GB2074891A - Growing Crystals - Google Patents

Abstract

According to this invention a process for growing a crystal from a compound in a solution in which the compound has a negative temperature coefficient of solubility by suspending a seed crystal of the compound in a saturated solution (3) of the compound, comprises directing infra- red radiation substantially on the seed crystal (1) or in the immediate vicinity thereof. The infra-red radiation is absorbed by the surface of the seed crystal, heating it to a higher temperature than the solution and causing preferential deposition of solute to occur thereupon and not on the walls of the vessel. The invention may particularly be used for growing crystals of berlinite. <IMAGE>

Description

SPECIFICATION Improved Process for Growing Crystals This invention relates to processes for growing crystals from solutions in which the crystals have a negative temperature coefficient of solubility, that is the crystals become less soluble as the temperature of the solution increases. This invention relates especially but not exclusively to hydrothermal growth of berlinite crystals.

Berlinite, that is to say aluminium orthophosphate, is one of the few known piezoelectric materials from which a suitably rotated single crystal can be cut to provide resonators having very small frequencytemperature coefficients. Reasonably sized crystals of berlinite occur rarely in nature and have proved difficult to prepare artificially.

Consequently the use of berlinite crystals has hitherto been inhibited.

It is known to grow crystals of berlinite by suspending a seed crystal in a saturated solution of aluminium orthophosphate in 6 molar orthophosphoric acid, then gradually heating the solution causing crystallisation of the solute. This method suffers from a disadvantage that the formation of berlinite crystals occurs preferentially where the solution is hottest, that is on the walls of the containing vessel.

According to this invention a process for growing a crystal from a compound in a solution in which the compound has a negative temperature coefficient of solubility by suspending a seed crystal of the compound in a saturated solution of the compound, comprises directing infra-red radiation substantially on the seed crystal or in the immediate vicinity thereof.

The infra-red radiation is absorbed by the surface of the seed crystal, heating it to a higher temperature than the solution and causing preferential deposition of solute to occur thereupon and not on the walls of the vessel.

Although a single source of infra-red radiation has been found adequate for the production of single crystals, a number of such sources, disposed around a crystal may be used simultaneously to produce more even growth on the faces of the crystal than may be obtained using a single source.

The solution may be conveniently contained in a vessel of glass or other infra-red transmissive material with the infra-red source or sources located outside said vessel.

Infra-red radiation is preferably focussed onto a crystal to maximise the selective heating of the latter. The radiation is conveniently focussed onto a crystal by reflection from a suitably shaped mirror or mirrors.

A quartz-iodine lamp has been found to be a suitable source of infra-red radiation.

A number of crystals may be grown simultaneously from one solution using a suitable number of sources of radiation.

This invention includes crystals grown by the above process.

The invention is further described by way of an example with reference to the accompanying drawing, which illustrates in sectional view an apparatus for hydrothermal growth of an individual berlinite crystal.

A seed crystal 1, chosen for its suitability of form and freedom from imperfections, is suspended by a platinium wire 2 in a solution 3 of 1.2 molar aluminium orthophosphate in 7 molar aqueous orthophospheric acid contained in a sealed glass vessel 4. The vessel is placed in an oven and heated to 1 500C. Infra-red radiation from a quartz-iodine lamp 5 focussed onto the crystal by a gold plated mirror 6. The crystal 1 is irradiated until the temperature of the surrounding solution 3, as determined by a thermometer (not shown) has reached 2000C and growth of the crystal is complete, after which the vessel is rapidly cooled, to prevent dissolution of the crystal, and the crystal removed. In a modified apparatus the crystal is withdrawn from the solution prior to cooling.

Claims

1. A process for growing a crystal from a compound in a solution in which the compound has a negative temperature coefficient of solubility, comprising suspending a seed crystal of the compound in a saturated solution of the compound, and directing infra-red radiation substantially onto the seed crystal or in the immediate vicinity thereof.

2. A process according to Claim 1 in which the compound is aluminium orthophosphate.

3. A process according to Claim 1 or 2 in which infra-red radiation is directed onto a seed crystal from more than one direction.

