</b> Liquid crystal (Liquid Crystal) is a kind of polymer material, because of its special physical, chemical, and optical properties, the middle of the 20th century began to be widely used in the thin and light display technology.
People are familiar with the state of matter (also known as phase) for gas, liquid, solid, more rusty plasma and liquid crystal (Liquid Crystal, referred to as LC). Liquid crystal phases are produced by combinations of molecules with special shapes, which can flow and have the optical properties of crystallization. The definition of liquid crystal is now broadened to include substances that can be liquid crystal phases at a certain temperature range and normal crystallization at lower temperatures. Liquid crystals are composed of organic compounds, which are compounds with carbon as the center. At the same time with two substances of liquid crystals, is a combination of intermolecular forces, their special optical properties, and sensitive to electromagnetic fields, extremely practical value.
In 1888, the Austrian scientist called Reinitzel, synthesized a strange organic compound, which has two melting points. Heating its solid crystals to 145 ° C, it melted into a liquid, only turbid, while all pure substances are transparent when melted. If you continue to heat to 175 ℃, it seems to melt again, into a clear and transparent liquid. Later, the German physicist Lehmann in the "middle ground" of the turbid liquid called crystal. It is like neither a horse nor a donkey mule, so some people call it the mule of the organic world. Since the discovery of liquid crystals, people do not know what it is used until 1968, it is used as a material in the electronics industry.
The most common use of liquid crystal display material is the electronic watch and calculator display board, why will display numbers? The original liquid optoelectronic display material, the use of liquid crystal electro-optical effect of electrical signals into characters, images and other visible signals. Liquid crystal in normal circumstances, its molecular arrangement is very orderly, appear clear and transparent, once added to the DC electric field, the arrangement of molecules is disrupted, a part of the liquid crystal becomes opaque, the color deepens, and thus can display figures and images.
The electro-optical effect of liquid crystals refers to its interference, scattering, diffraction, spinning, absorption and other optical phenomena modulated by the electric field.
Some organic compounds and polymers, in a solution of a certain temperature or concentration, have both the fluidity of a liquid and the anisotropy of a crystal, which is a liquid crystal. Liquid crystal photoelectric effect is controlled by the temperature conditions of the liquid crystal is called thermal liquid crystal; solvated liquid crystal is controlled by the concentration conditions. Liquid crystals for display are generally low molecular thermotropic liquid crystals.
According to the liquid crystal will change color characteristics, people use it to indicate the temperature, alarm gas. For example, liquid crystals can change color from red to green and blue as the temperature changes. This can indicate the temperature in a certain experiment. Liquid crystal meets hydrogen chloride, hydrocyanic acid and other toxic gases, will also change color. In the chemical plant, people hang the liquid crystal on the wall, once a trace of poisonous gas escapes, liquid crystal discoloration, to remind people to hurry to check, fill the leak.
There are many types of liquid crystals, usually classified according to the characteristics of the liquid crystal molecule's central bridge bond and ring. At present, more than 10,000 kinds of liquid crystal materials have been synthesized, of which thousands of commonly used liquid crystal display materials, mainly biphenyl liquid crystal, phenyl cyclohexane liquid crystal and ester liquid crystal. Liquid crystal display materials have obvious advantages: low driving voltage, tiny power consumption, high reliability, large amount of display information, color display, no flicker, no harm to the human body, automation of the production process, low cost, can be made into a variety of specifications and types of liquid crystal displays, easy to carry and so on. Due to these advantages. Computer terminals and televisions made of liquid crystal materials can be greatly reduced in size, etc.. Liquid crystal display technology has a profound impact on the structure of display imaging products, and promote the development of microelectronics technology and optoelectronic information technology.
