& lt/b & gt； Liquid crystal is a polymer material. Because of its special physical, chemical and optical characteristics, it has been widely used in thin and light display technology since the mid-20th century.
The familiar states of matter (also known as phases) are gas, liquid and solid, while the unfamiliar ones are plasma and liquid crystal (LC). Liquid crystal phase can only be produced by molecular combination with special shape. They can flow and have crystalline optical properties. Now the definition of liquid crystal is relaxed to include substances that can be liquid crystal phase in a certain temperature range and can crystallize normally at a lower temperature. The components of liquid crystal are organic compounds, that is, compounds centered on carbon. At the same time, liquid crystals with two substances are combined by intermolecular forces. Their special optical properties and sensitivity to electromagnetic fields have great practical value.
1888, an Austrian scientist named Leinitzel synthesized a strange organic compound with two melting points. When its solid crystal is heated to 145℃, it melts into a liquid, which is only turbid, while all pure substances are transparent when melted. If heated to 175℃, it seems to melt again and become a clear and transparent liquid. Later, German physicist Li Man called this turbid liquid "intermediate zone" crystal. It looks like a mule that is neither like a horse nor a donkey, so some people call it an organic mule. Since the discovery of liquid crystal, people didn't know its use until 1968.
The most common use of liquid crystal display materials is the display board of electronic watches and calculators. Why do they show numbers? It turns out that this kind of liquid crystal photoelectric display material uses the electro-optical effect of liquid crystal to convert electrical signals into visual signals such as characters and images. Under normal circumstances, the molecular arrangement of liquid crystals is very orderly and appears clear and transparent. Once the DC electric field is applied, the molecular arrangement is disrupted, and some liquid crystals become opaque and dark in color, so that numbers and images can be displayed.
The electro-optic effect of liquid crystal refers to the optical phenomena such as interference, scattering, diffraction, optical rotation and absorption modulated by electric field.
Some organic compounds and polymers, in a solution with a certain temperature or concentration, have both liquid fluidity and crystal anisotropy, which is called liquid crystal. Liquid crystals whose photoelectric effect is controlled by temperature conditions are called thermotropic liquid crystals; Lyotropic liquid crystals are controlled by concentration conditions. Liquid crystals used for display are usually low molecular thermotropic liquid crystals.
According to the color change of liquid crystal, people use it to indicate temperature, alarm poison gas and so on. For example, liquid crystal can change from red to green and blue with the change of temperature. This can indicate the temperature in the experiment. Liquid crystal will change color when it meets toxic gas such as hydrogen chloride and hydrocyanic acid. In chemical plants, people hang liquid crystal tablets on the walls. Once a trace of toxic gas escapes and the liquid crystal changes color, it reminds people to check and fill the loopholes quickly.
There are many kinds of liquid crystals, which are usually classified according to the characteristics of the central bridge bond and ring of liquid crystals. At present, more than 10000 kinds of liquid crystal materials have been synthesized, among which there are thousands of commonly used liquid crystal display materials, mainly biphenyl liquid crystal, phenylcyclohexane liquid crystal and ester liquid crystal. Liquid crystal display materials have obvious advantages: low driving voltage, low power consumption, high reliability, large display information, color display, no flicker, harmless to human body, automatic production process, low cost, and can be made into liquid crystal displays of various specifications and models, which is convenient to carry. Due to these advantages. Computer terminals and televisions made of liquid crystal materials can be greatly reduced in size. Liquid crystal display technology has a far-reaching impact on the structure of display imaging products, and promoted the development of microelectronics and photoelectric information technology.
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The history of liquid crystal
Crystal liquid crystal-liquid crystal as early as 1850, Prussian doctor Rudolf? Virchow and others found that the extract of nerve fibers contained an unusual substance. 1877, German physicist Otto? Otto lehmann observed the liquid crystal phenomenon for the first time with a polarizing microscope, but he didn't know the reason of this phenomenon.
Frederick, a plant physiologist at the German University in Prague, Austria? Friedrich Reinitzer studied the role of cholesterol in plants by heating cholesteryl benzoate, and observed the abnormal behavior of cholesteryl benzoate during the thermal melting process on March 1883+04. It melts at 145.5℃, producing a glossy turbid substance. When the temperature rises to 178.5℃, the luster disappears and the liquid is transparent. The clarified liquid cooled slightly, became turbid again, and instantly became blue, and the color was blue-purple just before crystallization began.
