Crystal

From Wikipedia, the free encyclopedia

This article is about crystalline solids. For the type of glass, see lead crystal. For other uses, see Crystal (disambiguation).

Quartz crystal. The individual grains of this polycrystalline mineral sample are clearly visible.

A crystal or crystalline solid is a solid material, whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification. The word crystal is derived from the ancient Greek word κρύσταλλος (krustallos), which had the same meaning, but according to the ancient understanding of crystal. At root it means anything congealed by freezing, such as ice.^[1] The word once referred particularly to quartz, or “rock crystal”.

Most metals encountered in everyday life are polycrystals. Crystals are often symmetrically intergrown to form crystal twins

Crystal structure

Insulin crystals grown in outer space Halite (sodium chloride) – a single, large crystal.

The process of forming a crystalline structure from a fluid or from materials dissolved in the fluid is often referred to as crystallization. In the old example referenced by the root meaning of the word crystal, water being cooled undergoes a phase change from liquid to solid beginning with small ice crystals that grow until they fuse, forming a polycrystalline structure. The physical properties of the ice depend on the size and arrangement of the individual crystals, or grains, and the same may be said of metals solidifying from a molten state.

Which crystal structure the fluid will form depends on the chemistry of the fluid, the conditions under which it is being solidified, and also on the ambient pressure. While the cooling process usually results in the generation of a crystalline material, under certain conditions, the fluid may be frozen in a noncrystalline state. In most cases, this involves cooling the fluid so rapidly that atoms cannot travel to their lattice sites before they lose mobility. A noncrystalline material, which has no long-range order, is called an amorphous, vitreous, or glassy material. It is also often referred to as an amorphous solid, although there are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the latent intense warmth of fusion.

Crystalline structures occur in all classes of materials, with all types of chemical bonds. Almost all metal exists in a polycrystalline state; amorphous or single-crystal metals must be produced synthetically, often with great difficulty. Ionically bonded crystals can form upon solidification of salts, either from a molten fluid or upon crystallization from a solution. Covalentlydiamond, silica, and graphite. Polymer materials generally will form crystalline regions, but the lengths of the molecules usually prevent complete crystallization. Weak van der Waals forces can also play a role in a crystal structure; for example, this type of bonding loosely holds together the hexagonal-patterned sheets in graphite. bonded crystals are also very common, notable examples being

Most crystalline materials have a variety of crystallographic defects. The types and structures of these defects can contain a profound effect on the properties of the materials.

Crystalline phases

See: Phase transformations in solids

• Polymorphism is the ability of a solid to exist in more than one crystal form. For example, water ice is ordinarily found in the hexagonal form Ice I_h, but can also exist as the cubic Ice I_c, the rhombohedral ice II, and many other forms.
• Amorphous phases are also possible with the same molecule, such as amorphous ice. In this case, the phenomenon is known as polyamorphism.
• For pure chemical elements, polymorphism is known as allotropy. For example, diamond, graphite, and fullerenes are different allotropes of carbon.

Special cases

A large monocrystal of potassium dihydrogen phosphate grown from solution by Saint-Gobain for the megajoule laser of CEA.

Gallium, a metal that easily forms large single crystals

Ice crystals                                                      Fossil shell with calcite crystals

Since the initial discovery of crystal-like individual arrays of atoms that are not regularly repeated, made in 1982 by Dan Shechtman, the acceptance of the concept and the word quasicrystal have led the International Union of Crystallography to redefine the term crystal to mean “any solid having an essentially discrete diffraction diagram”, thereby shifting the essential attribute of crystallinity from position space to Fourier space. Within the family of crystals one distinguishes between traditional crystals, which are periodic, or repeating, at the atomic scale, and aperiodic (incommensurate) crystals which are not. This broader definition adopted in 1996 reflects the current understanding that microscopic periodicity is a sufficient but not a necessary condition for crystals.

