DE1544331A1 - Method for growing garnet single crystals - Google Patents

Description

April 7 , 1970 Gzy / bü

UNION CARBIDE CORPORATION, '270 Park Avenue, New York, N.Y. 10017

Method for growing garnet single crystals

Single crystals of rare earth containing garnets are heretofore by melting flakes and by crystallizing from the JMflMe'lfce manufactured before. Those made by flaim melting Crystals often have strong internal tensions. The crystals obtained from the Schaelze by crystallization are in ■ Usually small and contain impurities. Impurities and irregularities, however, interfere with the light conduction of the crystals and prevent their use in lasers.

One object of the invention is the manufacture of single crystals made of garnet, which are pure and have no internal stress, so they are used in laser devices and for other technical purposes Purposes, can also be used as jewelry.

The invention relates to a method for growing single crystal len made of yttrium-doped aluminum garnet. The procedure consists in making a melt of the oxides of YttriUB and aluminum and from the oxides of praseodymium and / or neodymium produces the molten material on the for crystal formation The required temperature is heated, a seed crystal is introduced into the melt and this seed crystal is removed from the Melt at a rate of not more than 1.25 aa pulls out every hour. Here is a uniformly doped Obtained single crystal.

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One pulls the seed crystal from the melt at one speed out, which is less than the maximum allowable growth rate of the crystal. This will make that Penetration of excess amounts of doping elements into the crystal with the formation of a second phase in the crystal prevented. Between the melt and the growing surface of the crystal, a structure containing the doping ions forms,

standing layer from which the doping substances diffuse.

It is known to produce larger single crystals of various substances Pulling out a single crystal from a molten mixture of the substances forming the crystal. The speed at which it is withdrawn is as a rule dependent on the maximum allowable rate of crystal growth; this again depends on the heat transfer on the growth area. It has now been found that the Production of single crystals from yttrium-aluminum-garnet the growth rate must be much slower than that normal allowable growth rate to the occurrence to prevent a second phase in the finished crystal.

In the process of growing crystals from the melt consists usually movement between the solid crystal and the melt, for example achieved by rotating the crystal or by condensing the melt through convection currents. This consists of a fixed layer of molten material, often called the diffusion layer, and the Surface of the solid crystal is directly adjacent. The 'material in the bulk of the melt is caused by convection currents emotional; but in this layer atomic diffusion is the only means of hardening ions through the diffusion layer from or to the growth surface. While growing of the crystal harden the starting materials in the diffusion

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layer and are replaced by substances from the main amount of the melt, which are composed according to the composition of the melt. The content of doping ions in the solid crystal rarely corresponds to the content of the melt on these substances. The partition coefficient is the ratio of the concentration of the dopant in the crystal the concentration of the doping substance in the melt, that the equilibrium bit is in the crystal. When the distribution coefficient is greater than 1, i.e. when the concentration of the dopant in the crystal is higher than in the melt, then the diffusion layer gradually becomes depleted of dopants. Conversely, if the distribution coefficient is less than 1, i.e. if the concentration of Dopant in the liquid is greater than in that Crystal, then ill — t the concentration of the doping substances In the Diffuaionsachicht too. When the increase in concentration of the doping substances in the diffusion layer leads to a change and the formation of a second l & ase, then can these second phases are included in the crystal. These second phases cause a refraction and a reflection of the light in the crystal and thus impair the optical properties. Therefore, such crystals are unsuccessful to use as lasers because of the high threshold values for the laser effect must be exceeded. Excessive precipitation of doping additives can also make the crystal cloudy or opaque, which can affect its properties as Schouckstein impaired.

A gown to prevent excessively high concentration of Dopants in the diffusion layer is that ■ reduces the content of dopants in the melt. But if the melt does not contain enough dopants are and therefore only a few doping ions in the

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Crystal are built in, so this cannot contain enough active ions in the garnet lattice to have optimal properties for lasers or as gemstones. The inventive method allows the use of correct amounts of doping substances in the melt to obtain a highly doped crystal that still does not contain any second phases.

