GB2064500A - A crucible for flameless atomic absorption spectroscopy and a method of making it - Google Patents

Abstract

A pyrolytic graphite crucible of improved length of life when used in flameless atomic absorption spectroscopy comprises a hollow preform of pyrolytic graphite material which is formed by being deposited pyrolytically from the gaseous phase whilst controlling the gas pressure, the temperature and the deposition time so that a thickness which is slightly greater than a desired thickness is obtained, the outer surface is then mechanically machined to a desired thickness and at least one coating of pyrolytic produced graphite is applied to the outer machined surface and to the inner surface of the rough crucible thus obtained.

Description

SPECIFICATION A crucible for flameless atomic absorption spectroscopy and a method of making it The invention relates to a pyrolytic graphite crucible (hereinafter also referred to as "pyrographite" crucible or tube) for flameless atomic absorption spectroscopy (hereinafter referred to as "AAS"). The invention also relates to a method of making such a crucible, by the reactive deposition of a coating of pyrolytic graphite from the gas phase on to a spindle of a high-melting-point material, the shape and dimensions of said spindle corresponding to the shape and dimensions of the inner surface of the desired crucible, making allowance for the heat expansion occurring during deposition.

AAS crucibles are known for use as containers and heating devices for the analysis of samples to be analysed. Tubular bodies in particular are known for use as crucibles from German Patent Specification Nos 20 06 032 and 21 48 777. They generally consist of a high-temperature-resistant, electrically conductive material, since each sample to be analysed is usually heated by electrical resistance heating of the crucibles which are connected to an electric current. Other heating methods, e.g. inductive or radiation heating are, of course, also possible.

The preferred material for such crucibles is carbon, especially in the form of pure-spectrum electrical graphite.

It has been possible to improve the detection sensitivity to a number of elements, and in particular also the useful life, i.e. the number of times a crucible can be re-used, to an unexpectedly great extent by the application of thin coatings of uniformly oriented pyrolytic graphite to the graphite crucibles habitually used in AAS (Atomic Absorption with Electrothermal Atomisation; Prospectus issued by Pye Unicam Ltd., Cambridge, England). A long useful life is of considerable importance in a largely maintenance-free, automatic analysing apparatus. The present state of the art is such that it is probable that ordinary, i.e. uncoated graphite crucibles will increasingly be replaced by those coated with pyrographite.

Nevertheless there are various technical and chemico-physical reasons why a certain number of different types of crucibles which can differ in both shape and material, will continue in use. The known literature on this subject, especially that of various patent specifications, contains many proposals for crucibles of various shapes (German Patent Specification No. 22 21 1 84) and for crucibles of various materials, such as, tantalum and tungsten (Atomic Absorption with Electrothermal Atomisation: Prospectus issued by Pye Unicam Ltd., Cambridge, England), vitreous carbon (German Patent Specification No. 20 34 960) and even pyrolytic graphite (German Patent Specification No. 25 54 950).

It now seems somewhat surprising that investigations on and with crucibles consisting of pyrolytic graphite only (e.g. Analytical Chemistry 44 (1972)1718-1720) have shown no evidence of even approximately the same good results, particularly as such long useful lives, have been found with graphite crucibles merely coated with pyrographite.

One object of the invention is to increase the useful life of crucibles consisting only of pyrolytic graphite.

According to the present invention there is provided a pyrolytic graphite crucible for flameless atomic spectroscopy in which the crucible consists of a hollow preform of pyrolytic graphite whose outer surfaces are mechanically machined at least in places, and at least one pyrolytic graphite coating is additionally applied to the inner and outer surfaces of the hollow preform.

According to the method of making a crucible of the invention a coating of pyrolytic graphite is deposited from the gas phase onto a spindle of a high melting-point material, with the shape and dimensions of the spindle being such as to correspond to the shape and size of the inner surfaces of the desired crucible, taking into account the heat expansion which occurs during the pyrolytic reactive deposition of the coating, in which the gas pressure, temperature and deposition time of pyrolytic graphite from the gas phase are controlled during the reaction deposition in such a way that the thickness of the pyrolytic graphite coating becomes slightly greater than that required to correspond to the desired outside dimensions and wall thickness of the crucible, the hollow pyrolytic preform thus produced in pyrolytic graphite is brought to the desired outside dimensions and wall thickness by mechanical machining its outer surfaces and a coating of pyrolytic graphite is then additionally deposited to the inner and outer surfaces of the rough crucible thus obtained.