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History of Liquid Crystals
Liquid crystals, a crystalline liquid, were discovered as early as 1850 by the Prussian physician Rudolf Filschow. As early as 1850, the Prussian doctor Rudolf Virchow (Rudolf Virchow) and others found that the extract of nerve fibers contain an unusual substance. 1877, the German physicist Otto Lehmann (Otto Lehmann) found that the liquid crystals are a crystalline liquid. Otto Lehmann, a German physicist, first observed the phenomenon of liquid crystallization using a polarized light microscope, but he did not understand the cause of this phenomenon.
Friedrich Leinzer, a plant physiologist at the German University in Prague, Austria, was the first to observe liquid crystallization using polarized light microscopy. Reinitzer (Friedrich Reinitzer) in the heating of benzoic acid cholesterol lipid (Cholesteryl Benzoate) to study the role of cholesterol in plants, on March 14, 1883, observed cholesteryl benzoate in the heat melting of the abnormal behavior. It melted at 145.5°C, producing a cloudy substance with a luster. After the temperature rose to 178.5°C, the luster disappeared and the liquid became clear. This clarified liquid cooled slightly, the turbidity reappeared, momentarily blue, and again the color was blue-violet a moment before crystallization began.
Lenizer repeated to determine his findings, to the German physicist Lehmann for advice. At that time, Lehmann built a microscope with a heating function to explore the process of crystallization of liquid crystals cooling, and later even more on the polarizing lens, it is the most in-depth study of Lenizer's compounds of the instrument. From that time on, Lehmann's efforts were entirely focused on this class of substances. He first called them soft crystals, then crystalline fluids, and finally became convinced that the polarized light property was specific to crystallization, and that the name Fliessende kristalle (flowing crystals) was correct. The difference between this name and the liquid crystal (Flussige kristalle) was only one step. Reiniger and Lehmann were later credited as the fathers of liquid crystals.
The oxygen azide ether synthesized by L. gattermann, A Ristschke, was also identified by Lehmann as a liquid crystal. But in the 20th century, famous scientists such as tammann (G. tammann) thought that lehman and other observations, only very fine crystals suspended in the body of the colloid phenomenon. W. Nernst, on the other hand, thought that liquid crystals were just a mixture of isomers of compounds. However, the efforts of the chemist D. Vorlander (D. Vorlander) by the aggregation of experience to enable him to predict which class of compounds is most likely to show liquid crystal properties, and then synthesized to obtain such compounds, the theory was then proved.
Physical Properties of Liquid Crystals
When energized, liquid crystals conduct, arranging themselves in an orderly fashion so that light passes through them easily; when they are not energized, they arrange themselves in a chaotic fashion, preventing light from passing through them. Allowing liquid crystals to block or allow light to pass through like a gate. Technically speaking in simple terms, LCD panels contain two pieces of fairly delicate sodium-free glass material called Substrates with a layer of liquid crystal sandwiched between them. When a beam of light passes through this layer of liquid crystals, the liquid crystals themselves stand in a row or twist in an irregular pattern, thus blocking or allowing the beam of light to pass through smoothly. Most liquid crystals are organic complexes, consisting of long, rod-shaped molecules. In their natural state, the long axes of these rod-shaped molecules are roughly parallel. When liquid crystals are poured into a well-machined, slotted plane, the liquid crystal molecules follow the slots, so if those slots are perfectly parallel, the molecules are also perfectly parallel.
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Classification of liquid crystals
nematic
smectic
cholesteric
discotic
thermotropic LC)
Reproducible liquid crystals (reproducrant LC)
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Liquid crystal use
Liquid crystals should be stirred well before use before being perfused and used. Liquid crystals with solid chiral agent added should be heated up to 60 degrees Celsius, then cooled quickly to room temperature and stirred well. And in the process of use can not be static time is too long. Especially low threshold voltage liquid crystals, due to the low threshold voltage liquid crystals have these different characteristics, so in the use of these liquid crystals should pay attention to the following aspects:
Liquid crystal in the use of the former should be fully stirred, the liquid crystal should be deployed immediately into the production of the use of the liquid crystals, as far as possible to shorten the time of storage, to avoid the phenomenon of chromatography.
