramic from the LAS-System is a mechanically strong material and can sustain repeated and quick temperature changes. However, it is not totally unbreakable. Because it is still a brittle material as glass and ceramics are, it can be broken. There have been instances where users reported damage to their cooktops when the surface was struck with a hard or blunt object (such as a can falling from above or other heavy items).

The material has a very low heat conduction coefficient, which means that it stays cool outside the cooking area. It can be made nearly transparent (15–20% loss in a typical cooktop) for radiation in the infrared wavelengths.

In the visible range glass-ceramics can be transparent, translucent or opaque and even colored by coloring agents.

Today, there are two major types of electrical stoves with cooktops made of glass-ceramic:

·        A glass-ceramic stove uses radiant heating coils or infrared halogen lamps as the heating elements. The surface of the glass-ceramic cooktop above the burner heats up, but the adjacent surface remains cool because of the low heat conduction coefficient of the material.

·        An induction stove heats a metal pot's bottom directly through electromagnetic induction.

Glass-ceramics are used in medical applications due to their unique interaction, or lack thereof, with human body tissue. Bioceramics are typically placed into the following groups based on their biocompatibility: biopassive (bioinert), bioactive, or resorbable ceramics

One particularly notable use of glass-ceramics is in the processing of ceramic matrix composites. For many ceramic matrix composites typical sintering temperatures and times cannot be used, as the degradation and corrosion of the constituent fibres becomes more of an issue as temperature and sintering time increase. One example of this is SiC fibres, which can start to degrade via pyrolysis at temperatures above 1470K. One solution to this is to use the glassy form of the ceramic as the sintering feedstock rather than the ceramic, as unlike the ceramic the glass pellets have a softening point and will generally flow at much lower pressures and temperatures. This allows the use of less extreme processing parameters, making the production of many new technologically important fibre-matrix combinations by sintering possible

Packing of LAS Glass-Ceramics

Standard Packing:

Typical bulk packaging includes palletized plastic 5 gallon/25 kg. pails, fiber and steel drums to 1 ton super sacks in full container (FCL) or truck load (T/L) quantities. Research and sample quantities and hygroscopic, oxidizing or other air sensitive materials may be packaged under argon or vacuum. Solutions are packaged in polypropylene, plastic or glass jars up to palletized 440 gallon liquid totes Special package is available on request.

ATTs’ LAS Glass-Ceramics is carefully handled to minimize damage during storage and transportation and to preserve the quality of our products in their original condition.

Chemical Identifiers