A particularly good laser material is a yttrium-aluminum garnet, which contains various amounts of ions of the rare age Earth is doped, up to 50 atomic percent; the atomic percent of the dopant is the mole percent of the dopant «· rendcn ions divided by the sum of the mole percent of the doping ions and the yttrium ions. Pure yttrium-aluminum groups achaelzen congruent and can old from a melt Vachatimafeachwindifkelten gained about 12-13 »» / hour will. VJJl "on but" cultivate a garnet, since 1.5 Atoaprotent Neodya is endowed with 1.5 In the process, second phases enclosed in the crystal arise. Such products are not suitable for Laser or as a show file. Even if the concentration of Neodraa is reduced to 0.06 atomic percent, it is created nevertheless second phases, which the successful use of dea Prevent crystals as a laser. Larger amounts of dopant also increase the amount of the second phases.

The distribution coefficient for neodymium in yttrium-aluminum-garnet is around 0.25. As explained above, the alao grows Concentration of the neodymium in the diffusion layer very quickly after the start of crystal growth. The enrichment of the diffusion layer in neodymium can be achieved by the method according to the invention can be reduced by reducing the speed at which the crystal is pulled out of the melt will. ".

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This increases the speed of the first in the diffusion layer contained material is reduced, at the same time ■ with the influx of additional, neodymium-containing material from the melt. The excess of neodymium contained in the diffusion layer can therefore be extracted from the diffusion layer in diffuse in the bulk of the melt.

It was found that the pull-out speed of the crystal for obtaining a single crystal from doped yttrium-aluminum-garnet depends on the ionic radius doping rare earths and their relationship to the ionic radius the yttrium that is replaced by the doping ions; this speed is also dependent on the concentration of the dopant. If one wants to produce crystals doped with 1.5 atomic percent neodymium or praseodymium, then one must about 4 times the amount of doping ions in the melt to be available; the crystal must be pulled out of the melt at a speed of 1.25 Na / h. Want to produce crystals with a 3 atomic percent dopant »see above the maximum growth rate is 0.6 mm / h.

To carry out the process, you bring the starting materials, Yttrium oxide, aluminum oxide and the oxides of additives, in a suitable refractory container or crucible. This container or crucible should "consist of a" refractory material " its Scheelzpunkt above the required working temperature door is around 2Ö50 ° C. The crucible should also face the Must be inert. Iridium is a good crucible material.

The starting materials can be prepared using induction heating, resistance heating, Flames, arcs and hot gas streams melted will. The crucible is used for induction heating

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as a suscepter for an alternating electric field. Induction * - Currents are generated in the crucible and heat it to high levels Temperatures at which the yttrium oxide, the aluminum oxide and the additives are heated by conduction.

Induction heating can be carried out at atmospheric pressure or pressures above or below. But you can also heat the crucible by a direct electrical potential and let resistance currents run through the crucible. Likewise, a flame, for example made up of oxygen and hydrogen, _{can be directed at the crucible%} An electric arc can touch the crucible and the crucible can be heated instantaneously by resistance. Other possibilities . consist in the use of a gas stream heated by an arc.

After the enamel has formed, a ± * ae "larch convection" is stirred and thereby receives an even consistency. Nan can of course also use other means of stirring.

A monocrystalline seed crystal of the desired composition and the desired crystal orientation is then introduced in contact with the surface of the Schaelze. A small part of the seed crystal melts and a temperature gradient is created between the solid part of the seed crystal and the melt. Then you pull the seed crystal slowly out of the Schaelze, at the speed indicated above. The molten material solidifies on the surface between the solid seed crystal and the effusion layer. The temperature gradient in the plague phase affecting this surface immediately adjacent 1 is kept at a value which makes it possible to obtain the desired growth conditions. If you have the When the seed crystal pulls out, an elongated single crystal body is formed. The material is a yttrium aluminum garnet of the formula

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In the case of the doping atoae chosen, they are housed by the violent ytterium-aluminum-cranate. The doping crystal, for example Y-Al ₅ O,.. When pulling the seed crystal, it should also be rotated. The speed of rotation is generally 60 revolutions per minute.

