Besides synthetic ruby, synthetic sapphire, synthetic spinel and synthetic strontium titanate are also produced by flame melting. In the artificial products industry, this method and its cultivated products occupy a very important position.
I. Working principle and device
Flame melting method is also called flame method, or Werner method. As the name implies, it is a method of melting raw materials into a melt by flame, and then growing crystals (see Figure 4- 1- 1), which is an important method of growing gems.
Figure 4- Schematic diagram of crystal growth device by flame melting method (1- 1)
The device for growing gems by flame melting method consists of a feeding system, a gas combustion system, a furnace body and a descending system.
The feeding system adds the raw material powder into the sieve-shaped flat-bottomed container, and under the regular vibration of the vibrator, the powder falls into the oxygen flow evenly and equally through the sieve bottom.
Gas combustion system This is the main part of molten powder. When oxygen and hydrogen burn through the burner, the temperature can be as high as 2400 ~ 2500℃, and the highest can be as high as 2900℃.
The role of the furnace body is to create heat preservation conditions, so that the crystal temperature will drop slowly. The furnace requires streamline, which is convenient for gas flow and does not accumulate powder.
During the descent, the rotating platform descends evenly, and the average platform descends 1cm per hour.
The flame melting method has a history of 100 years. It occupies a very important position in the artificial products industry, and it is a method with the largest output and scale and the longest use history.
Second, the preparation of raw materials
The preparation of raw materials for growing gems is the first step to grow high-quality crystals. Now take synthetic corundum as an example to illustrate.
Many years' practical experience in synthesizing gemstones by flame melting at home and abroad has proved that γ-Al2O3 powder with pure and uniform particles, high dispersibility, suitable packing density and good fluidity is the most ideal raw material for growing and synthesizing corundum gemstone crystals. According to different uses and performance requirements, corundum gems with different colors and properties can be obtained by adding different colorants and modifiers into γ-Al2O3. Therefore, in order to obtain the required physical and chemical properties of powder, how to choose a reasonable dopant and how to determine the roasting degree of gem raw materials are two key links in the preparation process of γ-Al2O3 powder.
1. Doping of raw materials
For gems with different colors and uses, different metal ions need to be doped, and the doping amount of metal ions is determined according to the following factors: ① the requirements for different colors, photoelectric properties and uses of gems; (2) Requirements on the structure and physical properties (such as hardness and toughness) of gemstones; ③ Loss on ignition of doped ions during the growth of gem powder and crystal.
Table 4- 1- 1 lists the relationship between dopant and color in synthetic corundum raw materials. It is very important to control the doping amount, too little color is too light; Too much will make the gem easy to break.
Doping is adding different reagents to aluminum ammonium sulfate alum. The reagents used for doping are shown in Table 4- 1-2.
The burning loss of chromium ions in the process of melt crystallization of synthetic ruby is very obvious. From a set of experimental data of chromium ion loss, it can be seen that when the mass fraction of chromium ion is 0.875%, the mass fraction of chromium ion after roasting is reduced to about 0.80% ~ 0.82%, and the loss on ignition is about 7.4%. After flame melting crystallization, the mass fraction of chromium ion decreased to about 0.388%, and the loss on ignition was about 55.7%. Therefore, when determining the doping amount in raw materials, the loss on ignition of dopants in the process of flame melting crystallization must be fully considered.
Table 4- 1 to 1 Dopants and Doping Amount Used in Synthetic Corundum
Table 4- 1-2 reagents for various dopants
2. Decomposition process of aluminum ammonium alum
The preparation process of γ-Al2O3 is the thermal decomposition process of aluminum ammonium alum. The basic process is to repeatedly crystallize aluminum ammonium alum to remove impurities, dehydrate at 200℃ to form anhydrous porous aluminum ammonium alum, and then bake to volatilize H2O, SO3, NH3, etc. To form Al2O3.
After the complete decomposition of ammonium aluminum alum, amorphous alumina is first formed, and then it is gradually transformed into γ-Al2O3 powder, in which the temperature at which ammonium aluminum alum is completely decomposed into alumina must be accurately controlled. China scientific and technical personnel have made a systematic determination of this. The experimental results show that the roasting temperature is lower than 950℃ and the holding time is 2 hours.
Third, crystal growth.
Before crystal growth, the seed crystal should be prepared as required, and the seed crystal must be cut directionally (oriented with an orienter or polarized light). For gem crystals, the growth direction mainly considers production efficiency and crystal quality. The angle between the growth axis and the optical axis of synthetic corundum crystals is about 60, so the grown crystals have high yield and are not easy to crack.
The growth process of gem crystal can be divided into four stages: ① The initial crystal bud grows on the seed crystal. In the early process, the seed crystal is usually a part of ceramic body or crystalline crystal sintered by powder. At present, the seed crystal method has been used to replace the spontaneous growth of crystal buds. For example, when growing synthetic rubies, the seed crystal adopts synthetic rubies. ② Enlarge the area or diameter of the seed crystal. ③ Equal channel growth: after the crystal is enlarged to a certain size, it is in the equal channel growth stage and remains until the end of growth, becoming an inverted pear-shaped crystal. In equal channel growth, the growth surface of pear crystal should always be in the most suitable growth temperature zone, which is the so-called crystallization focus. The best crystallization condition is to leave a 2 ~ 3 mm thick melting layer on the top of pear crystal, so that the powder falling on this layer can be completely melted, and then solidify and crystallize under the melting layer when the crystal bar descends. (4) Annealing treatment: after the synthetic crystal is put into a high-temperature furnace according to regulations, the furnace temperature is slowly raised to a predetermined temperature, and then annealing at a constant temperature for a long time. Because of the large thermal stress of synthetic gem crystals without annealing, if the heating speed is too fast, the crystals will often crack due to thermal shock during heating. Usually the heating time is 5 ~ 10 hour. After the temperature rises to a predetermined temperature, keep the temperature for dozens of hours, and then slowly cool it to room temperature. High temperature annealing near the melting point should strictly control the temperature to prevent crystal reflux.
