Luminescent Materials And Applications
The luminous process of luminescent materials is the process of converting absorbed energy into light radiation. It is another kind of radiation besides thermal radiation, and the duration of this radiation is greater than that of light.
The production process of the luminescent materials can be referred to the Luminescent tape.
The definition of luminescence
This definition of luminescence consists of two parts:
(1) Light emitting from burning objects (such as incandescent light, solar light, etc.), without heat;
(2) The luminescence is different from the reflection and scattering of light, and the reflection and scattering of light has no lasting time, and the luminescence still lasts for a period of time after the excitation is stopped.
Classification of luminescent materials
It is necessary to ascertain whether a material is emitting without obvious boundaries. Under general excitation conditions, the material that does not luminescence also has weak luminescence under very strong energy excitation.
Some materials need to improve purity, luminescence can be stronger, and some materials need to be added to a certain amount of activator to have a stronger luminescence.
Materials that can glow under various forms of energy are called luminescent materials (or phosphors).
According to the different modes of excitation energy, the classification of the luminescent materials is the photoluminescence materials which are luminescent by ultraviolet light, visible light and infrared light. Photoluminescence materials are divided into long afterglow luminescent materials, light emitting materials and multiphoton luminescent materials, according to their different luminescent properties and application range.
A cathode ray luminescent material excited by an electron beam;
An electroluminescent material that is excited by an electric field;
A chemiluminescent material that causes luminescence by a chemical reaction between two or more than two chemical substances;
X – ray luminescent materials irradiated by X rays;
A radioluminescence material that is irradiated by natural or artificial radioactive materials.
Considerations for the purchase of reflective materials for reference: How to buy Photoluminescent Tape
The concept of luminescence of luminescent materials
Some cathodoluminescence phosphors, such as radar phosphors, also have long afterglow after cathode ray ceasing. These materials contain sulfides and some fluoride. The afterglow time is longer than 1s, even up to 30s.
Luminescent phenomena and luminescent materials have played an important role in the national economy and people’s lives.
Long afterglow luminescent material is a kind of luminescent material that absorbs the excited light energy and stores it and stops after light excitation, then releases the stored energy in the form of light, and it can last several or even more than ten hours.
The process of absorption, photoluminescence, storage, reemission and infinite repetition is similar to that of battery charging, discharging, recharging and redischarging. Therefore, we suggest that long afterglow luminescent materials should be called light storage luminescent materials.
A brief history of luminescent materials
The earliest records of the luminescent materials in the West were in the jewels inlaid on the goods operated by the Italy Poloni shoe store.
The gemstone is made by calcining of ore and carbon, and it can be glowing in the dark. It was very precious at that time.
In 1603, the Italian man, Casciarolus, got some red minerals in the dark when he roasted the gold in the local ore.
Then by the analysis of barium sulfate containing components of stone, by reduction roasting may become a part of barium sulfide, which is in fact after artificial preparation of barium sulphide storing type luminous material.
In 1609, Brand discovered the substance that gives white light in the air. It named it “phosphors”. Since then, inorganic luminescent pigment is called phosphor, sometimes it is also called luminophor.
There are two kinds of luminescence: phosphorescence and fluorescence.
Phosphorescence is derived from phosphor. It refers to the phenomenon of long duration of light after excitation and stop. The light from long afterglow material is a typical phosphorescence.
The term refers to the fluorescence decay time is relatively short, generally refers to the phenomenon of less than 10-8s.
At present, people generally do not strictly distinguish between fluorescence and phosphorescence, which is called fluorescence when the time of afterglow is short to the indistinguishable by the human eye. The weak luminescence of afterglow is sometimes called phosphor, and the light emitting materials, which have a short afterglow time, are sometimes called phosphors.
From 1600 to 1700, many people, including Goethe, studied the luminescence of noctilucent materials.
Since 1764, the British people with oysters and burn out of blue luminescent material mixed sulfur, which should be the calcium sulfide luminescent materials.
It can be said that the discovery of the luminescent phenomenon begins with the light emitting material. Many natural ores themselves are light emitting materials, and people have already begun to use this kind of material to make various products.
Therefore, the light emitting materials are the first kind of luminescent materials to be found and applied.
A systematic and scientific study of inorganic phosphors began in nineteenth Century. In 1852, Stokes and Becquerel, Verneuil and Lenard defined the first fluorescence definition and published the famous Stokes law.
At Sidot in 1866, the luminescence of ZnS was discovered, and since then, ZnS has become the basis of many important industrial phosphors.
On the basis of the study of uranyl salt in 1867, two kinds of afterglow mechanism were deduced, that is, the Hyperbolic Attenuation and exponential decay.
In 1887, Verneuil realized that heavy metal impurities played an important role in the luminescence of inorganic substances.
In 1898, Mr. and Mrs. Curie extracted radium from uranium and radium bitumen, and radio luminescence materials began to be applied in all aspects.
In 1903 and 1904, Lenard and Klatt reported the relationship between activator and luminescence, first using flux and discussing its effect.
Urbach (1926), Randall and Wilken deduce the different trap depths based on the different time constants of the afterglow attenuation.
The formal production and application of light storage luminescent materials began in the early twentieth Century. The need for military and air defense in the Second World Wars also promoted the research and application development of these materials, but all these materials were sulfide series.
Before 1970s, there had been no reports of non sulphide light storage materials.
After that, though there were some brief descriptions about the long afterglow properties of aluminate system, it was not until 1990s that the aluminate system luminescent material with better optical storage property than sulphide was launched.
