Flux crystal growth has a long history. /kloc-In the middle of 0/9th century, some people in western Europe used this method to synthesize rutile and emeralds. Due to the rise of ruby synthesis by flame melting, this method was once ignored, but in recent decades, due to the development of science and technology, it has been widely used in gem growth. The flux method can not only synthesize rubies, but also emeralds and spinel.
The flux method for growing gem crystals has many advantages. Compared with other methods, it has strong applicability and can find suitable flux for almost all gem materials. The flux method requires low temperature, and many refractory compounds, compounds that are volatile or change valence at the melting point, or compounds melted by different components can grow from the melt solution. In addition, because it is similar to the crystallization of minerals in magma, the inclusions of synthetic gem crystals are very similar to those of natural gems, so it is highly valued by gem synthesizers. The flux method requires relatively low temperature, so the equipment is relatively simple, and the components from the heating element to the temperature measurement are easy to configure. The disadvantage of this method is that the growth period is long, and some fluxes are corrosive and toxic, which is easy to pollute the environment.
First, the principle of crystal growth by flux method
Flux method, as its name implies, must have flux. As a flux, a basic requirement is that the crystal to be grown can be dissolved after melting, and it is not easy to decompose and volatilize. Polar compounds such as PbF2, PbO and Bi2O3 are the best materials with low melting point and strong solubility. In addition, B2O3 and Ba O-B2O3 are also commonly used. Some complex compounds such as tungstate, molybdate, cryolite, etc. Sometimes it is chosen as flux.
The choice of flux should be based on several principles:
1) has excellent solubility in the crystal to be grown, and the solubility changes greatly with the change of temperature, which makes the crystal easy to grow.
2) In a wide temperature range, the grown crystal is the only stable phase, that is, the flux and crystal components cannot form intermediate compounds.
3) Flux has low viscosity and high boiling point.
4) Low volatility, low toxicity and easy removal.
Commonly used fluxes and their properties are shown in table 4- 1-8.
Table 4- 1-8 Common Fluxes and Their Characteristics
The basic principle of growing gem crystals by flux method can be illustrated by the crystal phase diagram of binary components, as shown in Figure 4- 1-6. Assume that the melting point of gem component A is t A, and the melting point of flux as low melting point component B is T B. Mix component A and component B in the mixing ratio of X, and after heating and melting, both component A and component B are melted into molten liquid. At this time, the melting point of X as a mixed component is P. When the temperature drops, component A crystallizes at Q, which is equivalent to the temperature of T Q.. When the temperature decreases again, the composition ratio of the melt changes along TAQE and finally reaches the composition of point E, which is called low melting point. In this process, component A continuously precipitates or grows into crystals. It can also be seen from the figure that the addition of component B makes the crystallization temperature of component A at TQ point obviously lower than that of component A, that is, after adding component B with low melting point into component A, the melting point and crystallization point of component A are reduced from t A to TQ, so that gem crystals with high melting point can be grown at a lower temperature. Because component B plays a role in lowering the melting point, it is called flux. Because component B is usually inorganic salt, the flux method is also called molten salt method or flux method.
Figure 4- 1-6 binary * * crystal phase diagram
It can also be seen from the phase diagram that the lower part of the phase diagram is the desolvation area of solid phases A and B, which is very common in gem crystals. In the process of ruby and sapphire discoloration, TiO2 _ 2 _ 2, Fe _ 2O _ 3, Cr _ 2O _ 3 etc. Is desolventized, and there is tetragonal desolventization in the cubic phase of the synthesized cubic zirconia crystal, which becomes a separated crystal during annealing.
Secondly, the growth and synthesis of ruby crystal by flux method.
Ruby synthesized by flame melting method has large output, good crystallization and full color, but compared with natural ruby, it is easy to identify. In order to grow artificial ruby close to natural ruby, people focus on flux method. Because the inclusions and growth habits of ruby synthesized by flux method are similar to those of natural ruby formed in magma, the synthetic ruby is almost genuine, which is also the reason why the synthetic ruby by flux method lasts for a long time.
Common problems in the process of synthesizing ruby by flux method are:
1) nucleation control, especially slow cooling method, sometimes the nucleation is out of control and the crystal is not long.
