Brief introduction of the first author: Chen, the first and second members of the Artificial Products Professional Committee of China Insurance Association, the third senior consultant, and a researcher at Southwest Institute of Technical Physics.
I. Introduction
Natural sapphire, the mineral name is corundum, the matrix is alumina (Al2O3), and the Mohs hardness is 9, which is second only to diamond. It is one of the best mineral gems on earth. Pure corundum gem is colorless, and different trace impurities can present various colors. Due to historical reasons, red corundum gems are usually called rubies, while corundum gems of other colors are generally called (colored) sapphires, such as colorless sapphires, blue sapphires, yellow sapphires, pink sapphires, orange sapphires, green sapphires and purple sapphires.
Among all kinds of colored sapphires, "Padparadsha" sapphire is the most precious. The Chinese website of the International Association of colored gems (ICA) reported that in 2004, due to the limited production and strong demand of natural pink sapphires, the international market price of natural pink sapphires of 2-3 carats rose from the original $ 300-400 per carat to $600.
It is understood that in the past two years, the jewelry market has increasingly favored pink materials, and Italian jewelry design is also full of pink tones. Consumers' love for pink jewelry directly leads to the price increase of pink sapphire. At present, the market is in urgent need of large-size pink sapphire, but the supply is insufficient. In the international gem market, a synthetic pink sapphire has appeared. This paper will discuss the growth, preparation method and gemological characteristics of this synthetic pink sapphire.
Second, the coloring mechanism of pink sapphire
As we all know, pure corundum gem crystals are colorless, and a small amount of coloring elements can be added to corundum gem crystals to make them appear in various colors. Common colors of corundum gemstones are red, blue, green, yellow, orange and purple. The color of corundum gem is related to the type, content and combination of impurity elements. Table 1 shows the relationship between the color of natural sapphire and impurity elements and their valence states.
Table 1 Color of natural corundum gemstone and combination of colored ions or colored ions
As we all know, as mentioned above, adding a small amount of Cr (Cr) and Ti (Ti) to corundum crystal can make it appear red. When chromium (Cr) is doped alone, corundum gem appears very pure red. When Cr (Cr) is doped with other coloring elements [such as Fe (iron), Ti (titanium)] or other color centers, corundum gems will turn into other reds, including orange red to orange pink, that is, pink corundum gems. However, the true chromogenic ions of natural pink "Padparadsha" sapphire are still unclear.
The synthetic pink sapphire discussed in this paper is similar to the natural "Padparadsha" sapphire, and it is a kind of sapphire doped with trivalent titanium ions. In fact, it is a by-product of our research and development of laser material Ti: sapphire. This titanium-doped sapphire laser crystal is used to generate tunable laser in high-tech field (Chen et al., 1993).
It is known that titanium ions in various valence states exist in titanium-doped sapphire crystals. In principle, according to the different crystal growth conditions or heat treatment conditions, bivalent, trivalent and tetravalent titanium ions may appear. Different valence states of titanium ions will make sapphire appear different colors.
The experiment shows that the relative proportion of different valence titanium ions in titanium-doped sapphire crystal can be changed by changing the growth conditions or heat treatment conditions. Titanium ion is a kind of ion which is easy to change valence. During the growth of a single titanium-doped sapphire, the valence state of titanium ions and the partial pressure of oxygen in the growth environment have the following reversible chemical equilibrium reactions:
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From the above two chemical equilibrium equations, it can be seen that the direction of the reaction (or the valence state of titanium ions) is related to the partial pressure of oxygen in the gem growth environment. When the partial pressure of oxygen in the environment decreases, the reaction proceeds in the positive (or right) direction, otherwise it proceeds in the opposite direction.
In the presence of trivalent titanium ions, the decomposition temperature or the corresponding equilibrium oxygen partial pressure of the above two chemical formulas can be expressed by the following relationship (Shigeo Kimura et al., 1998):
lgp(O2)=t/200—22 3 (3)
Where: p(O2) is the equilibrium oxygen partial pressure in the presence of trivalent titanium ions, and the unit is atmlatm= 10 1325Pa.
