What is glass ceramics?

Crystals and new gems are also called microcrystalline jade or ceramic glass. It is a new building material developed by combining glass and stone technology. Because it can use ore, industrial tailings, metallurgical slag, fly ash, coal gangue and so on. As the main raw materials for production, and there is no pollution in the production process, the product itself has no radioactive pollution, so it is also called environmental protection products or green materials.

Glass-ceramics has triple advantages of glass, ceramics and natural stone, which is superior to natural stone and ceramics. It can be used for building curtain walls and indoor high-grade decoration, and can also be used as structural materials for machinery, insulating materials for electronics and electricians, backplane materials for large-scale integrated circuits, heat-resistant containers for microwave ovens, chemical anticorrosion materials and wear-resistant materials for mines. It is a promising new material in 2 1 century.

Second, the composition of glass-ceramics

The glass with or without nucleating agent with specific components is subjected to crystallization heat treatment under controlled conditions, so that the original single glass phase becomes a composite material with evenly distributed microcrystalline phase and glass phase. The difference between glass-ceramics and ordinary glass is that the former is partially crystalline and the latter is completely amorphous. The surface of glass-ceramics can present natural stone patterns and color transparent body, while glass is transparent body with various colors and different processes.

The comprehensive properties of glass-ceramics mainly depend on three factors: the original composition, the size and quantity of microcrystals, and the nature and quantity of residual glass phase.

The latter two factors are determined by the crystallization heat treatment process of glass-ceramics. The original composition of glass-ceramics is different, and its crystal phase types are also different, such as β wollastonite, β Yingshi, fluorophlogopite, lithium disilicate and so on. Various crystalline phases endow glass-ceramics with different properties. Among the above crystal phases, β wollastonite crystal phase has the properties required for building glass-ceramics. Therefore, CaO-Al2O3-SiO2 system is usually selected as the original composition system of building glass-ceramics, and its general composition is shown in Table 1.

Table 1: composition of calcium aluminum silicon glass ceramics

Color \ Composition SiO2 Al2O3 B2O3 Cao ZnO Bao Na2O K2O Fe2O3 Sb2O3

White 59. 07. 0 1. 0 1. 5

Black 59.06.00.513.06.04.03.02.06.00.5

The main crystal phase precipitated from the above glass components after crystallization heat treatment is β-wollastonite (β-Cao, SiO2 _ 2).

Third, the performance of building glass-ceramics

List 2 properties of glass-ceramic decorative board for building and natural marble and granite (see next page).

Materials: glass ceramics, marble and granite.

characteristic

Mechanical properties Bending strength ① (MPa) 40 ~ 505.7 ~158 ~15.

Compressive strength (MPa) 341.367 ~100100 ~ 200.

Impact strength (Pa) 2452 2059 196 1

The elastic modulus (×104mpa) is 52.7 ~ 8.24.2 ~ 6.0.

Mohs hardness is 6,53 ~ 5 ~ 5.5.

Vickers hardness (100g) 600130130 ~ 570

Specific gravity 2.7 2.7 2.7

Chemical properties Acid resistance ② (1%H2SO4) 0.0810.00.10.

Alkali resistance ② (1%NaOH) 0.05 0.300.10

Seawater resistance ③ (mg/cm2) 0.080.190.17.

Water absorption ④(%) 0 0.3 0.35

Frost resistance (%)⑤ 0.028 0.23 0.25

Thermal characteristic expansion coefficient

( 10-7/30℃ -380℃) 62 80~260 80~ 150

Thermal conductivity (w/m.k)1.62.2 ~ 2.32.1~ 2.4.

Specific heat 0.190.180.18.

Whiteness of optical characteristics (L degree) 89 59 66

Diffuse reflectance (%) 80 42 64

Positive reflectivity (%) 4 4 4

As can be seen from Table 2, architectural glass-ceramics are superior to natural marble and granite in material dimensional stability (influence of thermal expansion coefficient, etc.). ), wear resistance (hardness), frost resistance, gloss durability (acid and alkali resistance), strength (bending resistance, impact resistance). Glass-ceramics have the same composition as glass, and have good compatibility with silicone structural adhesive and weather-resistant adhesive.

Because glass-ceramics are composite materials composed of transparent, translucent and opaque phases, the light incident on glass-ceramics is reflected not only from the surface, but also from the inside of the material, which is soft and deep, and produces an optical effect as bright as diamonds.

Homomorphic glass has no water absorption, anti-freezing, anti-rust, silicone oil and other infiltration, does not dissolve, and is easy to attach dust. Even if dust is attached, it is easy to clean and has self-purification.

Glass-ceramics have high strength and stable strength, and are not dispersed like natural granite. The structure is uniform, the isotropic strength is consistent, and there is no anisotropy (layering and irrationality) like granite.

The cambered surface or curved surface of glass-ceramics can be heated to about 760℃ ~ 800℃. Therefore, compared with natural stone, it has the advantages of uniform strength, simple process and low cost.

When producing white or brightly colored glass-ceramics, mineral raw materials and chemical raw materials are generally used, which can have no color difference and can also be simulated into natural stone materials of various colors. These colors are formed by high-temperature heating with metal oxides that do not change color. They have good weather resistance and will not change color and fade.

Because of its excellent performance, glass-ceramics have been widely used in the exterior wall and interior decoration of hotels, restaurants, shops, airports, stations, theaters and other high-end buildings at home and abroad, and are new building materials in the 2 1 century.

