GB2222400A - Doped elements - Google Patents

Abstract

A doped element, in particular a microcystalline doped glassy or ceramic element, is produced by forming a porous element, by a sol-gel process and subsequent solvent removal, and subsequently incorporating dopant particles into the pores of the element by elution of a colloid containing the dopant.

Description

DOPED ELEMENTS This invention relates to doped elements, in particular microcrystalline doped glassy or ceramic elements and methods of producing them.

Colloidally coloured glass, such as is used for cut-off filters, is well known. Since the properties of the final glass are determined by the size of the inclusions providing the colour, the processing needs to be performed in a manner which leads to size consistency. Crystallite regrowth is a disadvantage in the processes normally employed to manufacture colloidally coloured (doped) glass since it adversely affects the inclusion size consistency.

According to the present invention there is provided a method of producing a doped element comprising the steps of forming a porous element by a sol-gel process and subsequent solvent removal, and incorporating dopant into the pores of the element by elution of a colloid.

The present invention is based on sol-gel processing. A sol, or liquid colloid, changes into a gel, or semi-rigid colloid, on standing. After preliminary processing or "low" temperature sintering, an air dry "gel" which has an open pore structure can be obtained. The porosity of this body is a function of both the derivation of the material and the process route. For certain process paths it is known that the porous body can have a very well defined pore diameter, with values in the range 5nm to > l0Onm.

From the band theory of semiconductors and from both ancient and modern glass theory, it is known that crystallites and/or small particles of semiconductors and metals can give high colour to glass, as in the colloidally coloured glass referred to above. It is a particular element of the processing to control the size of these inclusions in order to determine the properties (mostly optical) of the final glass.

The processing proposed herein uses both colloid formation and the defined porosity bodies to yield a product whose optical and/or electro-optic properties are determined by "low" temperature colloid formation followed by elution, with the porous body, to yield the final product.

The processing involved will be more fully appreciated from the following example. A porous body is formed first as follows. A solution is made up of the following molar ratios. One part of tetramethyl orthosilicate (TMOS) in two parts dimethyl formamide (DMF) is mixed in a suitable vessel. In another vessel one point nine (1.9) parts methanol (MeOH) is added to ten parts water with approximately five ten thousands (0.0005) parts of a catalyst, for this example nitric acid (HN03). The latter solution is added carefully to the former to yield an homogenous solution. The vessel is then closed and placed into an environmental chamber at 40-60 0C to gel. The wet gel is then further processed, thermally, to remove residual solvent. This process yields a coherent porous material of good (well-defined) pore size.With the starting materials quoted the porous material is a matrix of transparent silica. If required the porous material may be baked before further processing.

A colloid is now prepared, for example, as follows by admixture of milimolar solutions of tungsten tetrachloride (WCl4) and sodium sulphide (Na2S) in ethanolic solution. An additive to support the colloid need not be employed.

The next stage in the processing is elution (absorption) of the crystallite in the colloid by the controlled porosity body for which processing the porous body (matrix) may be simply immersed in the prepared colloid.

Depending on the dimensions of the matrix diffusion alone may be sufficient to incorporate the crystallite in the matrix. Alternatively, additional/alternative processing may be necessary, such as high pressure, to incorporate the crystallite in the matrix. With the colloid example quoted above the final product is a layered chalcogenide, one of the family of transition metal chalcogenides, incorporated in a silica matrix.

As an alternative to separate formation of the colloid as described above, the colloid can be formed directly in the porous body. By this method the pores can be more easily filled with crystallite than requiring diffusion from a prepared colloid. Following elution the impregnated body is heated at a temperature close to the solvent boiling point to drive off the solvent. Alternatively a reduced pressure may be used for solvent removal, or there may be use of both heat and pressure control such as a hypercritical drying method. For any post elution processing of the crystallite example given above, there is a maximum permissable temperature of44000C.

However formed, elution of the colloid by the controlled porosity body results in a structure in which the "particle" (inclusion) size distribution is reduced in comparison with previous processing methods and in particular a sharp cut-off to large size particles will be achieved due to the pore size being well controlled.

It should be noted that the insitu colloid formation method may result in a product with different properties to that produced with a separately formed colloid. By suitable control of the elution a body with a particular particle size distribution spread along it may be achieved. Such a body may be usefully exploited at a later stage due to its "graded" properties, such as non-linear optical effects.

Since the pore size and thus the incorporated particle (inclusion) size is well controlled, the optical properties will be improved since the band-edge of the particle (its optical absorption edge) will be sharper than achieved with previous processing, as in colloidally coloured glass used for cut-off filters, and, furthermore, may manifest the phenomena associated with quantum size effects if it is of a size comparable with the electron-hole pair interaction. In this latter case, the electro-optic properties of the inclusion will be greatly enhanced, and thus active as well as passive optical materials can be produced.

In the above described manner, inclusions of diverse materials may be made into a substrate at good concentrations (approximately 5%) without encountering the difficulties associated with redox processes or crystallite regrowth typical of the processes normally employed to manufacture colloidally doped glasses. In addition, the tight size control and versatile preparative route make the process ideally suited to the production of non-linear optical or electro-optical materials as well as passive optical materials. In particular, the low temperature route allows considerable control of the exact nature of the dopant, almost irrespective of the host's chemistry, during pre-processing.

A particular application currently envisaged is in the field of integrated optics with silica on silicon devices. A sol-gel can be spin coated onto a substrate then processed to yield a porous structure as described above and doped by the incorporation of inclusions by elution of a suitable colloid. A planar structure can thus be achieved using electronics type production techniques of photolithography, etching etc. The refractive index of a waveguide in an integrated optic device can be controlled to required levels, due to the electro-optic effect by using the appropriate pore size and inclusion material, in a particularly easy manner.

Large index changes can be achieved. In view of the achievable electro-optic effect, active integrated optic devices are obtainable.

Whereas the matrix described above is silicon, other glasses or ceramics may form the matrix.

The dopants can be chosen from any of the metal chalcogenides, such as "II-VI" compounds as well as lead sulphide/selenide, routes to GaAs. InSb and other "III-V semiconductors should also prove facile. Dyes and/or other electro-optic organic materials could be incorporated in the pores if the matrix was presintered to avoid surface tension engendered collapse of the porous material. In all cases it is the processability of the sol via spin coating which is the key to applicability. Silica could be extended by addition of boron/sodium containing oxides, but these multi-component oxide glasses/materials might be less well characterised.

Claims (10)

1. A method of producing a doped element comprising the steps of forming a porous element by a sol-gel process and subsequent solvent removal, and incorporating dopant into the pores of the element by elution of a colloid.

2. A method as claimed in Claim 1 wherein the colloid is formed prior to elution by the porous element.

3. A method as claimed in claim 1 wherein the colloid is formed directly in the porous element by co-reacting precursors of the colloid in the presence of the porous element.

4. A method as claimed in any one of the preceding claims and including the step of processing the doped element following elution whereby to remove colloid solvent.

5. A method as claimed in any one of the preceding claims wherein the porous element comprises a layer formed on a substrate as a result of coating the substrate with a sol-gel and subsequent solvent removal.

6. A method as claimed in any one of the preceding claims and wherein the elution of the colloid is controlled whereby the incorporated dopant particle size is distributed along the element.

7. A method as claimed in any one of the preceding claims wherein the porous element is of silica or a ceramic.

8. A method as claimed in any one of the preceding claims wherein the dopant comprises crystallites and/or small particles of semiconductors or metals.

9. A method of producing a doped element as claimed in claim 1 and substantially as herein described.

10. A doped element produced by a method according to any one of the preceding claims.