After repeatedly confirming his findings, Lenin sought advice from the German physicist Lehman. At that time, Lehman built a microscope with heating function to discuss the cooling and crystallization process of liquid crystal, and later it was equipped with a polarizer, which was the most in-depth instrument for studying Lenezer compounds. Since then, Lehman's energy has been completely focused on such substances. At first, he called it soft crystal, and then renamed it crystalline liquid. Finally, he is convinced that polarized light is unique to crystals, and Fleeson de Cristal's name is correct. The name is only one step away from the liquid crystal (smooth ge crystal). Lenin Ze and Lehman were later called the fathers of liquid crystal.
Oxyazo ether synthesized by L. gattermann and A Ristschke was also identified as liquid crystal by Lehman. But in the 20th century, famous scientists such as G. tammann believed that Lehman's observation was only a phenomenon that extremely fine crystals were suspended in the body to form colloid. W. Nernst thinks that liquid crystal is only a mixture of tautomers of compounds. However, the efforts of chemist D. Vorlander enabled him to predict which compounds are most likely to show liquid crystal characteristics through aggregation experience, and then synthesize these compounds, so the theory was proved.
Physical characteristics of liquid crystal
When it is turned on after being electrified, the arrangement becomes orderly and light can easily pass through; When there is no electricity, the arrangement is chaotic, which hinders the passage of light. Let the liquid crystal block or let the light pass like a gate. Technically, a liquid crystal panel consists of two pieces of quite exquisite sodium-free glass materials, called substrates, with a layer of liquid crystal sandwiched between them. When the light beam passes through this layer of liquid crystal, the liquid crystal itself will stand in rows or distort irregularly, thus blocking or making the light beam pass smoothly. Most liquid crystals belong to organic compounds and consist of long rod-like molecules. In the natural state, the long axes of these rod-like molecules are roughly parallel. Pour the liquid crystal into a machined groove plane, and the liquid crystal molecules will be arranged along the groove, so if those grooves are very parallel, the molecules are also completely parallel.
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Classification of liquid crystals
Nematic phase
Smectic phase
cholesteric phase
Discoid
Thermotropic liquid crystal
Reproducing liquid crystal
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Use of liquid crystal
The liquid crystal should be fully stirred before use. The liquid crystal containing the solid chiral agent should be heated to 60 degrees Celsius, and then quickly cooled to room temperature and fully stirred. In addition, it should not be left for too long during use. Especially low threshold voltage liquid crystals, because low threshold voltage liquid crystals have these different characteristics, we should pay attention to the following aspects when using these liquid crystals:
The liquid crystal should be fully stirred before use, and the prepared liquid crystal should be put into production immediately, and the standing storage time should be shortened as much as possible to avoid chromatographic phenomena.
The prepared LCD should be stored in a cool cover and used up within one shift (eight hours) as far as possible. Unused liquid crystals need to be recycled and stirred before retesting. Generally speaking, as time goes on, the driving voltage will increase.
After the liquid crystal is taken out of the original bottle, the original bottle should be sealed in time to reduce the exposure time to air, which will increase the leakage current of the liquid crystal.
From PI curing to liquid crystal filling, it is best to fill the empty LCD box with low threshold voltage, and the production time is less than 24 hours. Generally, the filling speed is slow when filling liquid.
Low threshold voltage liquid crystal must be covered with a suitable hood when sealing, and it should be as far away from the ultraviolet light source as possible during the whole liquid crystal filling period except the curing period of sealant. Otherwise, the wrong direction and the increase of threshold voltage will appear near the ultraviolet light.
Liquid crystal is an organic polymer, which is easily soluble in various solvents or reacts with other chemicals. Liquid crystal itself is also a good solvent, so it should be as far away from other chemicals as possible during use and storage.
1922, Frenchman G. Friedel carefully analyzed the liquid crystals known at that time and divided them into nematic, smectic and cholesteric types. The source of the name, the first two are taken from the Greek thread and detective (soap); The names of cholesterol types are of historical significance. For example, according to modern classification, they belong to chiral type. In fact, Fried disagreed with the word liquid crystal. He thought "mesophase" was the most appropriate expression.
Discotic liquid crystals discovered only in 1970' s are nematic or columnar systems composed of highly symmetric undisturbed molecules. In addition to the classification of types, liquid crystals can be divided into thermotropic liquid crystals and lyotropic liquid crystals due to different conditions (conditions). Thermotropic liquid crystals are formed by heating and adding solvents respectively.
