Development trend of the latest inorganic fluoresc

2022-09-26
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Development trend of new inorganic fluorescent pigments

development trend of new inorganic fluorescent pigments

December 13, 2001

at present, Japan has become the development and production center of phosphor in the world. In our daily life,

, inorganic phosphors should be used for fluorescent lamps, color TV Braun tubes, luminous indicators, and medical supplies. In its

, during the manufacturing process of fluorescent lamps and Braun tubes, they need to be heated to 400~600 ℃ for surface treatment, so they need to use heat-resistant inorganic phosphors

fluorescent pigment is a special functional pigment that can not only fluoresce, but also be used as coloring pigment. It is used in printing, dyeing, plastic coloring, etc. The development and marketization of fluorescent pigments began with the second great

war. They are used in military signs, such as signal flags, attention signs, life-saving appliances, etc. After the war, it was extended to billboards or signs, detergents, cosmetics and other uses. At first, most of these fluorescent pigments were organic products, which can show bright colors under

sunlight. For this reason, organic fluorescent pigments are called daylight fluorescent pigment. However, organic fluorescent pigments have limited applications due to their aging over time and high valence

inorganic fluorescent pigments are confirmed to exist in natural ores, and the price is low. Because these ores are white, otherwise the lifting drive mechanism may fail. There is a color difference between the color of

in visible light and the color emitted when irradiated with ultraviolet light, so the luminous effect can also be improved. But

is that the luminous rate and tone of kairan fluorescent pigment will change subtly with the composition of ore. Therefore, inorganic fluorescent pigments should be manufactured manually. For example, in the early 1990s, we officially began to study light storage pigments. This is because

has shifted to the night for our current pace of life, and the city is becoming a city that never sleeps. Therefore, coatings containing UV emitting

inorganic fluorescent pigments are increasingly used at night and in the dark. For example, sidewalks or objects, indicator boards,

outer walls of buildings, some can absorb sunlight and fluorescent light, and some contain General inorganic fluorescent

pigments. When irradiating ultraviolet (invisible light) from day to sunset, these objects will show various bright colors

colors. It is precisely because of the development of these decorative urban landscape materials that the use of inorganic fluorescent pigments is further expanded

this paper will introduce the basic luminescence mechanism of inorganic phosphor, the preparation and composition of inorganic phosphor, light storage pigment and the application of this pigment in building materials, so as to deepen our understanding. In addition, the author of this paper has carried out a series of studies on inorganic fluorescent pigments using calcium compounds such as calcium phosphonate, lime (CaCO3, Cao) and calcium tungstate with low price and good thermal stability as crystallization mother system. Here, some of the results obtained are introduced

2. Luminescence mechanism of inorganic fluorescent pigments

luminescence refers to the phenomenon that a substance is excited by some external energy (UV, electronic line, X-ray, etc.), and the electronic state is formed by

, and then this excitation energy is emitted by light. Those that emit light at the same time as the external excitation are called fluorescence, and those that can continue to emit light after the excitation is stopped are called phosphorescence

therefore, luminescence will include fluorescence and phosphorescence

The biggest difference between fluorescence and phosphorescence is the length of light-emitting time. Recently, the light storage pigments we talked about belong to the phosphorescence category that can continue to emit light after the excitation stops. Therefore, phosphorescent pigment is also called light storing pigment and luminous pigment. For the luminescence mechanism of organic phosphors, please refer to the relevant literature

inorganic fluorescent pigments contain additive ions and defects (luminous centers) that can produce fluorescence, and these ions and defects should have appropriate concentrations in the parent crystal of

. Most of the parent crystals themselves cannot emit light to visible light. The thing that leads

into the luminescence center in the crystallization and can produce fluorescence is called activation. In the composition formula of the phosphor, it is separated by a colon. The left table

shows the matrix, and the right represents the elements or ions that form the luminous center. The energy used to excite inorganic fluorescent pigments is generally ultraviolet light

the emission spectrum and emission attenuation characteristics change with the type of emission center, as shown in Table 1. The luminescence center formed by the internal migration of ions is to replace some of the ions that make up the parent crystal with luminescence ions. Among them, the outermost layer is composed of S2

electrons (one S2 electron is excited to the p orbital), which usually increases the strong absorption of blue from the outside of purple

due to the allowable migration in s2-p. Rare earth ions with vacancies in the 4f orbit emit light due to the migration of

4 f-4f. Their luminescence spectrum is narrow in half amplitude and has a slightly longer decay time. The

luminescence of rare earth ions has a weak effect on 5S and 5p electrons due to the crystallization field, so the luminescence light

spectrum from these ions into the parent crystal is characterized by less changes

among rare earth ions, ce3+, eu2+, yb3+ belong to 4f-5d migration, and have lower energy than 4f-4f migration

The

5d orbital is strongly affected by the crystallization field, so the luminescence spectrum is strongly dependent on the parent. In addition, F-D migration belongs to

allowable migration, so the decay time of luminescence is short

The luminescence mechanism of the phosphor absorbing ultraviolet light is shown in Figure 1. (1) The parent crystal absorbs external energy from ultraviolet rays, etc