As soon as this new material is published, its excellent light storage performance has been recognized by people and is known as the second generation of light emitting materials.
Luminescence principle of luminescent materials
In the crystal lattice formed during the synthesis of luminescent materials, structural defects and impurity defects have the properties of luminescence. Structural defects are the formation of vacancies and ions between the lattice points, also known as lattice defects. The luminescence caused by the lattice defect of the material is called self-activated luminescence.
In the process of high temperature synthesis, the ions added into the matrix lattice formed the impurity defects in the high temperature synthesis process. The luminescence caused by this defect is called activated luminescence. The elements to be added are called activators, and also called luminescent centers. Most of the luminescent materials that have been used in practical application are mostly activated form luminescent materials.
For photoluminescence materials, the ultraviolet energy of the excited material can be absorbed by the matrix (also called intrinsic absorption).
The matrix absorbs energy, and its luminescence can be directly generated by the recombination of valence band electrons and holes, and it can also be generated by the luminescent center formed by lattice defects or impurity defects.
This type of luminescent material is called a “composite” luminescent material. The sulfide series is the representative of this kind of luminescent material. They are semiconductors, and the matrix lattice is the medium that produces the luminescence process. If the same activator is used to change the matrix component, the composition of the emission spectrum can be changed regularly in a wide range of wavelengths. When this kind of luminescent material is excited by ultraviolet light, the matrix absorbs energy first, and then transfers energy to the luminescent center to produce luminescence.
Therefore, in the process of absorbing energy, transmitting energy and emitting light, the composite luminescent material is lost and the efficiency of energy luminescence is low.
Ultraviolet energy can also be directly absorbed by the center of light when the material is excited. After absorbing the energy, the electrons in the center of the luminescence go to the high energy state from the low energy state. When the electron returns from the high energy state to the low energy state, it generates light. The materials that luminescent only relate to the electron transition of the luminescent center are called “characteristic” luminescent materials. The luminescent materials, such as silicate, phosphate and aluminates, are characteristic luminescent materials based on oxygen containing compounds.
The elements which are used as activators to form the center of luminescence are mainly transition elements, rare earth elements and mercury like elements. The luminescence process of the 5 materials is simpler than that of the composite material. The luminescence center directly absorbs the excitation energy, and the concentration is only a few percent. The energy between the center and the substrate is low, so that the material has high luminous efficiency.
A schematic diagram of the luminescence process, as shown in Figure 1:
Figure 1 energy level transitions in the production of fluorescence (a) and phosphorescence (b)
Electronic excited by external energy from the ground, “g” excited state “e”, namely, figure 2 (a) transition process (I).
When the excited electrons return to the ground state by photon emission, i.e. Figure 2 (a) in the process of transition (II).
The fluorescence transition (I) and (II) the time interval between less than 10-8s, and this process is independent of temperature.
If in the excited state of “e” and “g” between the ground state energy gap “exists in the” metastable “level” m “[see Figure 2 (b)], the energy level diagram structure has changed from” g “, the electronic excitation to the” e “may be bound in the metastable state of” m “so, until the” m “from” e “to return to gain enough energy to reach E level,” e “will be able to return to the normal electronic transitions of G”, accompanied by light emission.
In this way, the delay time of the phosphorescence is in fact corresponding to the time that the electron stays in the trap “m”.
In fact, the basic storage and luminescence principle of light storing luminescent materials is similar to the schematic diagram in Figure 2 (b). In the light storing luminescent materials, the metastable m corresponds to the trap energy level, and the energy E corresponds to the trap depth.
The light storage properties of the sulphide series and the aluminate system luminescent materials are related to the trap energy levels of different depths in these materials.
When ultraviolet light is excited, a trap energy with a certain depth of energy has captured a sufficient number of electrons from the excited state and stored.
After stopping the excitation of ultraviolet light, the electrons stored at the trap level gradually release 6 electrons at room temperature, and the electrons released are then transferred to the excited state, and the electrons grow back to the ground state from the excited state to produce afterglow.
Luminous principle of Luminescent Materials, reference glow film
Invention of non radioactive material
There is an urgent expectation that materials that can glow for a long time and do not contain radioactive materials will come out as soon as possible. This expectation cannot be achieved by the series of sulphide light storage materials.
The afterglow time after 1990s developed the aluminate light storage material has reached 2000min, the night light for long time, the actual effect luminous digital symbol has more than permanent luminescent materials, the development of light storing materials entered a new era.
Europe was the leader of phosphor technology and industry before World War II. After the Second World War, the United States surpassed Europe, and after 1980s, Japan began to gain the advantage in this industry. China has been catching up with Japan in recent years, and has made great progress in some aspects. It occupies the world advanced level in the research and industrialization of new light storing luminescent materials.
The application fields of new light storage luminescent materials are very extensive. They are widely applied in the fields of building decoration, transportation, military facilities, fire safety, daily life products and so on, and form the luminous industry chain. The market prospect is broad at home and abroad. Especially in the field of fire safety which is related to the safety of the people’s life and property, it plays an important role.
In September 2002, the American “Newsweek” in the “9 – 11” 1st anniversary with the theme of “Five Who Survived”, reported the survivors escape insider, which is making use of the Glow In The Dark Stairs with the nose and dark corridor in the building after the evacuation instructions in smoke.
The luminescent material is applied not only to the safety mark, but also to the thermal transfer printing, which can be used as a reference to the Luminous Heat Transfer Vinyl.