The liquid crystals should be covered with a light shield and deposited, and try to use up in a shift (eight hours), the liquid crystals need to be recycled and stirred to re-test the voltage and then used. Generally with the extension of time, the driving voltage will increase.
After the liquid crystal is taken from the original bottle, the original bottle should be capped in time to keep it shaded, and reduce the time of open exposure in the air. Generally, too long exposure in the air will increase the leakage current of the liquid crystal.
Filling low-threshold voltage LCD chip empty box is best from the PI solid baking to fill the liquid crystal process, the flow of the production time within twenty-four hours of the empty box, filling liquid operation generally use a relatively low filling speed.
Low-threshold voltage liquid crystal in the sealing must be covered with a suitable hood, and in the entire period of filling liquid crystal in addition to sealing glue curing period, to try to stay away from ultraviolet sources. Otherwise misdirection and increased threshold voltage will occur close to the UV light.
Liquid crystal is an organic polymer substance, it is easy to dissolve in various solvents or react with other chemicals, liquid crystal itself is also a good solvent, so try to stay away from other chemicals during use and storage.
In 1922, the Frenchman Friedel (G. Friedel) carefully analyzed the then known liquid crystals, and divided them into three categories: to column type (nematic), layer column type (smectic), cholesterol type (cholesteric). The origin of the name, the first two are taken from the Greek linear and detergent (soap); cholesteric type of the name has a historical significance, such as with the recent classification, they belong to the hand to column type. In fact, Fried disagreed with the term liquid crystal, and thought that 'intermediate phase' was the most appropriate expression.
Discotic liquid crystals, discovered in the 1970s, are columnar or columnar systems composed of overlapping protomolecules with high symmetry. In addition to type classification, liquid crystals are classified into thermotropic LC and lypotropic LC due to the different conditions (conditions) under which they are produced, which are generated by heating, adding solvents to form liquid crystals, and forming a liquid crystal phase.
An example of lyotropic liquid crystal generation is soap water. In high concentrations, the soap molecules are layered, and between the layers are water molecules. At lower concentrations, the combination is different.
In fact, a substance can have multiple liquid crystal phases. It was also found that the two liquid crystal mixtures heated to get isotropic liquid and then cooled, can be observed next to the column type, layer column type liquid crystals. This phase change of the substance, known as the reproducible liquid crystal (reproducrant LC). Liquid crystal molecular structure.
Stabilized liquid crystal phase is the van der Waals force between molecules. Because of the high density of molecular aggregation, repulsive anisotropy has a greater impact, but the attraction is to maintain the high density, so that the collective to the liquid crystal state of the force, hearing and attraction of mutual checks and balances is very important. Another example of molecules with polar groups, dipole interaction becomes important attraction.
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The uses of liquid crystals
One of the consequences of the arrangement of liquid crystal molecules is that they exhibit selective light scattering. Because the arrangement can be affected by external forces, liquid crystal materials have great potential for manufacturing devices. Chiral nematic liquid crystals, which range between two glass plates, can be treated with certain procedures to form different textures.
Steroidal liquid crystals, because of the spiral structure and selective reflection of light, the use of white light in the circular polarization, the simplest is based on the principle of color change made of thermometers (often seen in the fish tank thermometer). In the medical field, the detection of skin and breast cancers can also be done by applying steroid liquid crystals to the suspected areas and then comparing the color with normal skin (because cancer cells metabolize faster than normal cells, so the temperature will be higher than normal cells).
Electric and magnetic fields have a huge impact on liquid crystals, and the dielectric behavior of nematic liquid crystal phases is the basis for all kinds of photovoltaic applications (the use of liquid crystal materials to create displays with an applied electric field has been developing rapidly since the 1970s. This is because they have many advantages such as small volume, low power consumption, low operating voltage, and easy design of multi-color surface panels. However, because they are not light-emitting displays, their clarity in the dark, viewing angle and ambient temperature limitations are not ideal. In any case, the screens of TVs and computers are manufactured with liquid crystal material, which is very favorable. Large screens were constrained in the past by the need for high voltages, and the size and weight of the transformers were unspeakable. In fact, the color projection system, can also use chiral to the column type liquid crystal to manufacture such as polarized surface plate, filter, photoelectric adjuster.