An example of lyotropic liquid crystal formation is soapy water. At high concentration, soap molecules are layered and water molecules are between them. The concentration is slightly lower, and the combination is different.
In fact, a substance can have multiple liquid crystal phases. It was also found that after heating the mixture of two kinds of liquid crystals to obtain isotropic liquid, and then cooling, it can be observed that the secondary is nematic and nematic liquid crystals. This phase change substance is called near phase change liquid crystal. Molecular structure of liquid crystal.
The stable liquid crystal phase is an intermolecular van der Kawara Tsutomu phase. Because the molecular aggregation density is high, the repulsive anisotropy has a great influence, but the attraction benefit is to maintain high density, so it is very important to balance the collective strength, hearing and attraction to achieve liquid crystal state. Another example is that when molecules have polar groups, dipole interaction becomes an important attraction.
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Use of liquid crystal
One of the results of liquid crystal molecular arrangement is selective light scattering. Because this arrangement will be affected by external forces, liquid crystal materials have great potential in manufacturing devices. Chiral nematic liquid crystals between two glass plates can form different textures after a certain procedure.
Steroid liquid crystal selectively reflects light due to its spiral structure. The simplest thermometer (common thermometer in fish tanks) is made by using circular polarization in white light and according to the principle of color change. In medical treatment, skin cancer and breast cancer can also be detected by applying steroid liquid crystal to suspicious parts and then comparing with normal skin color (because cancer cells metabolize faster than ordinary cells, the temperature will be higher than ordinary cells).
Electric field and magnetic field have great influence on liquid crystal, and the dielectric behavior of nematic liquid crystal phase is the basis of various photoelectric applications (displays made of liquid crystal materials with external electric field have developed rapidly since 1970' s). Because they have many advantages, such as small size, low power consumption, low working voltage and easy design of multi-color panels. However, because it is not a light-emitting display, the clarity, viewing angle and environmental temperature limit in the dark are not ideal. In any case, TV and computer screens are made of liquid crystal, which is very beneficial. In the past, the large screen was subject to the demand of high voltage, and the volume and weight of the transformer were indescribable. In fact, color projection electric system can also use chiral nematic liquid crystal to make polarizer, filter and photoelectric regulator.
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liquid crystal panel
There is a close relationship between LCD and LCD. Many factors such as the output, advantages and disadvantages of LCD are related to the quality, price and market trend of LCD itself. Among them, the LCD panel is related to the response time, color, viewing angle, contrast and other parameters that players value most. From the LCD panel, we can see the performance and quality of this LCD. Kobayashi is looking for information about LCD panels on the Internet. As long as it is aimed at the current mainstream LCD panel, let everyone have a bottom in mind when purchasing LCD monitors.
VA type: VA type LCD panel is widely used in current display products. When used in high-end products, 16.7M color (8bit panel) and large viewing angle are its most obvious technical features. At present, VA panels are divided into MVA and PVA.
MVA type: full name (multi-domain vertical alignment), which is a multi-quadrant vertical alignment technology. It uses protrusions to make the liquid crystal stand at a certain angle instead of the traditional vertical. When the voltage is applied to make the liquid crystal molecules become horizontal to let the backlight pass through, the speed is faster, which can greatly shorten the display time, and because the protrusions change the orientation of the liquid crystal molecules, the viewing angle is wider. The viewing angle can be increased to more than 160 degrees, and the reaction time can be shortened to less than 20 ms
PVA type: It is a panel type introduced by Samsung, and it is a vertical adjustment technology for images. This technology directly changes the structure of the liquid crystal cell, greatly improves the display efficiency, and obtains better brightness output and contrast than MVA. In addition, on the basis of these two types, two improved panels, S-PVA and P-MVA, have been developed. In terms of technical development, the viewing angle can reach 170 degrees, the response time can be controlled within 20ms (accelerated to 8ms GTG through Overdrive), and the contrast can easily exceed the high level of 700: 1. Most products of Samsung's own brand are PVA LCD panels.
IPS type: IPS type LCD panel has the advantages of large viewing angle, fine color and so on, and looks transparent, which is also a way to identify IPS type LCD panel. Many liquid crystal displays in Philips use IPS panels. S-IPS is the second generation IPS technology, which introduces some new technologies to improve the gray inversion phenomenon of IPS mode at some specific angles. Independent panel manufacturers of LG and Philips are also LCD panels featuring ips technology.