Fourth, the production process of glass-ceramics

There are two production processes for building glass-ceramics, namely calendering and sintering, and the process flow is as shown in the figure:

At present, glass-ceramics for building are all sintered, and no nucleating agent is added. Its basic principle is that glass is an amorphous solid. From the thermodynamic point of view, it is metastable and has higher internal energy than crystal, so it can be transformed into crystal under certain conditions. From the kinetic point of view, the viscosity of glass melt increases sharply during cooling, which inhibits the formation of crystal nucleus and the growth of crystal, and prevents the growth and growth of crystal. Glass-ceramics for building use the heterogeneous crystallization mechanism of nucleating agent, make full use of the possibility of thermodynamics and the inhibition of kinetics, make this opposing physical process form a new balance under certain conditions, and thus obtain a new material.

Glass ceramics produced by sintering process have the following hot spots and difficulties:

First, glass melting: Glass-ceramics with other colors are usually melted in a well-sealed crucible except for glass-ceramics colored by solar powder. Its production cost and quality are superior to those of crucible furnace. However, the glass-ceramic tank kiln can not copy the general glass tank kiln, but should be convenient for discharging, feeding and stopping the furnace.

The second is crystallization heat treatment: only after crystallization heat treatment can glass-ceramics be formed. The process parameters and specifications of heat treatment have an important influence on the type, size and quantity of main crystal phase, product bursting, flatness, bubble size and quantity, output, gas consumption and cost. Crystallization furnace is also different from ordinary heat treatment furnace and ceramic barbecue oven, and its temperature field and structure should be suitable for the characteristics and technology of crystallization heat treatment of glass-ceramics.

Thirdly, according to the aesthetic requirements of architects, how to conveniently and realistically modulate glass-ceramics of various colors, prevent spontaneous explosion and blowholes, increase specifications and varieties, improve the flatness of large-area plates and reduce costs is a hot and difficult point to further promote the application of architectural glass-ceramics.

As can be seen from the above introduction, glass-ceramics is also a new product with high technology content. Abroad, the United States and Russia have taken the lead in developing and using glass-ceramics, and some countries in Japan, Western Europe and the Asia-Pacific region are also developing new glass-ceramics products. At present, three companies in China have mass-produced building glass-ceramics. It is understood that the production capacity is about 500,000 square meters. However, due to product specifications, varieties, colors and prices, it can not meet the requirements of the construction market. In addition, the promotion and publicity of glass-ceramics as a new building material is not enough, and only a few projects in China use glass-ceramics, such as Ku in Guangdong, Beijing New Airport Terminal and Dalian International Center in the Great Hall of the People. China imports a large number of high-grade stones from abroad every year to meet the demand of the domestic market. Replacing natural stone, especially importing high-grade natural stone, is a potential urgent demand in the construction market. Glass-ceramics will not only be widely used in building interior decoration, but also be worth developing and popularizing in building stone curtain wall.

Verb (abbreviation of verb) Key points of glass-ceramic curtain wall

1. Glass-ceramics are brittle materials, and their openings are easily broken after construction, so it is impossible to completely copy the joints of natural stone curtain walls. Generally speaking, the short groove and through groove structure of natural stone curtain wall is not suitable.

2. As a curtain wall panel, the glass-ceramic panel is required to be resistant to rapid cooling and rapid heating. The test method is: the specification is 100mm×80mm× plate thickness, with 5 samples in each group, and the samples are cooled in room temperature water. Then gently tap each part of the sample with a hammer. If the voice becomes hoarse and there are cracks and edges on the surface, it is judged as unqualified.

3. Although the glass-ceramic plate is required to withstand rapid cooling and rapid heating, in order to prevent the damage of fragments to people when the curtain wall panel is broken, a layer of glass fiber (FRP) is attached to the back of the glass-ceramic plate with multi-component plate grease for safety.

4. The requirements of ordinary glass-ceramic plate for curtain wall are as follows:

(1) The standard value of bending strength is not less than 40MPa. The test method shall be carried out in accordance with GB 9966.2.

(2) Rapid cooling and rapid heating will not cause cracks.

(3) The length tolerance is 0.5mm, the flatness11000 and the thickness tolerance1mm..

(4) no edges and corners, no pores. There are no visible impurities on the surface.

(5) The glossiness of the mirror plate is not more than 85 optical selection units.

(6) The color difference of plates with the same color and batch number is not more than 2.0CIE 1AB color difference unit.

(7) The manufacturer of glass-ceramics for curtain wall panels shall provide: type test report; The factory inspection report of this batch of plates shall at least indicate the inspection results of bending strength, length, thickness and flatness tolerance, rapid cooling and heating resistance, color difference and gloss; And provide 10 quality guarantee, etc.

5. The glass-ceramic curtain wall must be tested for four properties (wind pressure resistance, water tightness, air tightness and in-plane deformation) at 100%. Only after passing the test can the construction be carried out.

In a word, there are not many glass-ceramics used for building curtain walls in China. In the future, in addition to the hot and difficult points in the popularization and application of glass-ceramic materials, it is also very urgent and important to strengthen the development and research on the joints and structures, processing technology and mechanical properties of glass-ceramic curtain walls. Besides measuring its curvature, it is better to measure its fracture toughness, so that the strength of glass-ceramic curtain wall can lay the foundation for fracture mechanics design.