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LCD panels
LCD panels and LCD monitors have a very close relationship, LCD panel production, quality and a variety of factors are linked to the quality of LCD monitors themselves, the price and market direction. Among them, the LCD panel is related to the player's most important response time, color, viewing angle, contrast and other parameters. From the LCD panel can be seen from the performance of this LCD monitor, how the quality? Xiaolin on the Internet to find the LCD panel information, as long as it is for the current mainstream LCD panel, so that we have a bottom in the heart when buying LCD monitors.
VA-type: VA-type LCD panels in the current monitor products are more widely used, used in high-end products, 16.7M color (8bit panel) and large viewing angle is its most obvious technical characteristics, the current VA-type panels are divided into two kinds: MVA, PVA.
MVA-type: the full name is (Multi-domain) Vertical Alignment), is a multi-quadrant vertical alignment technology. It is the use of protrusions so that the liquid crystal at rest is not the traditional upright, but biased towards a certain angle at rest; when the voltage is applied to let the liquid crystal molecules change into a horizontal in order to allow the backlight to pass through the faster, so that you can significantly shorten the display time, but also because of the protrusions to change the orientation of the liquid crystal molecules to make the field of view angle is more extensive. The increase in viewing angle can be up to 160 degrees or more, and the response time can be reduced to less than 20ms.
PVA type: Samsung launched a panel type, is a kind of image vertical adjustment technology, the technology directly change the structure of the liquid crystal unit, so that the display efficiency greatly improved can get better than MVA brightness output and contrast. In addition, on the basis of these two types and extend the improved S-PVA and P-MVA two types of panels, in the development of technology tends to be more up, the viewing angle of up to 170 degrees, the response time is controlled within 20 milliseconds (using Overdrive acceleration up to 8ms GTG), and the contrast can easily exceed the high level of 700:1, Samsung's own brand of the majority of the product for the The PVA LCD panels are the most common type of LCD panels in Samsung's product lineup.
IPS-type: IPS-type LCD panels have a large viewing angle, color fine and other advantages, look more transparent, which is also a way to identify IPS-type LCD panels, PHILIPS many LCD monitors use IPS-type panels. S-IPS, on the other hand, is the second generation of IPS technology, which in turn introduces some new technologies to improve the phenomenon of gray-scale reversal of the IPS mode at some specific angles. LG and Philips autonomous panel makers are also launching LCD panels with IPS as a technical feature.
TN type: This type of LCD panel is used in entry-level and mid-range products, affordable, inexpensive, and chosen by many manufacturers. In terms of technology, compared with the first two types of LCD panels in the technical performance is slightly inferior, it can not show 16.7M brilliant colors, can only reach 16.7M colors (6bit panel) but the response time is easy to improve. Viewing angle is also subject to certain restrictions, the viewing angle will not exceed 160 degrees. Now on the market in general within 8ms response time of the products are mostly used in the TN LCD panel.
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Liquid Crystal Display
Liquid Crystal Display, or LCD (Liquid Crystal Display), is a flat, ultra-thin display device, which consists of a certain number of color or black-and-white pixels placed in front of a light source or reflective surface. Liquid crystal displays consume very little power and are therefore favored by engineers for battery-operated electronic devices.
Each pixel consists of the following parts: an array of liquid crystal molecules suspended between two transparent electrodes (indium tin oxide), and two polarization filters whose polarization directions are perpendicular to each other. Without the liquid crystals between the electrodes, the light passing through one of the filters would inevitably be blocked by the other, and the direction of polarization of the light passing through one of the filters is rotated by the liquid crystals to allow it to pass through the other one.