2) Unwanted growth habits. Because the (000 1) plane of ruby grows slowly and grows into (000 1) flake, the utilization rate is low and the crystallization is incomplete.
3) There are too many inclusions containing flux inside, which destroys the crystallization integrity.
Taking Na3AlF6 as a flux, the growth process of ruby synthesized by flux seed crystal rotation method is explained below.
Growth process of (1) artificial ruby crystal
As mentioned above, the growth principle of flux method can be explained according to the binary phase diagram, and the mixing ratio can be determined according to the binary phase diagram of Na3Al F6-Al2O3, and the mixing ratio of Na2AlF6 and Al2O3 can be selected within the range of Al2O3 mass fraction 13% ~ 20%. The growth temperature is between 980 ~ 1050℃.
Mixing Na3Al F6 and Al2O3 (AP grade), adding Cr2O [3w (Cr2O3) = 1% ~ 3%], mixing, briquetting and melting evenly. The growth furnace is shown in Figure 4- 1-7, and its heating element is high-temperature electric furnace wire. The temperature field can be adjusted by the distribution of furnace wire or changing crucible.
Fig. 4- 1-7 growth furnace for ruby synthesis by flux method
The crucible is 85mm(d)×85mm(h), and the charging weight is1000 g g. After melting, the saturation temperature of the solution is measured by seed crystal downward test method, and it is kept at about 20℃ higher than the saturation temperature for 4 ~ 5 hours to ensure the melt is fully melted, and then the seed crystal is gradually reduced to 0.5 ~1℃.
(2) Effect of melt volatilization on crystal growth
Because the crystal growth is carried out in an open system, 6% of the flux Na3AlF6 volatilizes in a molten state during each growth process. A large number of F, Na ions and their compounds in the melt are quite active, and it is easy to overflow from the surface, resulting in rapid evaporation of the melt. In addition, due to surface evaporation, the surface saturation of the melt is high, and the probability of surface nucleation is high, which makes the quality of the grown crystal poor. Therefore, when using seed crystal to grow, the seed crystal should be placed below the liquid surface to make the crystal fully develop, and the proportion of flux should be increased to compensate for the loss caused by evaporation.
(3) The main varieties and identification characteristics of ruby synthesized by flux method.
At present, there are several kinds of rubies synthesized by flux method in the international jewelry market, such as Chatham, Lamla, Cnis, Kasha and Duross.
1. Chatham Synthetic Ruby
Chatham synthetic ruby was put on the market on 1960. It is the earliest synthesis of ruby by flux method, and its characteristics are as follows.
1) crystal characteristics Chatham synthetic ruby is a kind of synthetic ruby containing natural seeds, and the early growing single crystal is characterized by natural seeds.
2) There are strong red fluorescence and chalk red fluorescence under fluorescent ultraviolet lamp.
3) Internal characteristics: ① Flux inclusions can be seen, and coarse flux residues are often seen in Chatham synthetic rubies, which are often torn, fine-meshed and feathered; (2) platinum metal sheet. At present, most of the flux-synthesized rubies have not seen platinum metal pieces, but platinum metal pieces can still be seen in Chatham's synthetic rubies. These platinum metal pieces and flux residues form some similar natural landscapes, which constitute the identification basis of Chatham artificial ruby. Platinum metal sheets are hexagonal and triangular with serrated edges. (3) The seed crystal, observed under the microscope, is often seen in the blue phantom seed crystal of the ruby synthesized by Chatham, and some light blue-red-purple boundaries can be seen on its edge; ④ Transparent crystal: a granular colorless transparent crystal is occasionally seen in the ruby synthesized by Chatham, and it is confirmed by analysis (Kerre, 1982) that this crystal is chrysoberyl.
2.Cnis synthesizes rubies.
Knischka synthetic ruby is a flux method for synthesizing ruby from natural seed crystals, which was invented by an Australian engineer. Its characteristics are as follows.
1) crystal characteristics The ruby synthesized by K nischka is in the form of spindle crystal (see Figure 4- 1-8), in which there are hexagonal biconical surfaces besides the bottom axial plane c{000 1} and the rhombic plane.