TN type: This type of LCD panel is used in entry-level and mid-range products, which is affordable and cheap, and many manufacturers choose it. Technically, compared with the first two types of LCD panels, the technical performance is slightly inferior. The gorgeous color of 16.7M can't be displayed, but it can only reach the color of 16.7M (6bit panel), but the response time can be easily improved. The viewing angle is also limited, and the viewing angle will not exceed 160 degrees. At present, TN liquid crystal panels are mostly used for products with response time less than 8ms in the market.
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liquid-crystal display
LCD(Liquid Crystal Display (LCD) is an ultra-thin flat panel display device, which is composed of a certain number of color or black and white pixels and placed in front of a light source or a reflector. Liquid crystal displays are favored by engineers because of their low power consumption, and are suitable for electronic devices that use batteries.
Each pixel consists of the following parts: a row of liquid crystal molecules suspended between two transparent electrodes (indium tin oxide), and two polarization filters with perpendicular polarization directions. If there is no liquid crystal between the electrodes, the light passing through one filter will be blocked by the other filter, and the polarization direction of the light passing through one filter will be rotated by the liquid crystal, so that it can pass through the other filter.
Liquid crystal molecules themselves are charged. If a small amount of charge is added to the transparent electrode of each pixel or sub-pixel, the liquid crystal molecules will be rotated by electrostatic force, and the passing light will also rotate and change a certain angle, so that it can pass through the polarizing filter.
Before the charge is applied to the transparent electrode, the liquid crystal molecules are in an unconstrained state, and the charge on the molecules makes these molecules form a spiral or ring (crystal shape). In some liquid crystal displays, the chemical surface of the electrode can be used as a seed, so the molecules crystallize at a desired angle. The light passing through one filter rotates after passing through the liquid chip, so that the light can pass through another polarizer, and a small part of the light can be absorbed by the polarizer, but other devices are transparent.
After the charge is applied to the transparent electrode, the liquid crystal molecules will be arranged along the direction of the electric field, thus limiting the rotation of the polarization direction of the transmitted light. If the liquid crystal molecules are completely dispersed, the polarization direction of the transmitted light will be completely perpendicular to the second polarizer, so it will be completely blocked by the light. At this time, the pixel will not emit light. By controlling the rotation direction of the liquid crystal in each pixel, we can control the light that illuminates the pixel more or less.
Many liquid crystals will turn black under the action of alternating current, destroying the spiral effect of liquid crystals, and when the current is turned off, the liquid crystals will become bright or transparent.
In order to save electricity, LCD adopts multiplexing mode. In the multiplexing mode, the electrodes at one end are connected in groups, each group of electrodes is connected to a power supply, and the electrodes at the other end are also connected in groups, and each group is connected to the other end of the power supply. The grouping design ensures that each pixel is controlled by an independent power supply, and the electronic equipment or the software driving the electronic equipment controls the display of pixels by controlling the on-off sequence of the power supply.
The indicators for testing LCD display include the following important aspects: display size, response time (synchronization rate), array type (active and passive), viewing angle, supported colors, brightness and contrast, resolution and screen aspect ratio, and input interfaces (such as visual interface and video display array).
brief history
The first operable LCD was based on dynamic scattering mode (DSM), and George? The team led by Hailmann developed this kind of LCD. Hailmann founded Optel Company, which developed a series of liquid crystal displays based on this technology. 1970 12, the rotational nematic field effect of liquid crystal is registered as a patent by Santer and Hoffman-lerouke Central Laboratory in Herfrich, Switzerland. 1969, James? Ferguson discovered the rotational nematic field effect of liquid crystal in Ohio University, and registered the same patent in the United States in February 197 1. 197 1 year, his company (ILIXCO) produced the first LCD based on this feature, which quickly replaced the DSM LCD with poor performance.