The liquid crystal molecules themselves have an electrical charge, and by adding a small amount of charge to the transparent electrodes of each pixel or sub-pixel, the molecules of the liquid crystal will be rotated by electrostatic forces, and the light passing through will be rotated at the same time, changing a certain angle, so that it will be able to pass through the polarizing filter.
Before the charge is applied to the transparent electrodes, the liquid crystal molecules are in an unconstrained state, and the charge on the molecules causes these molecules to form a helix or a ring (crystalline), and in some LCDs, the chemical surfaces of the electrodes act as crystalline seeds for the crystals so that the molecules crystallize at the desired angle, and the light rays that pass through one filter rotate in a polarized line of defense after passing through the liquid chip, allowing the light to pass through the other polarized filter. light can pass through another polarizer, a small portion of the light is absorbed by the polarizer, but the rest of the device is transparent.
After adding an electric charge to the transparent electrode, the liquid crystal molecules will be aligned in the direction of the electric field, thus limiting the rotation of the direction of polarization of the light through the liquid crystal molecules, if the liquid crystal molecules are completely dispersed, the direction of polarization of the light through the liquid crystal molecules will be completely perpendicular to the second polarizer, and therefore be completely blocked by the light, at this time the pixel does not emit light, through the control of the direction of rotation of the liquid crystals in each pixel, we can control the light that illuminates the pixel. By controlling the direction of rotation of the liquid crystal in each pixel, we can control the amount of light that illuminates the pixel, either more or less.
Many LCDs turn black under alternating current, which destroys the spiral effect of liquid crystals, and turn off the current and the LCD becomes bright or transparent.
In order to save power, LCD displays use multiplexing, in multiplexing mode, one end of the electrodes grouped together, each group of electrodes connected to a power supply, the other end of the electrodes are also grouped together, each group is connected to the power supply at the other end of the grouping design to ensure that each pixel is controlled by an independent power supply, electronic devices or software that drives the electronic devices by controlling the power supply on/off sequence The electronics or software driving the electronics controls the on/off sequence of the power supply, and thus the display of the pixels.
The metrics for examining an LCD monitor include the following important aspects: display size, response time (sync rate), array type (active and passive), viewing angle, supported colors, brightness and contrast, resolution and screen aspect ratio, and input interfaces (such as visual interfaces and video display arrays).
A brief history
The first operational LCD was based on Dynamic Scattering Mode (DSM), which was developed by a group led by George Heilman of RCA. The LCD was developed by a team led by George Heilmann of RCA, who founded Optech, a company that developed a series of LCDs based on this technology. In December 1970, the rotating nematic field effect of liquid crystals was patented in Switzerland by the Central Laboratory of Szent and Helfrich Hoffmann-Leroux. In 1969, James K. Furgeson's patent was registered at the Ohio State University in the U.S. In 1969, James Ferguson discovered the rotating nematic field effect of liquid crystals at Kent State University (Ohio University) and registered the same patent in February 1971 in the U.S. In 1971, his company (ILIXCO) produced the first LCD based on this characteristic, which soon replaced the poor performance of the DSM-type LCDs.
The display is realized by utilizing the basic properties of liquid crystals. Natural light is "filtered" into linearly polarized light through a polarizer, and because the twisted pitch of liquid crystal molecules in the box is much larger than the wavelength of visible light, when the liquid crystal molecules along the surface of the orientation film are aligned in the same direction or orthogonal to the linearly polarized light incidence, the direction of its polarization will be twisted by 90° through the entire liquid crystal layer and then ejected out from the other side, and the orthogonal polarizer plays a role in transmitting light. The orthogonal polarizer plays the role of light transmission; if a certain value of voltage is applied to the liquid crystal box, the long axis of the liquid crystal begins to tilt along the direction of the electric field, when the voltage reaches about 2 times the threshold voltage, in addition to the electrode surface of the liquid crystal molecules, all the liquid crystal molecules between the two electrodes in the liquid crystal box become rearranged along the direction of the electric field, at which time the function of 90 ° spinning disappears, and the spinning effect is lost between the orthogonal sheet oscillator, so that the device Can not transmit light. If you use a parallel polarizer is the opposite.