One of the essential features of pyrolytic graphite is a marked anisotropy in its physical properties, as frequently described in the literature. This anisotropy, which is dictated in turn by a high degree of orientation of the tightly-packed microcrystalline graphitic layer lattice structure, extends to the chemical behaviour. A look at a slice of pyrolytic graphite taken paraliel to the direction of growth, i.e. in a direction virtually perpendicular to the lamellar layering, clearly shows that the layer, when regarded either along, or against, the direction of growth, looks different from place to place, and is thus nonuniform. It may therefore be expected that some "topographical" variation exists in the properties of the layer connected with this non-uniformity.

During the development of the invention, a series of tests showed that the rate of oxidation of pyrolytic graphite, for instance, varies widely depending whether the samples have their naturally grown, outer surfaces or their "inner" surfaces originally in contact with the substrate (short substrate side) exposed to the oxygen-containing gas (air). It was found that the reaction-controlled temperature range of about 700 to 10000 C, the rate of burning is a factor of 1.6 to 3.0 when it takes place on the substrate side, i.e. in the same direction as the growth of the layer, than in the opposite direction.

Together with the previously good experiences with coated crucibles, this significant discovery has led to the design of the AAS crucible of the invention, the manufacture of which will now be described by way of example in detail below.

A coating of pyrolytic graphite is deposited by a method known per sue employing the reactive deposition from the gas phase (CVD method) on a cylindrical spindle acting as a substrate, which consists of a high-melting-point material such as tantalum, but preferably of electrical graphite or vitreous carbon with the most highly polished surface possible. The diameter of the substrate spindle used is of such a size that it corresponds to the inside diameter of the crucible to be made, making allowance for the heat expansion occurring at the deposition temperature (about 20000 C). The thickness of the deposited layer, which may be controlled via the gas pressure and temperature and especially the deposition time within a certain tolerance range, must be adjusted to a value slightly greater than the desired outside diameter of the crucible.Once coating is completed and the whole has cooled to room temperature, the coating may be slid off the substrate. This gives a hollow cylindrical body of pyrolytic graphite with a smooth inner surface, which is further processed as described below to form a rough crucible. The substrate spindle may generally be used again for further coating processes.

The hollow pyrolytic graphite cylinder obtained is now processed further into a rough crucible. It is best cut to length on a lathe using a hard-metal tool for example, a chisel or on a diamond wheel. A fastrunning grinding machine is best used for grinding to the desired wall thickness. It is usually necessary to make one or more holes in the side of the crucible. This is best done before grinding, using an ordinary drill. After these machining processes the rough crucible is cleaned (e.g. using a degreasing medium in an ultrasonic bath), dried and taken to the pyrolytic graphic coating station.

The rough crucible manufactured as described above is now coated with one or more thin layers of pyrolytic graphite in a known way. One method of performing this step in the process is proposed, for instance in German Patent Application P 28 25 759.2-52. This post-coating is one essential feature of the invention. It results in the following: All the "reactive centres" on the inner surfaces of the crucible on the substrate side are blocked in the same way as the uncovered active surfaces and centres produced by drilling, cutting and grinding.

The crucible is thus given the chemical passivity of the crucible proposed in the German Patent Application referred to above together with its high quality and long useful life.

Sealing is brought about by the second or multiple post-coating which also seals off the interlaminary cracks and fissures often present in pyro-graphite items, with their negative effects on the operation and useful lives of the crucibles.

The "sealing" results in a layer which is oriented so that the c-axes corresponding to the direction of growth are perpendicular to the base (rough crucible) everywhere. This also means that the electrical resistance of the crucible is increased, especially at the annular terminal areas which also form the contact surfaces for the current flow. The same applies to the heat resistance at the contact surfaces.

Both effects are desirable, the first on account of the favourable current/voltage ratio in the heating power, and the second because it prevents heat flowing away from the crucible.