(2) the energy absorbed by the parent crystal is transferred to the luminescent ions. Luminescent ions are excited from the substrate state (E0) to the excitation

state (E2) (energy transfer and excitation of luminescent ions). (3) The excitation energy

of the excited luminous ions emits part of the energy with the lattice vibration or heat of the parent crystal, and the energy gradually decreases to a stable excitation state

(E1). (4) Light is emitted from the emission level and returns to the substrate state (E0) (emission). Coal prices continue to rise unilaterally. Therefore, the energy difference between E1 and e0

is luminous energy. However, the excitation energy of most substances will be lost due to the extinction of thermal mitigation phenomenon, impurities,

lattice defects and the ion concentration of activator, so the energy emitted by luminescence will be smaller than the original

(Stokes law)

The potential curve (coordinate model) of activator ions in the phosphor parent crystal is shown in Figure 2. The horizontal

coordinate in Figure 2 is the distance re (equilibrium core spacing) between the luminescent cation and the nearest anion. Irradiating ultraviolet light and so on can push the luminous ionization from its base state to the excited state. At this time, it is difficult to move relative to the nucleus, and the movement of electrons is obviously accelerated, so when the position of the nucleus remains unchanged, only electrons are excited (Frank Condon principle)

the vibration (thermal vibration) between activator and similar ions in crystallization, if expressed in quantization, as shown by the horizontal line

in Figure 2, has a jumping energy level

in Figure 2 (b), the vibration from a to B is equivalent to the excitation state (E2) in Figure 1. Then the energy is given to

to the parent crystal, and at the same time, it falls to the C energy level, and then falls from C to D. at this time, it emits light with an energy equivalent to c~d

the characteristic of this potential curve is that the lowest point of the excitation state curve (equilibrium nucleus spacing) is inside the basal state curve

relative to Fig. 2 (b), Fig. 2 (a) is the lowest point of the excitation state curve of general substances. On the outer

side of the substrate state curve, the specially obtained excitation energy B can easily be

added to the substrate state through the intersection s of the potential curve of the substrate state and the excitation state, and it is lost without luminescence. That is, excitation energy B is consumed by lattice vibration (thermal energy)

(thermal relaxation phenomenon)

3. preparation and components of inorganic fluorescent pigments

3.1 preparation

in order to prepare inorganic fluorescent pigments, it is generally necessary to have a host, activator and flux

the preparation of inorganic phosphor is to mix the matrix, activator and melting aid, put them into heat-resistant crucibles such as quartz or alumina, calcine them at a set temperature (1000~1600 ℃ with rare earth elements and about 1000 ℃ with ZnS system) with an electric furnace, and then prepare the phosphor after cooling

in order to produce bright phosphors, it is necessary to pay attention to raw materials (purity, particle size), mixing uniformity, calcination temperature and atmosphere control, phosphor particle refinement and coating, etc

3.2 parent crystal

inorganic phosphors can be classified into phosphate, silicate, aluminate, tungstate, oxide,

sulfide, etc. according to the parent composition. The most suitable matrix will be selected according to the purpose. Sulfide, aluminate salt and silicate are commonly used as fluorescent pigments. As a parent crystal, it should generally have the following characteristics

(1) ions as luminous centers can be uniformly added and added to the necessary amount

(2) when adding the luminous center, it is necessary to reduce the skew and defects caused by different ion radii

(3) the relative luminescence is transparent

(4) the absorption efficiency of excitation energy is high, and it can be effectively transferred to the luminous center

(5) good chemical and physical stability, fully resistant to service conditions

the author of this paper used calcium compounds as the parent crystallization of inorganic fluorescent pigments, which has attracted people's attention, because it can almost meet the above conditions (1) ~ (5). Details are described later

3.3 activator

rare earth is a typical activator. The concentration of activator is usually 1mol for rare earth activated phosphor, and mol/mol for Ag, Cu and other activated ZnS phosphors

saying that rare earth is actually a bit untrue, because it is neither rare nor earth (metal oxide). On the earth

ball, except for a few exceptions, these elements are richer than gold, silver, mercury and tungsten. Usually, rare earth refers to scandium, yttrium

and lanthanides with atomic numbers from 57 to 71. In Japan, because the resources of rare earth elements depend on foreign countries, there is a basic research on the reuse of rare earth elements in inorganic

phosphors. Table 2 lists the electronic structure of the elements in the f-segment and the ionic half

usually, the adjacent elements in the periodic table are not only different in nuclear charge, but also different in valence electron form of each atom

and rare earth elements have nothing to do with atomic number, especially the sixth electron layer outside the nucleus, which has the same number and type of

valence electrons. With the increase of the positive charge of the atomic nucleus, there is an unfilled f-sub orbital in the fourth electron layer, which can add electrons. Rare earth elements have almost the same chemical properties, because their valence electron layer basic phase

the ion radius of rare earth elements will vary slightly with the coordination number. Except for scandium, other elements

are in the range of 0.2A. The radii of alkaline earth ions such as ca2+, sr2+ are similar to those of bi3+, tl3+. The unique property of rare earth elements is that the 4f orbital in the inner layer is not enough to enter electrons. Due to the existence of the inner dissatisfied 4f orbital and the same configuration of the outermost electrons, the ligand field is out of bounds

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