Fig. 4 Ruby crystal synthesized by-1-8cnis card.
Systematic Gemmology (Second Edition)
2) Ruby synthesized by fluorescence Knischka shows strong red fluorescence under long-wave ultraviolet lamp, and the characteristics of short-wave ultraviolet fluorescence are the same as those of long-wave.
3) The absorption spectrum is in the range of 400 ~ 700 nm of UV-Vis absorption spectrum. There are obvious iron absorption peaks of 468.5nm, 475nm and 476.5nm and absorption peaks of 659.2nm, 668nm, 692.8nm and 694.2nm in Knischka synthetic ruby, which are the same as natural rubies. However, natural ruby lacks an absorption peak of 270nm between 250 and 400 nm, which can be used as the basis for Knischka to distinguish synthetic ruby from natural ruby (see Figure 4- 1-9).
Fig. 4- 1-9 UV-Vis absorption spectrum of natural ruby synthesized by flux method.
4) Internal characteristics: ① Flux inclusions, the residual flux in the ruby synthesized by Knischka often forms some strange clouds and veils, and may also have irregular tubes with obvious shrinkage bubbles and solidified flux glass with high refractive index; (2) negative crystals. Another characteristic of ruby synthesized by Cnis Card is that there are a large number of negative crystals with large volume and different shapes. These negative crystals are dispersed or aggregated in clusters, and the biconical negative crystals appearing in groups are distributed at the end of the long crystal tube, which is regarded as the identification basis of Cnis's synthetic ruby. (3) Platinum flakes, compared with rubies synthesized by other fluxes, platinum flakes synthesized by Knischka are mostly twisted hexagons and triangles; (4) Seed crystal, the seed crystal of Cnis synthetic ruby is collected from natural rubies in India and other places, so it can be found that there are natural inclusions and flux inclusions under the microscope; ⑤ Chemical composition, its trace elements are mainly chromium and iron, and a small amount of titanium and copper.
3.Lamla synthesized ruby.
Ramaura synthetic ruby is a kind of synthetic ruby with spontaneous nucleation, which was introduced in 1983. Its characteristics are as follows.
c { 000 1 }； r { 10 1 1 } d { 0 1 12 }
1) crystal characteristics Isotropic rhombohedral crystals often appear in rubies synthesized by R am aura, and three crystal planes are developed on them, namely, the bottom axial plane c{000 1}, the rhombohedral plane R {10 165438} and the negative rhombohedral plane; The bottom axial plane c is relatively small, interspersed with twins; Unlike rubies synthesized by D ouros, the interlayer twins often develop in rhombic crystals (see Figure 4- 1- 10).
Fig. 4- 1- 10 Ruby Crystal Synthesized in Lamla
2) Ruby synthesized by fluorescent Ramaura has obvious orange-red fluorescence under long-wave ultraviolet lamp due to the addition of some rare earth elements. Short-wave ultraviolet fluorescence is the same as long-wave ultraviolet fluorescence, but the fluorescence intensity is slightly lower, and a few samples can have blue-white fluorescence.
3) Internal characteristics: ① Flux inclusions: Flux residues in Ramaura synthetic rubies are often orange-yellow, and a few are colorless or white. The residual flux is distributed along some directions of the crystal, forming some regular parallel arrangement or hexagonal network patterns, and some small flux aggregates are arranged in a ladder shape with obvious "turtle cracks" inside; (2) Platinum flake, the ruby synthesized by Ranaura rarely contains platinum flake; (3) Color and ribbon: Ramaura synthetic rubies are mainly purplish red, rose red and red, and color unevenness is manifested in almost every gem. This inhomogeneity is usually manifested as a spindle-shaped and triangular pattern color block. When the gem is rotated, the earthy tones of the color block is obviously enhanced, which can be distinguished from the "honey-like" structure of natural Burmese ruby. (4) The rich patterns formed by the growth line of the ruby synthesized by growth line and Ramaura under the oil immersion microscope have become an important identification basis for this gem. There are roughly two forms of growth lines, including nearly straight parallel growth lines and slightly curved parallel growth lines. Several different forms of growth lines intersect at a certain angle, forming an irregular growth phenomenon; ⑤ Chemical composition: In addition to Cr, Fe and Ti, the ruby synthesized by Ramaura also contains a small amount of K and Ca, and Pb exists in energy spectrum analysis, because the main fluxes used are lead oxide (PbO), lead fluoride (PbF2), bismuth oxide (Bi2O3) or lanthanum oxide (La2O3).
