Monday, January 3, 2011

【 Weak current College 】 TFT-LCD LCD monitor working principle】



LCD monitor refers to a mass produced by using the LCD monitor. today, liquid crystal display this name refers to the use of most of the notebook computer, or a desktop computer application monitor-it is thin-film transistor liquid crystal display for its English name-Thin-filmtransistorliquidcrystaldisplay, short of TFTLCD. from its English name we can know, this kind of monitor its composing major has two features, one is the thin film transistors, another is the LCD itself. we start with the LCD itself.

Liquid crystal (LC, liquidcrystal) classification

We generally think that substances like water has three States, namely solid-state liquid with gaseous. in fact, the three States of matter is for water, for different substances, there might be other different state exists. we have to talk about in liquid crystal State, it is between solid and liquid between a State, this State is only one phase of the process of change (see Figure 1), as long as the material with the above procedure, namely in solid and liquid between this state exists, it will call the physicist-liquid crystal

The liquid crystals for the first time, now has spent more than 100 years in the year 1888, was Austria's botanist FriedrichReinitzer found, in its observations on the plant separation and purification of acid cholesterol (cholesterylbenzoate) the thawing of the Act found that this compound heating to 145.5 degrees ° c, melting solid will, rendering a cross between solid and liquid melt flow between half white turbid liquid. this condition will always maintain the temperature rise to 178.5 ° c, to form a clear peers of liquid (isotropicliquid). the next year, in 1889, the study of phase transition and thermodynamic equilibrium of Germany physicist O.Lehmann, compounds for more detailed analysis of the polarized light microscope, he found that half of this thick whitish fluid, liquid compounds with heteroscedastic of Crystal-specific dual refractive index (birefringence) of optical properties, optical paradoxical nature (opticalanisotropic). it seems to be such a Crystal liquid named LCD. Since then, scientists will this new-found in nature, referred to as the fourth State of matter-LCD (liquidcrystal). it at a particular temperature, will have both liquid and solid features.
General water, solid in lattice because heating, heat absorption and undermine the lattice started, when the temperature exceeds the melting point is solubility into liquid. thermally induced Nematic Liquid Crystals are different (see Figure 2), when its solid state after heated, and will not be directly into liquid, will be dissolved form liquid crystal state. when your continuous heating time, will be dissolved into liquid (peers of liquid)-this is the so-called second dissolution phenomena. liquid crystal State as the name implies, it has solid state of lattice, and liquid fluidity. when liquid crystal just found, because many types, different research fields of the liquid crystal will have different classifications in 1922 by G.Friedel polarizing microscope the observed results, the LCD roughly divided into three classes NematicSmectic and Cholesteric-but if it is arranged according to molecular orderliness to points (see Figure 3), it can be divided into the following four categories:

1. layered LCD (Sematic):

Its structure is made of liquid crystal rod-like molecules together to form a layer with a layer of the structure. each level of molecules long axis parallel-and this long axis of the direction for each layer of the plane is a vertical or inclination. because of its structure is very similar to the Crystal, also known as do nearly crystalline. its order parameter S (orderparameter) approach 1. layered-type liquid crystal layer and layer of bond fracture due to temperature and, therefore, layers and layers easier sliding. However, each layer within a molecular bond stronger, so less interrupted. therefore the monolayer, thearrangement not only ordered and viscous larger. If we use the phenomenon of giant concept to describe the physical properties of liquid crystals, we can put a group of regional average of liquid crystal molecules to a Director (director), this is a group of regional average direction of liquid crystal molecules to layered LCD but, due to its liquid crystal molecules form layered structure, therefore also the Director of different then classification out different layered LCD. when its long axis of the liquid crystal molecules are vertical stand, call it "SematicAphase". If liquid crystal molecules of the long axis of the direction of a certain standing tilt angle (tilt), call it "A SematicCphase"., C, and other letters to name, this is in accordance with the order of discovery to call, and so on, should be the existence of a "SematicBphase" is later found Bphase. but is in fact a deformation Cphase, because Cphase structure of chiral if the belt is Bphase. i.e. ChiralsematicCphase is SematicBphase (please see Figure 4). the structure of a layer with a layer of liquid crystal molecules, in addition to every layer of liquid crystal molecules have the inclination angle, a layer of a layer between the skew angle also formed as helical structure.

2. line LCD (Nematic):

The word is Greece Nematic, representative of meanings and English thread is the same. mainly because Visual observation of the LCD, look like silk General drawings. such liquid crystal molecule in space has a dimension of rules of order, all the major axis liquid crystal molecular Rod will choose a particular direction (the Director) as spindle and parallel arrangement. Furthermore, unlike like layered LCD with a layered structure with layer comparison of Nematic Liquid crystalsComparison of disorderly arranged them, that is, the order parameter S a layered-type liquid crystal smaller. Furthermore its viscosity is relatively small, therefore more vulnerable to mobile (it mobility mainly formolecular long axis orientation easier for freedom movement). Line LCD is now of TFT LCD display common TN (Twistednematic) type LCD.