; T is the melt temperature in℃. For sapphire grown by Czochralski method, the growth temperature is about 2050℃. According to the above formula, the equilibrium oxygen partial pressure of trivalent titanium ion is10-9 ~10-15 ATM. However, in this oxygen partial pressure range, in addition to trivalent titanium ions, tetravalent titanium ions or divalent titanium ions can also exist at the same time. Further, according to the experimental results (Shigeo Kimura et al., 1998), the optimal equilibrium oxygen partial pressure range for trivalent titanium ions should be10-12-/kloc-0-. For the heat treatment of titanium gemstone after growth, the heat treatment temperature is about 1950℃, and the equilibrium oxygen partial pressure of trivalent titanium ions is also about10-9 ~10-15 ATM. Therefore, only trivalent titanium ions exist in the optimal equilibrium oxygen partial pressure range of about10-12 ~10-13 ATM.
Under the normal neutral atmosphere growth condition, because the vacuum degree in the crystal growth furnace before aeration is about 10-8atm, the partial pressure of oxygen in the furnace is basically in the same order of magnitude, so it can be considered that the partial pressure of oxygen in the furnace is also about 10-8atm. According to the above discussion, the "original" titanium sapphire grown under this condition will contain both trivalent titanium ions and tetravalent titanium ions. In the experiment, the Ti-doped sapphire grown under these conditions is light red with purple tone (Figure1). According to the above discussion, by changing the growth or heat treatment conditions, the partial pressure of oxygen in the furnace is within the optimal range where only trivalent titanium ions exist, and most of the tetravalent titanium ions in sapphire can be converted into trivalent titanium ions, and the titanium-doped sapphire obtained after this process treatment presents a beautiful pink color (Figure 2, Right, Figure 3 and Figure 4).
Third, experiment.
The specific preparation method of pink sapphire is briefly introduced below (Chen et al., 1993).
1. Seasoning
What we want to grow is a sapphire colored by trivalent titanium ions. In order to prevent other coloring elements from affecting the color of corundum gem, we use high-purity raw materials: 99.999% alumina (Al2O3) and 99.99% titanium tetroxide (Ti2O3). Al2O3 with a purity of 99.999% was purchased from Dalian Ruier Precision Ceramics Co., Ltd., a Sino-foreign joint venture. In a self-made high-temperature pure hydrogen atmosphere furnace, titanium dioxide with purity of 99.99% produced by Shanghai Chemical Reagent General Factory was reduced to prepare titanium dioxide with purity of 99.99%.
During batching, high-purity titanium trioxide (Ti2O3) powder is first mixed into high-purity alumina (Al2O3) powder according to the mass fraction of 0.5% ~ 2%, then evenly mixed, pressed into blocks by an oil press, and then sintered at 65438 0300℃ for 24 hours to make polycrystalline blocks and put them into the furnace.
2. Gem growth
In order to grow pink sapphire, we adopted the standard Czochralski crystal growth process. The crystal pulling furnace is SJ-763 laser single crystal growth furnace produced by Xi 'an University of Technology. The medium frequency power supply (4kHz) is produced by Xi 'an Power Electronics Research Institute with a rated output of 50kW. Fill the material with iridium crucible and carry out medium frequency induction heating. The crucible size is φ 120 RNM× 120 mm, the charging time is about 4 ~ 5 kg, and the heating power is about 12 ~ 14 kW. The growth direction of sapphire crystal is 90 degrees to the C axis, the pulling speed of sapphire is about 0.4 ~ 2 mm/h, and the atmosphere in the growth furnace is nitrogen plus helium. It usually takes 25 days to grow a titanium-doped sapphire laser crystal, and the size of the grown titanium-doped sapphire crystal is about 60 mm×120 mm.
Figure 1 "Original ecology" titanium-doped sapphire is pink with obvious purple tone.
3. Heat treatment
The "original ecology" titanium-doped sapphire grown by the above method is pink with obvious purple tone (the depth of red is positively correlated with the titanium doping amount, as shown in Figure 1 and Figure 2 on the left). Its absorption spectrum is shown as curve a in fig. 5.