4. Ruby synthesized in Duross
Douros synthetic ruby is a kind of spontaneous nucleation and seedless flux synthetic ruby introduced by 1993. Its main features are as follows.
1) crystal features There are two common crystal forms: rhombohedral and tabular. Only three crystal planes can be seen on the crystals of the two shapes, namely, the bottom axis plane c{000 1}, the positive rhombohedral and the negative rhombohedral, and twins are interspersed in the tabular crystals. The rhombic crystal shape is shown in Figure 4-1-1.
Figure 4-1-1/artificial ruby crystal with rhombic dolomite
2) The uncut synthetic rubies in Duross have all kinds of fluorescence. Because of its low Cr content, it can show no fluorescence or very weak fluorescence at its edge. Part of the outer layer shows yellow-yellow-orange fluorescence under long-wave ultraviolet, and medium-strong blue-white fluorescence under short-wave ultraviolet. However, most finished rubies show stronger red fluorescence than natural rubies.
3) Internal characteristics: ① flux inclusions, the ruby synthesized by Douros is pure inside, and a small amount of residual flux mainly appears in two forms, namely, the dispersed coarse circle, strip or veil formed by the polymerization of fine flux droplets, and the flux is mostly bright yellow. With the decrease of temperature, many holes are left in the center of flux contraction, and the edge presents a "mosaic" structure; (2) Color and ribbon: Ruby synthesized by Douros can be dark red, purple red, reddish purple, etc. And the color distribution is uneven. Pale red-colorless ribbons can be seen at the edge of plate crystal and the junction of twins, and purple or blue-purple acute triangle color blocks can be seen in diamond crystal and plate crystal; ③ Growth line: under the oil immersion microscope, the D surface of rhombic crystal has obvious arc umbrella outline, which constitutes the identification basis of synthetic ruby in Duross (see Figure 4-1-12); ④ Chemical composition: Ruby synthesized by Douros contains elements such as Ti, Fe, Ni and V besides Cr, and its flux component is Pb(NO3)2, so there is Pb in the test. The chemical composition of ruby synthesized in Duross is slightly different on different crystal faces, so that the refractive index values on different crystal faces also change slightly. It can be measured that the refractive index and birefringence of the finished ruby are greater than that of the natural ruby. No = 1.772 ~ 1.774, NE = 1.762 ~ 1.764, and the birefringence is 0.010 ~ 0.0/2.
Fig. 4- 1- 12 ruby diamond crystal synthesized by Dorothy.
Schematic diagram of D-plane umbrella-shaped growth zone (under oil-immersed orthogonal polarization) 1- orange growth zone at crystal edge; 2- red growth area; 3— Deep red umbrella-shaped growth area
3. Growth of Emerald Crystal by Flux Method
The history of emerald synthesis by flux method can be traced back to 1848. J.J.Ebelmen used molten H2BO3 as flux and natural emerald powder as raw material, which was dissolved and cooled to obtain flaky crystals. Generally speaking, however, emeralds grown by flux method reported by P.Heufefeuille and H.Perry in 1888 and 1900 are regarded as the beginning of synthesizing emeralds, and this method has been adopted by later generations. The flux used is Li2Mo2O7 (or Li2O+Mo O3+X2O5). Emerald powder was added and melted at 800℃. After 14 days, the emerald crystal with the size of 1mm (light yellow green containing Fe and green containing Cr) was grown. When it exceeds 800℃, it becomes beryl (Be2SiO4).
Later, Espig in Germany studied the synthesis of emeralds by flux method, but it was C.Chatham and P.Gilson who really put the synthetic emeralds into commercial production.
1 Synthesis of Emerald Crystal. Espig flux method
The synthetic emerald of Espig is shown in Figure 4- 1- 13.