The invention is explained below with reference to the drawing 10 1st a crucible on a rack 12. Coils 14 for induction heating surround the crucible and heat it. the Melt 16 in the crucible is heated by conduction from the walls of the crucible. Conduction strums take care of the circulation within the melt. In the present case, the Melt from an "Oeaiaeh" er oxides of yttrium, aluminum and neodymium. A crystal 18 is made from the melt pulled upwards, rotating around its longitudinal axis. The growth surface 20 is shown, for example, conical. Due to the convection currents within the melt and the rotation of the continuously withdrawn crystal creates a stagnant layer 22 of molten material between the growth surface of the crystal and the actual enamel ·. In this stagnant diffusion layer, the doping ions migrate through diffusion and not through the convection currents. Since the wachatuma speed is sufficiently slow, the unused doping ions can escape from the diffusion slot into the actual melt diffuse back. This avoids an excessive concentration of doping ions in the diffusion layer. A single-phase crystal is created.

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.,., \ Example I

To produce a crystal from yttrium-aluminum-garnet which was doped with neodymium, the procedure was as follows. In the crucible 32.51 g of powdered Y ⁰ were * "25 * 47 g Al.CL · (crushed sapphire) and 2.02 g Nd 0 is introduced in powder form. This amount corresponded to a content of 1.5 atomic percent 3-vertigen neodymium in the crystal. Pure yttrium aluminum garnet was used as the seed crystal. The temperature of the melt was kept between 2035 and 2050 C. The growth rate of the crystal was 1.25 mm per hour. The crystal was rotated at 60 revolutions per minute. A crystal was obtained with a length of 3.8 cm and a diameter of 0.5 cm. The crystal contained no second phases, no inclusions and was slightly blue in color.

A laser rod was made from this crystal. The rod had a diameter of 3 ma and a length of 30 aa old spherical ends coated with a multilayer dielectric bit. A 4-Riasen laser was made from it; The Laseratab was driven at room temperature with a mercury arc lamp with a threshold value of 1.96 Jeules. You could also use the laser continuously at room temperature operated with a volfram lamp of 350 W.

If the washing speed of crystals of this type is increased, inclusions of second phases arise, which make the crystals unusable for La β purposes.

A kit Pr & eeodfn doped Krlasst «31 manufactured by Yttri uB-Aluminiua-ar * mA litir & o as follows. Used in an Ir-I

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dium crucible 32.529 g of powdered Y ₀ O ₃ , 25.492 Al ^ O, (crushed sapphire) and 2.044 g of Pr ^ O ₁₁ in powder form, which corresponded to a content of 1.5 mol percent PrgO., ^ in the melt. Pure yttrium aluminum garnet was used as the seed crystal. The temperature of the melt was maintained at about 2030 ° C. The crystal was allowed to grow at a speed of 1.25 mm per hour while rotating at 60 revolutions per minute. A crystal was obtained about 3/8 per length. The crystal was single-phase: and had a pale yellow color.

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Claims (4)

- ίο - claims 1. Method of growing single crystals from doped yttrium ---. Aluminum garnet, characterized in that a melt of the oxides of yttrium and aluminum minium and the oxides of praseodymium and / or neodymium is made, this melt is brought to the temperature required for crystal formation introducing a seed crystal into the molten material, and this seed crystal of the Sch & elze with a 1.25 mm atündlich not (Iberschreitenden _speed; · extracts. 2. The method according to claim 1, characterized in that the Temperatu is th. Gehal- Schaelze temperature of about 2030 to about 205O ⁰ C. 3. The method according to claim 1 or 2, characterized in that a starting mixture of 32.51 g Y2O3, 25.47 g A1 “O- and 2.02 g! Fd ^ O- is used. 4. The method according to claim 1 or 2, characterized in that that one uses an Auagangegeaiech of 52.529 g ^ ₂ ⁰ V ² ^ f 492 g and 2.044 g PTgO _- ., - . BATH ORIGINAL 009S26 / 164 *