4. A process according to any preceding claim in which infra-red radiation is focussed onto the seed crystal.

5. A process according to any preceding claim in which a quartz-iodine lamp acts as a source of infra-red radiation.

6. A process for growing a crystal substantially as described in the example.

7. A crystal grown by a method according to any preceding claim.

6. Apparatus for growing a crystal substantially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improved Process for Growing Crystals This invention relates to processes for growing crystals from solutions in which the crystals have a negative temperature coefficient of solubility, that is the crystals become less soluble as the temperature of the solution increases. This invention relates especially but not exclusively to hydrothermal growth of berlinite crystals. Berlinite, that is to say aluminium orthophosphate, is one of the few known piezoelectric materials from which a suitably rotated single crystal can be cut to provide resonators having very small frequencytemperature coefficients. Reasonably sized crystals of berlinite occur rarely in nature and have proved difficult to prepare artificially. Consequently the use of berlinite crystals has hitherto been inhibited. It is known to grow crystals of berlinite by suspending a seed crystal in a saturated solution of aluminium orthophosphate in 6 molar orthophosphoric acid, then gradually heating the solution causing crystallisation of the solute. This method suffers from a disadvantage that the formation of berlinite crystals occurs preferentially where the solution is hottest, that is on the walls of the containing vessel. According to this invention a process for growing a crystal from a compound in a solution in which the compound has a negative temperature coefficient of solubility by suspending a seed crystal of the compound in a saturated solution of the compound, comprises directing infra-red radiation substantially on the seed crystal or in the immediate vicinity thereof. The infra-red radiation is absorbed by the surface of the seed crystal, heating it to a higher temperature than the solution and causing preferential deposition of solute to occur thereupon and not on the walls of the vessel. Although a single source of infra-red radiation has been found adequate for the production of single crystals, a number of such sources, disposed around a crystal may be used simultaneously to produce more even growth on the faces of the crystal than may be obtained using a single source. The solution may be conveniently contained in a vessel of glass or other infra-red transmissive material with the infra-red source or sources located outside said vessel. Infra-red radiation is preferably focussed onto a crystal to maximise the selective heating of the latter. The radiation is conveniently focussed onto a crystal by reflection from a suitably shaped mirror or mirrors. A quartz-iodine lamp has been found to be a suitable source of infra-red radiation. A number of crystals may be grown simultaneously from one solution using a suitable number of sources of radiation. This invention includes crystals grown by the above process. The invention is further described by way of an example with reference to the accompanying drawing, which illustrates in sectional view an apparatus for hydrothermal growth of an individual berlinite crystal. A seed crystal 1, chosen for its suitability of form and freedom from imperfections, is suspended by a platinium wire 2 in a solution 3 of 1.2 molar aluminium orthophosphate in 7 molar aqueous orthophospheric acid contained in a sealed glass vessel 4. The vessel is placed in an oven and heated to 1 500C. Infra-red radiation from a quartz-iodine lamp 5 focussed onto the crystal by a gold plated mirror 6. The crystal 1 is irradiated until the temperature of the surrounding solution 3, as determined by a thermometer (not shown) has reached 2000C and growth of the crystal is complete, after which the vessel is rapidly cooled, to prevent dissolution of the crystal, and the crystal removed. In a modified apparatus the crystal is withdrawn from the solution prior to cooling. Claims

1. A process for growing a crystal from a compound in a solution in which the compound has a negative temperature coefficient of solubility, comprising suspending a seed crystal of the compound in a saturated solution of the compound, and directing infra-red radiation substantially onto the seed crystal or in the immediate vicinity thereof.

2. A process according to Claim 1 in which the compound is aluminium orthophosphate.

3. A process according to Claim 1 or 2 in which infra-red radiation is directed onto a seed crystal from more than one direction.

4. A process according to any preceding claim in which infra-red radiation is focussed onto the seed crystal.

5. A process according to any preceding claim in which a quartz-iodine lamp acts as a source of infra-red radiation.

6. Apparatus for growing a crystal substantially as hereinbefore described with reference to the accompanying drawing.

6. A process for growing a crystal substantially as described in the example.

7. A crystal grown by a method according to any preceding claim.