A new type of bioceramic developed in recent years is alumina single crystal. Alumina single crystal is also called gem. Single crystals of alumina used as bioceramics are colorless and transparent (called white gemstones). Different additives have different colors, such as ruby, sapphire and so on.
1. Characteristics of alumina single crystal
Many properties of alumina single crystal, such as mechanical strength, hardness, acid and alkali resistance, are better than alumina polycrystalline ceramics. Moreover, its biocompatibility, stability in the body, and abrasion resistance are also better than polycrystalline ceramics. The most significant difference is still reflected in the mechanical strength. Therefore, alumina single crystals have been widely used in the medical field. Especially used in parts requiring high strength, wear resistance and corrosion resistance.
The chemical composition of alumina single crystal is the same as that of polycrystal, and the crystal type is α-Al2O3 (that is, corundum type). Therefore, the difference in performance between alumina single crystal and polycrystal can be mainly explained from the structural difference, that is, the structure of alumina single crystal is complete, with few defects, and no fragile grain boundary phase. Therefore, microcracks and crack propagation are not easy to appear under stress, so the strength is high. The structural integrity is also reflected in acid and alkali resistance and biostability (or biocompatibility).
There are few single crystals in nature, and the currently used alumina single crystals are all prepared by artificial methods.
2. Production method of alumina single crystal
There are many production methods of alumina single crystal, including pulling method, guided mold method, vapor chemical deposition method, flame melting method and so on.
(1) Lifting method
Tila method is also called Krask method or quotation method. That is, the raw materials (alumina and additives) are loaded into the crucible, the crucible is placed in the single crystal furnace (high-frequency induction furnace), the raw materials are completely melted by heating, and the seed crystal mounted on the seed rod is immersed in the melt In contact with the liquid surface, adjust the temperature precisely, slowly (at a speed of 1.0~4mm/min) to pull the crystal rod upwards, and rotate at a certain speed (the crucible speed is 10r/min, the speed of the seed rod 25r/min, but their directions are opposite) so that the crystallization process continues on the solid-liquid interface until the crystal growth reaches the original length.
(2) Guided mode method
Referred to as EFG method. In the intended growth of the single crystal material melt, put a hollow mold (guided mold) with the same top surface as the cross-sectional shape of the crystal to be grown. The mold material should be able to fully wet the melt without reaction. Due to the capillary phenomenon, the melt rises to the top surface of the mold to form a thin melt surface, and the seed crystal is immersed in it to pull out a single crystal with the same cross-section as the top of the mold.
(3) Vapor chemical deposition method
This method is also called the CVD method. Evaporate metal hydroxides, halides or metal organics into the gas phase, or use appropriate gas as a carrier, transport them to a lower temperature zone where they can condense, and deposit them on a certain substrate through chemical reactions to form thin-film single crystals .
(4) Flame melting method
Flame melting method is also called Werner Ye method. This method has simple and economical equipment and technology for growing single crystals, and does not require expensive iridium crucibles as containers, so it is still a commonly used method.
The initial raw material for preparing alumina single crystal in this method is the same as that of high-purity alumina ceramics, which is ammonium aluminum sulfate, but it must be the raw material obtained from industrially synthesized ammonium aluminum sulfate after multiple recrystallizations and purification.
Ammonium aluminum sulfate is heated to between 200~900℃, first dehydrated to become anhydrous alum, then gradually changed to aluminum sulfate, and then desulfurized into an amorphous compound, and finally formed gem-making raw material γ-Al2O3, if the temperature rises again, It becomes α-Al2O3. Therefore, when γ-Al2O3 is obtained from the ammonia decomposition of aluminum sulfate, the internal temperature of the crucible is required to be very uniform. If the temperature is uneven, partly high or low, the grain size of the decomposed γ-Al2O3 will be inconsistent. Some even partly transform into α-Al2O3 or contain a small amount of amorphous or unsulphurized material, which is unfavorable for single crystal growth.
Flame melting growth furnace mainly includes: flame combustion device, crystallization furnace. The flame is the heat source that melts the powder and creates an appropriate temperature field, and the most commonly used is the chlorine-oxygen flame. The raw materials are loaded in the hopper, descended through the inverted oxyhydrogen flame nozzle, melted and deposited on the refractory support column in the holding furnace to form a melting layer, which is lowered and supported while crystallized. The distance between the crystal growth surface and the nozzle of the flame spray gun is a parameter that needs to be strictly controlled, generally 15cm. The crystal prepared by this method grows fast and the process is simple, but the crystal has many defects.