High Temperature Materials. By Docent. N. Menad. Luleå University of Technology ( Sweden )

Transcription

1 Ch. Amorphous Structures Course KGP003 High Temperature Materials By Docent. N. Menad Dept. of Chemical Engineering and Geosciences Div. Of process metallurgy Luleå University of Technology ( Sweden )

2 Amorphous Materials An amorphous solid is a solid in which there is no long-range order of the positions of the atoms. (Solids in which there is long-range atomic order are called crystalline solids.) Most classes of solid materials can be found or prepared in an amorphous form. Ex: common window glass is an amorphous ceramic, many polymers (such as polystyrene) are amorphous, and even foods such as cotton candy are amorphous solids. Amorphous materials are often prepared by rapidly cooling molten material. The cooling reduces the mobility of the material's molecules before they can pack into a more thermodynamically favourable crystalline state. Amorphous materials can also be produced by additives which interfere with the ability of the primary constituent to crystallize. For example addition of soda to silicon dioxide results in window glass and the addition of glycols to water results in a vitrified solid. Some materials, such as metals, are difficult to prepare in an amorphous state. Unless a material has a high melting temperature (as ceramics do) or a low crystallization energy (as polymers tend to), cooling must be done extremely rapidly. Amorphous solids can exist in two distinct states, the 'rubbery' state and the 'glassy' state. The temperature at which the transition between the glassy and rubbery states is called their glass transition temperature or Tg.

3 Amorphous structures of inorganic glass, even called vitreous state, can be defined as follows: An isotropic material that does not have long range 3-dimensional atomic periodicity (after 20 Å) and has a viscosity greater than poise. The difference between a glass and its corresponding liquid can be demonstrated by the volume/temperature relationship during cooling shown in the diagram to the right. During cooling of a melt to its crystallization temperature (melting temperature) Tm, the volume follows line a. When the crystallization doesn t occur the volume follows line b and a super-cooled liquid is formed. After further cooling, line b changes to line c with an inflection point at the glass transition temperature, Tg, resulting in the formation of a glass. Due to the high viscosity of super-cooled melts, cooling must occur extremely slowly if the cooling curve is to show a distinct inflection point. Otherwise, the volume/temperature relationship follows a curve like line d.

4 The Glass Transition Temperature When a crystalline material is heated, the melting temperature (TM) is well defined. But on cooling, it is often possible to undercool the liquid so that nucleation and growth do not occur. At a temperature below TM, the material can become rigid and act like a solid, while preserving the amorphous atom arrangement. This is called the glass transition temperature T g. A plot of volume of density is often used to illustrate this, since the expansion coefficient of the liquid, crystalline solid, and amorphous solid are all generally different. In real materials, T g depends on cooling rate, and many other secondary factors, and may not show such a sharp transition as indicated here

5 Glass can be classified as a super-cooled liquid in which crystallization has not occurred. There are quite many similarities between glass and liquids. Glass structure is however much more complicated than crystal structure. Glass has properties that are difficult or impossible to simulate in crystalline material. These properties are also continuously variable in a way that is impossible for a crystalline material as well.

6

7

8 A kinetic theory which describes the transition of a melt to a glass is based on the slow nucleation of crystal grains and subsequent polycrystalline growth. The rate of nucleation is, as well as crystal growth rate, temperature dependent but the rate controlling functions contain conflicting terms that lead to the different functions having distinct rate maxima, see adjacent diagram.. As soon as a nucleation has begun, a crystal will start to grow at a certain rate depending on the rate of atomic diffusion to the crystal surface. Crystal growth is therefore dependent on the viscosity of the melt. Crystal growth rate reaches a maximum, which for glass, often lies at a different temperature than the maximum nucleation rate. Also noteworthy, is that the activation energy for nucleation as well as crystal growth within glass systems is higher than the Gibbs free activation energy for viscous flow or self-diffusion This phenomenon can be explained by the fact that bonds within a molecular unit must be broken to allow for crystallization to occur Tendency to glass formation increases with: Increased cooling rate Increased surface tension melt/crystalline phase Increased transformation temperature Decreased melt volume Decreased grain density