Furthermore, we put the "original ecology" titanium-doped sapphire in the pure hydrogen reducing atmosphere of 1920 ~ 1950℃, and then heat-treat it at constant temperature for 48 hours, and get a beautiful pink sapphire (see the right of Figure 2). The absorption spectrum is shown as curve b in fig. 5.
Four. discuss
"Original ecology" titanium-doped sapphire is pink with obvious purple tone, and its absorption spectrum has a strong and broad absorption peak at about 400 ~ 580 nm in purple-blue-green region, and its tail extends to about 580 ~ 700 nm in yellow-orange-red region, and further extends to 700 ~ 900 nm in near infrared region, forming a wide absorption band without obvious structure.
After the above heat treatment, the absorption spectrum of "original ecology" sapphire has the following changes: the absorption peak in the purple-blue-green region is enhanced at about 400 ~ 580 nm; However, the broad absorption band with a peak of about 750nm is basically eliminated, so the yellow-orange-red transmittance of about 580 ~ 700 nm is enhanced, and the near-infrared wide absorption band of 700 ~ 900 nm is also greatly weakened. In addition, the ultraviolet absorption below 400nm is also weakened. Accordingly, the purple tone in the color of the "original" titanium-doped sapphire is basically eliminated and turned into a very pure pink (Figure 2, right and Figure 3).
Fig. 2 Color change of pink sapphire before and after heat treatment.
Left is the color before heat treatment (with purple tone), and right is the color after heat treatment (with purple tone removed).
Fig. 3 pink sapphire crystal blank after heat treatment (surface not polished)
Fig. 4 Pink sapphire blank (surface polished) and processed faceted gemstone.
Fig. 5 changes of absorption spectrum of synthesized titanium-doped sapphire before and after heat treatment
(According to Kokta, 1986)
Identification and discussion on gemological characteristics of verb (abbreviation of verb)
The artificial pink sapphire in this paper has been appraised by Jewellery College of China Geo University and Beijing National Jewellery Quality Supervision and Inspection Center. The results are as follows:
The matrix crystal is sapphire; The color is pink (pink-orange pink), transparent and with strong glass luster; Mohs hardness 9, relative density 4.023; It is an optically anisotropic body, and the black cross interferogram of uniaxial crystal can be seen under cone light. The refractive index is 1.764 ~ 1.758, and the birefringence is 0.006. The high refractive index remains unchanged and the low refractive index moves, which is the negative optical property of uniaxial crystal. Strong dichroism, light pink/pink orange, light yellow-green under the color filter, with pink flash. There is no characteristic absorption spectrum under the hand-held spectroscope, which is inert under long-wave ultraviolet light and moderately blue-white under short-wave ultraviolet light.
In addition, a large number of dispersed deformed bubbles (micron level) can be observed under the microscope (Figure 6).
Fig. 6 The scattered small bubbles (left) and the enlarged deformed small bubbles (right) observed by the pink sapphire synthesized in this paper under the microscope.
Further energy spectrum analysis shows that the synthesized pink sapphire material contains trace amounts of S (sulfur) and Ti (titanium), as shown in Figure 7.
Appraisal conclusion: The color of synthetic pink sapphire is similar to that of "Padparadsha" sapphire.
Energy spectrum of synthetic pink sapphire.
Acknowledgement: Professor Yan from the Jewelry College of China Geo University (Wuhan) provided a lot of help for the writing of this article. Gemmological tests were carried out on the pink sapphire synthesized in this paper by China Geo University (Wuhan) Jewelry College and Beijing National Jewelry Quality Supervision and Inspection Center. Thank you!
refer to
Chen et al. Crystal growth and properties of titanium gemstone. Laser technology, vol. 17, No.2, P 107 ~ 1 1.
Maoxing Kimura et al. 1988. Growth method of high quality titanium sapphire single crystal. Japanese patent Zhao 63-274694.
Kokta M R. 1986。 The method of enhancing the fluorescence of Ti:al2o 3 aluminum titanate laser crystal by annealing, OUS patent 4,587,035.