While the term “crystal” has a precise meaning within materials science and solid-state physics, colloquially “crystal” refers to solid objects that exhibit well-defined and often pleasing geometric shapes. In this sense of the word, many types of crystals are found in nature. The shape of these crystals is dependent on the types of molecular bonds between the atoms to determine the structure, as well as on the conditions under which they formed. Snowflakes, diamonds, and table salt are common examples of crystals.

Some crystalline materials may exhibit special electrical properties such as the ferroelectric effect or the piezoelectric effect. Additionally, light passing through a crystal is often refracted or bent in different directions, producing an array of colors; crystal optics is the study of these effects. In periodic dielectric structures a range of unique optical properties can be expected as seen in photonic crystals.

Crystalline rocks

Inorganic matter, if free to take that physical state in which it is most stable, tends to crystallize. There is no practical limit to the size a crystal may attain under the right conditions, and selenite single crystals in excess of 10 m are found in the Cave of the Crystals in Naica, Mexico.^[2]

Crystalline rock masses have consolidated from aqueous solution or from molten magma. The vast majority of igneous rocks belong to this group and the degree of crystallization depends primarily on the conditions under which they solidified. Such rocks as granite, which have cooled very slowly and under great pressures, have completely crystallized, but many lavasglassy matter is frequent. Other crystalline rocks, the evaporites such as rock salt, gypsum and some limestones have been deposited from aqueous solution, mostly owing to evaporation in aridclimates. Still another group, the metamorphic rocks which includes the marbles, mica-schists and quartzites; are recrystallized, that is to say, they were at first fragmental rocks, like limestone, shale and sandstone and have never been in a molten condition nor entirely in solution. The high temperature and pressure conditions of metamorphism have acted on them erasing their original structures, and inducing recrystallization in the solid state.^[3] were poured out at the surface and cooled very rapidly; in this latter group a small amount of amorphous or

Properties

Crystal	Particles	Attractive forces	Melting point	Other properties
Ionic	Positive and negative ions	Electrostatic attractions	High	Hard, brittle, good electrical conductor in molten state
Molecular	Polar molecules	London force and dipole-dipole attraction	Low	Soft, non-conductor or extremely poor conductor of electricity in liquid state
Molecular	Non-polar molecules	London force	Low	Soft conductor

Amorphous solid

From Wikipedia, the free encyclopedia

“Amorphous” redirects here. For amorphousness in computational systems, see amorphous computing. For amorphousness in science fiction, see amorphous creature.

The amorphous structure of glassy Silica (SiO₂) in two-dimensions. No long-range order is present, however there is local ordering with respect to the tetrahedral arrangement of Oxygen (O) atoms around the Silicon (Si) atoms.

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 crystallines or morphous). Most classes of solid materials can be found or prepared in an amorphous form. For instance, common window glass is an amorphous solid, many polymers (such as polystyrene) are amorphous, and even foods such as cotton candy are amorphous solids.

In principle, given a sufficiently high cooling rate, any liquid can be made into an amorphous solid. Cooling reduces molecular mobility. If the cooling rate is faster than the rate at which molecules can organize into a more thermodynamically favorable crystalline state, then an amorphous solid will be formed. Because of entropy considerations, many polymers can be made amorphous solids by cooling even at slow rates. In contrast, if molecules have sufficient time to organize into a structure with two- or three-dimensional order, then a crystalline (or semi-crystalline) solid will be formed. Water is one example. Because of its small molecular size and ability to quickly rearrange, it cannot be made amorphous without resorting to specialized hyperquenching techniques.

Amorphous materials can also be produced by *something* 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. As the cooling is performed, the material changes from a supercooled liquid, with properties one would expect from a liquid stateglass transition temperature or T_g. material, to a solid. The temperature at which this transition occurs is called the

Definition

It is difficult to make a distinction between truly amorphous solids and crystalline solids if the size of the crystals is very small. Even amorphous materials have some short-range order at the atomic length scale due the nature of chemical bonding. Furthermore, in very small crystals a large fraction of the atoms are located at or near the surface of the crystal; relaxation of the surface and interfacial effects distort the atomic positions, decreasing the structural order. Even the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline structures on these length scales.