Display principle
Using the basic characteristics of liquid crystal to realize display. Natural light is "filtered" into linearly polarized light after passing through a polarizer. Because the twisted pitch of liquid crystal molecules in the box is much larger than the wavelength of visible light, when linearly polarized light with the same or orthogonal arrangement direction of liquid crystal molecules along the surface of the alignment film is incident, its polarization direction will be twisted by 90 degrees after passing through the whole liquid crystal layer and will be emitted from the other side, and the orthogonal polarizer plays the role of light transmission. If a certain voltage is applied to the liquid crystal cell, the long axis of the liquid crystal begins to tilt along the electric field direction. When the voltage reaches about 2 times of the threshold voltage, the liquid crystal molecules between the two electrodes in the liquid crystal cell are all rearranged along the electric field direction except the liquid crystal molecules on the electrode surface. At this time, the function of 90 optical rotation disappears, and the optical rotation between vibrating plates of orthogonal plates is lost, which makes the device unable to transmit light. If a parallel polarizer is used, the opposite is true.
In this way, the liquid crystal box is powered on or off, so that the light changes its transmission shielding state, thus realizing display. When the upper polarizer and the lower polarizer are orthogonal or parallel, the display displays a normally white or normally black mode.
Transmission and reflection display
LCD can be displayed by transmission or reflection, depending on the position of its light source. The transmissive LCD is illuminated by a light source behind the screen, while the viewing is on the other side (front) of the screen. This type of LCD is mainly used in applications requiring high brightness display, such as computer monitors, PDA's and mobile phones. The power consumption of lighting equipment used to illuminate LCD is often higher than that of LCD itself.
Reflective liquid crystal displays, commonly used in electronic clocks and computers, sometimes reflect external light back to illuminate the screen through the scattering reflective surface on the back. The contrast of this liquid crystal is very high, because the light has to pass through the liquid crystal twice, so it has to be cut twice. Not using lighting equipment significantly reduces power consumption, so devices that use batteries last longer. Small reflective liquid crystal displays are often used in pocket calculators because of their low power consumption and sufficient power supply from photovoltaic cells.
Transflective LCD can be used as transmissive and reflective. When the external light is sufficient, the LCD works as a reflection type, and when the external light is insufficient, it can work as a transmission type.
color display
In color LCD, each pixel is divided into three units, or sub-pixels, and additional color filters are marked with red, green and blue respectively. The three sub-pixels can be controlled independently, and the corresponding pixels produce thousands or even millions of colors. Old cathode ray tubes used the same method to display colors. According to needs, the color components are arranged according to different pixel geometric principles.
Common LCD dot spacing
Common LCD point distance table:
12. 1 inch (800× 600)-0.308mm.
12. 1 inch (1024× 768)-0.240mm.
14. 1 inch (1024×768)-0.279 mm.
14. 1 inch (1400× 1050)-0.204 mm.
15 inch (1024× 768)-0.297mm.
15 inch (1400×1050)-0.218mm.
15 inch (1600×1200)-0.190mm
16 inch (1280× 1024)-0.248 mm.
17 inch (1280× 1024)-0.264 mm.
17 inch widescreen (1280× 768)-0.2895mm.
17.4 inch (1280× 1024)-0.27 mm.
18 inch (1280×1024)-0.285438+0mm.
19 inch (1280× 1024)-0.294 mm.
19 inch (1600× 1200)-0.242 mm.
19 inch widescreen (1440× 900)-0.283mm.
19 inch widescreen (1680×1050)-0.243mm.
20-inch widescreen (1680×1050)-0.258mm.
20. 1 inch (1200×1024)-0.312mm.
20. 1 inch (1600×1200)-0.255mm.
20. 1 inch (2560× 2048)-0.156mm
20.8 inches (2048×1536)-0.207mm.
2 1.3 inch (1600× 1200)-0.27 mm.
2 1.3 inch (2048× 1536)-0.2 1 mm
22-inch widescreen (1600× 1024)-0.294 mm
22.2 inches (3840×2400)-0. 1245 mm
23-inch widescreen (1920×1200)-0.258mm.
23. 1 inch (1600× 1200)-0.294 mm.
24-inch widescreen (1920×1200)-0.27mm.
26-inch widescreen (1920×1200)-0.287mm.
Not only 20-inch flat-panel LCD, but also 17-inch, 23-inch and 24-inch widescreen LCD displays basically have the problem that the text is too small. Monitors suitable for Internet surfing and word processing include 15 inch, 19 inch, 19 inch widescreen, 22 inch widescreen and 26 inch widescreen. Their point spacing is large, and the text display size is appropriate.
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Advantages of LCD screen
The radiation of LCD screen is negligible, which is equivalent to several watts of light bulb. The radiation to human body is very small.