3. cholesterol LCD (cholesteric):

The name of the source, because they are mostly composed of cholesterol derivatives generated. but there are no cholesterol structure of liquid crystal will have this LCD liquid crystal phase. such as shown in Figure 5, if you put it in a layer with a layer of separation, would very much like line LCD. However in the Z axis, will find it Director as a layer with a layer of like spiral-like distribution, and when it points to the vector to rotate 360 degrees desired molecular layer thickness is called pitch. just because each layer is very similar to line LCD, also called Chiralnematicphase. taking cholesterol LCD, and Director of vertical distribution of liquid crystal molecules, because it points to the different vector would have different optical or electrical differences, thus creating a different characteristic.

4. the disc-shaped liquid crystal (disk):

Also known as columnar liquid crystals to one of the LCD, it is long like disc-shaped (disk), but their order is like a columnar (discoid).

If we are to points of high and low molecular weight it can be divided into the polymer liquid crystal (polymerliquidcrystal, aggregate number of liquid crystal molecules into) and two low-molecular liquid crystal-in this classification, TFT LCD display is part of the application of low-molecular liquid crystal-if the formation of liquid crystal State, you can split because temperature formation of liquid crystal State thermally induced Nematic (thermotropic), and because the concentration of lyotropic liquid crystal state of Nematic Liquid crystals (lyotropic). taking before mentioned, classification, layered LCD LCD with linear as most of the heat-induced liquid crystal, is formed as the temperature changes in liquid crystal state.-for a lyotropic liquid crystal, need to consider the molecules dissolved in solvents of low when concentration, molecular is messy in solvents which peers of solution, but when the concentration increases greater than a critical concentration, because the molecule is not enough space to form a messy start distribution, part of the molecules together to form a rule in order to reduce the space of impeding. therefore form a party (anisotropic) solution so lyotropic liquid crystal formation is the liquid crystal molecules in appropriate solvents reaches a critical concentration, will form a lyotropic liquid crystal state. LCD has one of the best example is SOAP. when bubbles in the water and will not immediately into a liquid, but its water bubble long after the formation of a white-like substance that is its liquid crystal state.

Electrical and optical properties of liquid crystals

Due to the structure of liquid crystal molecules for different parties (Anisotropic), so the photoelectric effect because direction differently, in short the liquid crystal molecules in permittivity and refraction coefficient etc photoelectric characteristics have different parties, which we can use these properties to change the incident light intensity in order to form a gray-scale, to apply to the display components on the following we would like to discuss is the electrical optical liquid crystal belongs to related properties with, some of the following:

1. the permittivity ε (dielectricpermittivity):

We can separate the dielectric factor into two directions of the component, namely ε//(and Director of parallel component) and ε ⊥ (and links to the components of the vector vertical). when ε ε//> is called ⊥ permittivity vary depending on the side of the LCD is positive-type, can be used in parallel coordination. ε ε//< ⊥ is called permittivity vary according to the parties of a negative-only available in the LCD, in the vertical coordination can be required of the photoelectric effect. when applied electric field, the liquid crystal molecules from different sides of permittivity is positive or negative, to decide steering of liquid crystal molecules are parallel or perpendicular to the field, to decide whether or not the light penetration. Now on common TFTLCD TN-LCD mostly belong to dielectric factor is of permittivity LCD. when different parties of Δ ε (= ε ε//-⊥) is bigger, the LCD's critical voltage (thresholdvoltage) will be smaller. this LCD can low voltage operation.

2. the refraction coefficient (refractiveindex):

Since most of the liquid crystal molecules by rod or saucer-shaped molecules are formed, with molecular long axis parallel or perpendicular to the direction of physics characteristics will have some differences, the liquid crystal molecules are also referred to as the exclusive party of crystals. And permittivity, refraction coefficient also pursuant to point to a vector vertical and horizontal direction, into two direction vector. respectively n//and n ⊥.

In addition to the single-axis (uniaxial) Crystal, originally has two different definitions of refraction coefficient to no one, it is for the refraction coefficient ordinaryray, so he shortened into no. and ordinaryray refers to its light weight of the electric field is perpendicular to the axis of that of another is a ne, it means for the refraction coefficient extraordinaryray, extraordinaryray refers to its light weight of the electric field is parallel to the optical axis. at the same time also defines double refractive index (birefrigence) Δ n = ne-no for the above two refractive index difference.

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