The transition from the liquid state to the glass, at a temperature below the equilibrium melting point of the material, is called the glass transition. The glass transition temperature, Tg, is the temperature at which an amorphous solid, such as glass or a polymer, becomes brittle on cooling, or soft on heating. More specifically, it defines a pseudo second order phase transition in which a supercooled melt yields, on cooling, a glassy structure and properties similar to those of crystalline materials e.g. of an isotropic solid material ^[1]. Tg is usually applicable to wholly or partially amorphous solids such as common glasses and plastics (organic polymers). Below the glass transition temperature, Tg, amorphous solids are in a glassy state ^[2] and most of their joining bonds are intact. In inorganic glasses, with increased temperature more and more joining bonds are broken by thermal fluctuations so that broken bonds (termed configurons) begin to form clusters. Above Tg these clusters become macroscopic large facilitating the flow of material. In organic polymers, secondary, non-covalent bonds between the polymer chains become weak above Tg. Above Tg glasses and organic polymers become soft and capable of plastic deformation without fracture. This behavior is one of the things which make most plastics useful ^[3]. It is important to note that the glass transition temperature is a kinetic parameter, and thus parametrically depends on the melt cooling rate. Thus the slower the melt cooling rate, the lower Tg. In addition, Tg depends on the measurement conditions, which are not universally defined ^[4]. The glass transition temperature is approximately the temperature at which the viscosity of the liquid exceeds a certain value (about 10¹² Pa·s). The transition temperature depends on cooling rate, with the glass transition occurring at higher temperatures for faster cooling rates. The precise nature of the glass transition is the subject of ongoing research. While it is clear that the glass transition is not a first-order thermodynamic transition (such as melting), there is debate as to whether it is a higher-order transition such percolation type transformation ^[5], or merely a kinetic effect.

Amorphous thin films

Amorphous phases are important constituents of thin films, which are solid layers of a few nm to some tens of µm thickness deposited upon an underlying substrate. So-called structure zone models were developed to describe the microstructure and morphology of thin films as a function of the homologeous temperature T_h that is the ratio of deposition temperature over melting temperature ^[6][7]. According to these models, a necessary (but not sufficient) condition for the occurrence of amorphous phases is that T_h has to be smaller than 0.3, that is the deposition temperature must be below 30% of the melting temperature. For higher values, the surface diffusion of deposited atomic species would allow for the formation of crystallites with long range atomic order.

Regarding their applications, amorphous metallic layers played an important role in the discussion of a suspected superconductivity in amorphous metals ^[8]. Today, optical coatings made from TiO2, SiO2, Ta2O5 etc. and combinations of them in most cases comprise of amorphous phases of these compounds. Much research is carried out into thin amorphous films as a gas separating membrane layer ^[9]. The technologically most important thin amorphous film is probably represented by few nm thin SiO₂ layers serving as isolator above the conducting channel of a metal-oxide semiconductor field-effect transistor MOSFET. Also, hydrogenated amorphous silicon, a-Si:H in short, is of technical significance for thin film solar cells. In case of a-Si:H the missing long-range order between silicon atoms is partly induced by the presence by hydrogen in the percent range.

The occurrence of amorphous phases turned out as a phenomenon of particular interest for studying thin film growth. Remarkably, the growth of polycrystalline films is often preceded by an initial amorphous layer, the thickness of which may amount to only a few nm. The most investigated example is represented by thin multicrystalline silicon films, where such an initial amorphous layer was observed in many studies, see for instance ^[10]. Wedge-shaped polycrystals were identified by transmission electron microscopy to grow out of the amorphous phase only after the latter has exceeded a certain thickness, the precise value of which depends on deposition temperature, background pressure and various other process parameters. The phenomenon has been interpreted in the framework of Ostwald’s rule of stages ^[11] that predicts the formation of phases to proceed with increasing condensation time towards increasing stability ^[8][10]. Experimental studies of the phenomenon require a clearly defined state of the substrate surface and its contaminant density etc., upon which the thin film is deposited.