The platinum crucible is180mm (depth) × 85mm (height), filled with 2.8kg of silicon dioxide, and platinum grid is added with a platinum tube. The upper SiO _ 2 ~ 4 is added every 4 weeks, and the lower raw materials are supplemented every 2 days.
Fig. 4-Growth Principle of Synthetic Emerald with Espig+0-13
The raw material formula consists of 2 parts of SiO2 _ 2, 2 parts of BeO, 4 parts of Al _ 2O _ 3 and a small amount of Li2CrO4. After the feed is heated to 800℃, after the raw materials are melted, Al2O3, BeO and Li2CrO4 diffuse upward due to the heat at the bottom, and SiO2 diffuses downward, meeting the emerald seeds under the grid. If it is supersaturated here, it will grow on the crystal nucleus, and a crystal with a size of 20mm can grow in 12 months. The seeds can be seeded or spontaneously nucleated.
Espig growth method is an early research achievement, and the grown crystals are small, with many inclusions and poor integrity. Hard to grind 1ct 1ct (carat) = 0.2g ..
Above the torus.
2. Chatham synthesizes emeralds
Chatham is an accomplished scientist in the synthesis of gem flux, and he has made great contributions to the synthesis of rubies and emeralds. Chatham also uses Li2O+2MoO3 as flux and slow cooling method when synthesizing rubies and emeralds, but the furnace and crucible are relatively large, and the typical growth cycle is 12 months. Because the growth process is confidential, there is no detailed technological process published, but from the point of view of the grown crystal clusters, it is spontaneous nucleation and slow cooling growth.
3.3 Synthetic Emerald. Gilson flux method
France Gilson is another company in the world that synthesizes emeralds by flux method and puts them on the market.
The synthesis of emerald by gilson flux method is divided into two steps. The first step is to optimize the seed crystal, as shown in Figure 4- 1- 14. Firstly, colorless beryl flakes are selected as seed crystals, and both sides grow to synthesize emeralds, and then the synthesized emeralds are cut off as seed crystals.
As shown in Figure 4- 1- 15, the intermediate grid divides the platinum crucible into two areas. In the hot zone, beryl blocks are used as raw materials, and the flux is Li2Mo2O7. In the hot zone, beryl molecules melt into the flux, and in the cold zone, beryl molecules precipitate and grow on the seed crystal in a supersaturated state. The temperature difference between these two areas is very small, mainly to maintain a low supersaturation. The fluid convection in the two regions can be driven by machinery. The typical growth process is 1 mm long per month, and the crystal with the size of 14 mm× 20 mm can cut out the gem with the weight of 18ct.
Fig. 4- 1- 14 schematic diagram of seed optimization
Fig. 4- 1- 15 schematic diagram of emerald synthesis by gilson method.
Flux method can also be used to grow YAG, SrTiO4 and other crystals, but these crystals have been rarely used in gem business in recent years, so they are not introduced again, and their principle is similar to that of synthetic ruby.
Four. Identification of gems synthesized by flux method
1. Inclusion characteristics
1) Solid inclusions in crystals grown by flux method often include crystal phase inclusions, flux inclusions, unmelted melt inclusions and crucible metal material inclusions. Crystal inclusions, such as beryl inclusions in synthetic emerald crystals. Flux inclusions are usually many, opaque and diverse, sometimes similar to those in natural gems. The crystals grown by flux method are more or less polluted by crucible materials, and there are some unmelted inclusions of molten raw materials.
2) The growth of gaseous inclusions in crystals by flux method is caused by the volatility of flux, and sometimes gaseous inclusions and solid inclusions coexist, which can form gaseous and solid inclusions.
Grow stripes
Flat growth lines can sometimes be observed in crystals grown by flux method, which is caused by the change of relative concentration of components or the periodic change of impurity concentration. The appearance of growth lines is also related to the existence of fine inclusions in crystals.
3. Chaos
Most crystals grown by flux contain spiral dislocations. When the screw dislocation terminates on the crystal plane, a growth hill or winding line will be formed on the surface, and a small inclusion center is often connected below the growth hill. Generally speaking, the dislocation density of gem crystals synthesized by flux method is low. Under the condition of stable growth, there are few growth hills on the crystal surface, sometimes only one.