Four, the characteristics of the flame melting crystal growth process
1) This method does not need a crucible, which not only saves crucible materials, but also avoids the pollution to the crucible in the process of crystal growth.
2)H2-O2 combustion can reach a very high temperature (above 2500℃), which is very suitable for the growth of refractory oxide crystals.
3) the growth speed is fast, which is beneficial to large-scale production and can reduce the cost.
4) The growth equipment is relatively simple, and large-size crystals can be grown. For example, the width of a gem pear crystal can reach 10 ~ 30mm and the length is 500 ~10 ~ 30 mm. Of course, this method also has some disadvantages, such as large temperature gradient, large crystal stress and easy cracking during growth.
Verb (abbreviation for verb) grows other gems by flame melting.
Flame melting method is not only used to grow colored corundum series gems, but also used to produce oxide crystals such as synthetic spinel, synthetic rutile and artificial strontium titanate, which are only briefly introduced here.
1. artificial strontium titanate
Artificial strontium titanate once appeared in the gem market as a diamond substitute, but it gradually decreased due to the appearance of artificial cubic zirconia. Artificial strontium titanate belongs to the equiaxed crystal system (perovskite family in structure) and hexagonal octahedral symmetry type.
195 1 year, Merker and others in the United States first grew strontium titanate crystals by flame melting, but because of the brittleness of this crystal, it has been impossible to grow large and complete optical crystals for a long time.
Preparation of raw materials for strontium titanate crystal growth: firstly, the double salt of strontium oxalate and titanium oxalate was synthesized, and then the double salt was roasted at high temperature to obtain SrTiO3 _ 3 powder.
In order to grow the crystal in a reducing environment, excess H2 is needed. In the process of nucleation, H2: oxygen =7: 1, and in the process of growth, H2: oxygen =5: 1. The grown crystal is black and shiny due to lack of oxygen, and then oxidized to colorless and transparent at 1600℃.
On the binary phase diagram of SrO-TiO _ 2, the ratio of SrO to TiO _ 2 is 1: 1. In practice, it is found that Sr is more volatile than Ti, so a little more Sr is added to the growth raw materials to make the final crystal reach the ratio of SrO: TiO _ 2 = 1: 1.
2. Flame melting growth of synthetic spinel
The ratio of synthesized spinel MgO MgO∶Al2O3 varies greatly, ranging from 1: 1 to 1: 2.5, and the maximum is 1:4.
The raw materials for growing and synthesizing spinel are magnesium carbonate and ammonium aluminum sulfate, which are mixed and roasted in proportion to form Al2O3-MgO mixture, and then the corresponding colorant is added. The refractive index of colorless synthetic spinel is 1.728, which is quite stable and can be used as a standard block for calibrating refractometers. Heat treatment of synthetic spinel at 950 ~ 1050℃ can eliminate thermal stress and improve hardness.
Synthetic spinel is easy to grow except red, among which blue (co-induced color) and light blue are more common. The red synthetic spinel must be in strict accordance with the ratio of 1: 1, and any deviation will turn into other colors, so it is difficult to grow. See Table 4- 1-3 for the doping and color of synthetic spinel.
Table 4- 1-3 Color and Doping of Synthetic Spinel
Synthetic spinel is often used to imitate aquamarine, olivine and tourmaline, but its properties are far from those of imitation gems, so it is easy to identify.
3. Flame melting growth of synthetic rutile
Synthetic rutile was once famous as a substitute for diamonds.
Artificial rutile belongs to tetragonal crystal system, with melting point 1840℃, density of 4.26g/cm3, Mohs hardness of 6-7, refractive index of 2.616-2.903 and dispersion of 0.33, which is too strong as a diamond substitute.
The raw materials used to synthesize rutile are the double salts of H2SO4, TiCl4 and (NH4)2SO4, and the ratio is (NH4) 2SO4: H2SO4: TiCl4 = 2:1.6:1. The composite salt is calcined and decomposed to obtain titanium dioxide. Because TiO2 is easy to deoxidize during the growth process, it is mixed with O2 during the growth process to keep it in the furnace. Even so, there is still no guarantee that TiO2 _ 2 will not lose oxygen at the melting point, so the grown crystal should be annealed under the oxidation condition of 1000℃ to make the crystal light yellow and transparent.
In 1950s, American National Lead Company and United Carbide and Carbon Company once produced this kind of crystal and sold it as a diamond substitute. The annual output once reached 150kg. However, after the appearance of artificial cubic zirconia (CZ), artificial rutile has rarely appeared in the jewelry market. Adding dopants to TiO2 can also grow color crystals, but it is rarely used.
Identification characteristics of gemstones synthesized by flame melting method with intransitive verb
1) gas inclusions can be found in gems grown by flame melting method. These bubbles vary in size, either individually or in groups. Bubbles are mostly spherical in shape, and there are also elongated and serrated variation bubbles. Sometimes a large number of small bubbles appear densely, forming cloud-like inclusions.
2) On the cross section of the crystal grown by flame melting method, dense arc-shaped growth bands or record-shaped bands can be seen, often accompanied by slender bubbles perpendicular to the stripe direction.
3) In the process of crystal growth, unmelted powder is sometimes wrapped in the crystal to form clastic inclusions.
4) Pear-shaped crystals grown by flame melting method are easy to crack in the middle and dislocate, leading to serious defects such as mosaic structure and crystal orientation distortion.
5) The gem crystals grown by flame melting method are large in size, uniform and bright in color.