(Sumber: www.wikipedia.org)

Geopolimer

Geopolimer dapat didefinisikan sebagai material yang dihasilkan dari geosintesis aluminosilikat polimerik dan alkali-silikat yang menghasilkan kerangka polimer SiO₄ dan AlO₄ yang terikat secara tetrahedral (Davidovits, 1994). Saat SiO₂ dan Al₂O₃ terikat secara tetrahedral dengan berbagi atom oksigen, harus ada ion positif (Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, NH⁴⁺) dalam lubang kerangka untuk menyeimbangkan muatan negatif dari Al³⁺ dalam bentuk koordinasi IV. Secara umum, geopolimer memiliki bentuk dasar polisialate dengan rumus empirik sebagai berikut

M_n{-(SiO₂)_z-AlO₂}_n, _wH₂O

dimana M adalah kation seperti ion kalsium, natrium atau kalium dan nz adalah 1, 2, 3. Polisialate memiliki fase amorf hingga semi kristalin. Berikut ini struktur 3 dimensi polisialate amorf hingga semikristalin adalah derajat polikondensasi,

Struktur 3 dimensi Polysialate (Davidovits, 1994)

Reaksi geopolimerisasi dapat digolongkan sebagai reaksi polikondensasi anorganik sebagaimana reaksi pembentukan zeolit. Sebagian besar sintesis zeolit juga dilakukan dalam kondisi basa menggunakan OH^- sebagai mineralising agent (Van Bekkum, et al., 1991). Menurut Van Jaarsveld et al. (1998), garam logam alkali dan/atau hidroksida diperlukan untuk melarutkan silika dan alumina sebagaimana reaksi katalisis dalam reaksi kondensasi. Secara umum skema reaksi geopolimerisasi adalah sebagai berikut

Skema reaksi geopolimerisasi (Davidovits, 1991)

Teknologi geopolimer memiliki keunggulan dalam kemampuannya menghasilkan binder kinerja tinggi dari berbagai sumber aluminosilikat. Berbagai riset telah dikembangkan dalam pembuatan geopolimer dari berbagai sumber aluminosilikat, seperti yang telah dilakukan oleh Davidovits (1989, 1991, 1993), Palomo et al. (1992), Barbosa et al. (2000), Cioffi et al. (2003), Kriven et al. (2003), dan Schmu¨cker and MacKenzie (2005) yang mengembangkan geopolimer dari metakaolinit. Di samping itu, Ikeda et al. (1998), Xu dan van Deventer (2000), Swanepoel and Strydom (2002), van Jaarsveld et al. (2002, 2003) dan Bakharev (2005) juga telah mengembangkan geopolimer dari sumber aluminosilikat yang lain, seperti fly ash.

Geopolimer sangat menarik karena sifat mekanik dan durabilitas yang mengagumkan (Palomo et al., 1992; Davidovits and Davidovics, 1988) juga stabilitas termal dan ketahanan terhadap asam. Schmu¨cker and MacKenzie (2005) telah membuktikan bahwa komposisi matriks geopolimer tidak berubah setelah dipanaskan pada 1200°C. Material geopolimer dengan kandungan Ca lebih rendah memiliki ketahanan terhadap asam lebih baik daripada material dari semen portland (Bakharev, 2005).

Geopolimer

Aplikasi lain dari geopolimer adalah sebagai bahan untuk imobilisasi bahan limbah berbahaya, material bangunan tahan api, panel untuk insulasi thermal, refractory dan aplikasi high-tech tahan api lainnya seperti interior pesawat dan otomotif. Bahkan sepanjang Grand Prix 1994 dan 1995, tim F1 Benneton telah mendesain perisai thermal yang unik dengan bahan dasar komposit geopolimer. Semua bagian di sekitar exhaust area dilapsi geopolimer untuk menggantikan titanium. Hasilnya, mereka juara pada tahun tersebut karena geopolimer mampu mengurangi getaran dan panas mesin. Sampai sekarang, sebagian besar tim F1 memakai material komposit geopolimer (Davidovits, 2002).

Macam2 aplikasi geopolimer

http://myscoutchemistry.wordpress.com/category/chem_is_try/