It is this way to use the liquid crystal box to energize or de-energize the method to make the light to change the state of its transmittance - cover, so as to achieve the display. When the upper and lower polarizers are oriented orthogonally or parallel, the display appears in constant white or constant black mode.
Transmissive and reflective displays
LCDs can be either transmissive or reflective, depending on where the light source is placed. Transmissive LCDs are illuminated by a light source behind the screen, while viewing is on the other side (front) of the screen. This type of LCD is mostly used in applications that require high brightness displays, such as computer monitors, PDAs, and cell phones. The lighting used to illuminate the LCD tends to consume more power than the LCD itself.
Reflective LCDs, commonly found in electronic clocks and computers, illuminate the screen by (sometimes) reflecting light back from the outside by a reflective surface that scatters behind it. This type of LCD has a high contrast ratio because the light is cut twice as it passes through the liquid crystal twice. The power consumption is significantly reduced by not using an illuminator, so the battery life of devices that use batteries is longer. Because a small reflective LCD consumes so little power that a bare battery is enough to power it, it is often used in pocket-sized calculators.
Semi-permeable reflective LCDs can be used as either transmissive or reflective. When there is a lot of external light, the LCD works as a reflective type, and when there is not enough external light, it works as a transmissive type.
Color display
In a color LCD, each pixel is divided into three cells, or subpixels, with additional filters labeled red, green, and blue. The three subpixels can be controlled independently, and the corresponding pixels produce thousands or even millions of colors. Older CRTs used the same method to display colors. The color components are arranged according to different pixel geometries as needed.
Common LCD dot pitch
Table of common LCD dot pitches:
12.1-inch (800×600) - 0.308 mm
12.1-inch (1024×768) - 0.240 mm
14.1-inch (1024×768) - 0.279 mm
14.1-inch (1400×1050) - 0.204 mm
15-inch (1024×768) - 0.297 mm
15-inch (1400×1050) - 0.218 mm
15-inch (1600×1200) - 0.190 mm
16-inch ( 1280×1024) - 0.248 mm
17-inch (1280×1024) - 0.264 mm
17-inch widescreen (1280×768) - 0.2895 mm
17.4-inch (1280×1024) - 0.27 mm
18-inch (1280× 1024) - 0.281 mm
19-inch (1280×1024) - 0.294 mm
19-inch (1600×1200) - 0.242 mm
19-inch widescreen (1440×900) - 0.283 mm
19-inch widescreen (1680×1050) - 0.243 mm
19-inch widescreen (1680×1050) - 0.243 millimeters
20-inch widescreen (1680×1050) - 0.258 millimeters
20.1-inch (1200×1024) - 0.312 millimeters
20.1-inch (1600×1200) - 0.255 millimeters
20.1-inch (2560×2048) - 0.156 mm
20.8-inch (2048×1536) - 0.207 mm
21.3-inch (1600×1200) - 0.27 mm
21.3-inch (2048×1536) - 0.21 mm
22-inch widescreen (1600×1024) - 0.294 mm
22.2-inch (3840×2400) - 0.1245 mm
23-inch widescreen (1920×1200) - 0.258 mm
23.1-inch (1600×1200) - 0.294 mm
24-inch widescreen (1920×1200) - 0.27 mm
24-inch widescreen (1920×1200) - 0.27 mm
26-inch widescreen (1920×1200) - 0.287 mm
It's not just 20-inch widescreen LCDs, but 17-inch, 23-inch widescreen, and 24-inch widescreen LCDs basically have the problem of having too little text. Suitable monitors for Internet surfing and word processing include the five sizes of 15-inch, 19-inch, 19-inch widescreen, 22-inch widescreen, and 26-inch widescreen, all of which have larger dot spacing and the right size text display.
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Advantages of LCD screens
The radiation from an LCD screen can be negligibly small, equivalent to a few watts of light bulb. The radiation to the human body is minimal.