9 Metallic Glass Some amorphous metallic alloys can be prepared under special processing conditions (such as rapid solidification, thin-film deposition, or ion implantation), but the term "metallic glass" refers only to rapidly solidified materials. Even with special equipment, such rapid cooling is required that, for most metals, only a thin wire or ribbon can be made amorphous. This is enough for many magnetic applications, but thicker sections are required for most structural applications such as scalpel blades, golf clubs, and cases for consumer electronics. Recent efforts have made it possible to increase the maximum thickness of glassy castings, by finding alloys where kinetic barriers to crystallization are greater. Such alloy systems tend to have the following inter-related properties: Many different solid phases are present in the equilibrium solid, so that any potential crystal will find that most of the nearby atoms are of the wrong type to join in crystallization. The composition is near a deep eutectic, so that low melting temperatures can be achieved without sacrificing the slow diffusion and high liquid viscosity seen in alloys with high-melting pure components. Atoms with a wide variety of sizes are present, so that "wrong-sized" atoms interfere with the crystallization process by binding to atom clusters as they form. One such alloy is the commercial "Liquidmetal", which can be cast in amorphous sections up to an inch thick

10 With the help of X-ray diffraction studies, the structure of glass is shown to be based on SiO4 tetrahedral units that are bound to each other in a random network with so called modified atom species spread throughout void spaces, as seen in the adjacent schematic. Here one notices that certain Si-O-Si bonds are broken and replaced by free O-endings and metal cations according to the following formula: GLASS STRUCTURE - Si O Si- + M2O -Si-O - M+ O - -Si- M+ A requirement in oxide glass formation is the possibility to form a 3-dimensional structural network with a comparable energy to that needed for a corresponding crystalline structural network. This requires that the primary coordinate number for every atom in the glass should be nearly identical to those in the crystalline material. Furthermore, the secondary coordinate should have only a very small contribution to the total structural energy. Consider SiO2 for example, the structural energy difference between Cristobalite and glassy SiO2 (Quartz glass) is only ~1%

11

12 Structures found from the glass beads demonstration. A perfect crystal (a), an amorphous structure (b), and a crystal with a vacancy (c) is shown

13 A B C The amorphous state obtained by a rapid quench from a random start to a temperature just above T= 0 The crystalline state obtained after a long slow cooling with occasional heating An intermediate state in the cooling process, which after reheating eventually resulted in a picture similar to B.

14 To fulfill these requirements, Zachariassen has proposed the following necessities for oxide glass formation. 1. Every oxygen atom should not be bound to more than two cations. 2. The coordinate number for oxygen ions positioned around the central cation should be small, i.e. less that Oxygen polyhedra can share a corner with each other in order to form a 3-dimensional network. However, the polyhedra can not share common edges or surfaces. 4. At least 3 corners of every polyhedron should be shared.

15 VISCOSITY OF GLASS The viscosity is a crucial factor in glass formation. Glass formation is favorable in a material when: 1. The viscosity is high at the melting point. 2. The viscosity below the melting point increases with decreasing temperature. The glass transition temperature, Tg, and the melting temperature, Tm, were introduced and which are also present in the given diagram. Normally, the two temperatures have the following relationship Tg 2/3*Tm Tg 2/3*Tm

16 The viscosity is, as mentioned, also very dependent on composition. In silicate glass, the viscosity decreases with increasing content of modified cations. In many cases the change is very pronounced. For example, for a quartz glass (pure SiO 2 ) at 1700 C the viscosity decreases 10 4 poise with an addition of as little as 2.5 mol% K 2 O. VISCOSITY OF GLASS This effect can be attributed to the presence of oxygen atoms that only have one bond and cause a weak link in the Si-O- network. In the adjacent diagram, the effect of replacing 8% of SiO 2 in a 74SiO 2-10CaO-16Na 2 O glass with different divalent oxides to the viscosity at 1400 C is shown. For Borsilicate glass, the change in viscosity with an addition of alkali oxides is more complicated. At high temperatures the viscosity decreases with increased alkali addition, whilst at lower temperatures, the viscosity increases with increased alkali addition. This phenomenon has yet to be explained theoretic.

17 Thermal Stress in Glass It is understandable that thermal stresses occur between materials with different thermal expansion behavior during heating and cooling. When considering glass materials, thermal stresses can occur when different parts of the glass, having different specific volumes, cool at different rates. This is the case when a glass product contains segments with diverse cross sections. Naturally, a thick material will cool faster than a thin material. A glass material that is cooled rapidly at the surface will suffer thermal stress between the surface and interior.

18 When reheating the glass the volume curve will follow the same curve as mentioned above but in the opposite direction.

19 The next diagram illustrates the behavior of 3 different glasses; A, B and C, with different specific volumes (i.e. different cooling rates) in terms of temperature vs. physical expansion. Glass A is a rapidly cooled glass, B a moderately cooled glass, C a slowly cooled glass. The diagram demonstrates that the thermal expansion coefficient is the same for all 3 glasses at temperatures up to 400 C, i.e. the curves are parallel. Above 400 C, glass A contracts up to roughly 560 C until the glass structure has reached equilibrium, i.e. the specific volume of the slowly cooled glass. On the other hand, glass C follows the normal glass cooling curve from the opposite direction which maintains that the structure in this glass is in equilibrium during the entire course of cooling. Viscosity of glass at different temperature Annealing temperature which is a characteristic property for every glass It is through the use of thermal treatment at temperatures near the annealing temperature that thermal stresses in glass can be eliminated. This is a standard practice in manufacturing of glassware with varying thickness, for example; drinking glass, glass bottles and so on.

20 GLASS CHEMICAL COMPOSITIONS Type of Glass Chemical composition, Wt.% SiO2 Al2O3 CaO MgO Na2O K2O PbO B2O3 Window glass Packing glass Borsilicate glass Lead glass (Crystal glass)

21 TYPES OF COMMERCIAL GLASS WINDOW GLASS PLATE GLASS BOTTLES AND CONTAINERS OPTICAL GLASS PHOTOSENSITIVE GLASS GLASS CERAMICS GLASS FIBERS MISCELLANEOUS TYPES OF GLASS RECYCLING GLASS

22 RECYCLING GLASS Scrap glass taken from the glass manufacturing process, called cullet, has been internally recycled for years. The scrap glass is economical to use as a raw material because it melts at lower temperatures than other raw materials, thus saving fuel and operating costs. Glass that is to be recycled must be relatively free from impurities and sorted by color. Glass containers such as bottles and jars are the most commonly recycled form of glass, and their colors are flint (clear), amber (brown), and green. Other types of glass, such as window glass, pottery, and cooking utensils, are considered contaminants because they have different compositions than glass used in containers. The recycled glass is melted in a furnace and formed into new products. Glass containers make up 90 percent of the total recycled glass used in the United States. The recycling rate for glass in 2000 was about 23 percent. Other uses for recycled glass include glass art and decorative tiles. Cullet mixed with asphalt forms a paving material called glassphalt.

23 Example Structural Evolution in Mechanically Alloyed Al-Fe Powder Mixtures The structural evolution in mechanically alloyed binary aluminum-iron powder mixtures containing 1, 4, 7.3, 10.7, and 25 at. pct Fe has been investigated using x-ray diffraction and electron microscopic techniques. The constitution (number and identity of phases present), microstructure (crystal size, particle size) and transformation behavior of the powders on annealing have been investigated. The solid solubility of Fe in Al has been extended up to at least 4.5 at. pct. compared to the equilibrium value of at. pct Fe at room temperature. A fully amorphous phase plus solid solution in the Al-10.7 at. pct Fe alloy; agreeing well with the predictions made using the semiempirical Miedema model. Debkumar Mukhopadhyay

24 Amorphous cogel with 3% titanium Example X-Ray Diffractometry TS-1 with 3% titanium TS-1 and VS-1 have a good crystallinity and show the typical reflexes of the orthorhombic MFI-structure

25 Example Heat flow at heating 10K/min (solid line) and subsequent cooling with 10K/min