The "sealing" layer also provides additional mechanical rigidity. It is therefore possible to make very thin-walled crucibles which are yet sufficiently stable. The risk of flaking is also virtually eliminated.

it is best for the mass production of the hollow bodies and for their subsequent coating to use the hot-wall pyrolysis method taught in German Patent Specification Nos. 16 67 649 and 16 67 650, which permits the simultaneous deposition of coatings on a number of substrates. Here, for the sake of uniformity, the coating should be performed in at least two steps; depending on the coating thickness wanted. This applies at least to the sealing process, while variations in the wall thickness in the production of the performs may be compensated by subsequent mechanical machining, provided that the wall thicknesses produced when the performs are made are oversized.

It should be noted that the method is not limited solely to precisely cylindrical substrates. Other internai shapes may be made, e.g. ones with conically widening ends or ones widened in steps. All that is necessary is to make the substrate core of two or more separable parts, preferably with a centring pin.

Here, undercuts, which would render it impossible to remove the substrate components from the enveloping coating, must be avoided.

Accordingly. as known per se, the AAS crucible consists only of pyrolytic graphite, although the pyrolytic graphite perform is again given a sealing coating. The directions of growth of the preform and the sealing are parallel to the outer and anti-parallel to the inner cylindrical surface. This not only blocks some increased chemical reactivity of the surface facing the substrate, but also seals off the interlaminar cracks and fissures which nearly always exist or are produced during operation, thus preventing the ingress of any substance under analysis thereinto. This effect of the invention may be taken as the explanation why the improvements in the properties have not been observed in previous uses of crucibles consisting only of pyro-graphite which have been achieved, for example, in the coating (sealing) of ordinary graphite tubular crucibles with intact thin pyrographite coatings. The latter applies especially to the extraordinarily long useful lives of the crucibles of the present invention. In addition to these features, the multi-coating crucibles of the invention also have electrical, thermal and mechanical advantages.

In this connection, the following finding is noteworthy: If pyrolytic graphite is used as a substrate for further coatings, the new coatings grow on the outer, naturally produced final surface of the substrate (of pyrolytic graphite) epitaxially, i.e. the crystalline structure is maintained and generally there is no detectable separation line. This finding applies only to this one surface and only then as long as it is not altered by any process, whether physical, chemical or mechanical. Any gas adsorption between the individual coating steps is insignificant, since the gases are virtually totally desorbed again during the following coating process, i.e. in the pumping and intense heating stages.

New layers grow on all other surfaces, especially the inner surface originally bonded to the substrate, normally without epitaxy.

The invention will be explained in greater detail with reference to the accompanying drawing and the following embodiments which are given by way of example.

The sole figure shows a cross-section through an AAS crucible for horizontal operation.

The crucible consists of a perform 1 of pyrolytic graphite covered with an enveloping coating 2 of pyrolytic graphite. The crucible has contact surfaces 3 at its ends. A hole 4 drilled in the wall of the crucible is used to insert the substance 5 to be analysed. The measuring beam when the crucible is in use passes along line 6-6.

EXAMPLE 1 Deposition was performed using the hot-wall pyrolysis process on a cylindrical graphite spindle 6.1 mm in diameter and with a polished surface in an atmosphere of propane at a total pressure of p = 2.3 mbar and a temperature of 20000 C. At the end of the process and after cooling to room temperature, it was possible to draw from the substrate spindle a pyrographite cylinder with the following dimensions: length (1) 6 cm outside diameter 7.4 mm inside diameter 6.2 mm average wall thickness a = 600 ,um.

The properties of this pyrographite cylinder included: specific gravity y = 2.1 g cm-3 resistance over length (I) less than 0.1 ohm specific resistance less than 10-3 ohm cm ultimate tensile stress at greater than or equal to 52 kp 2 406 kp/cm2 an axial load (P) EXAMPLE 2 Forty substrate spindles of graphite (electrical graphite) were coated simultaneously in the conditions set out in Example 1 and forty pyrographite cylinders were manufactured in this manner.

They were reduced to a length of 2.8 cm by machining on a lathe. A hole 1.5 mm in diameter was then made in the centre of the side wall of the cylinder. The rough crucibles with an outside diameter of 7.4 mm and an inside diameter 6.2 mm were machined on a grinding machine to various outside diameters and thus to wall thicknesses from 500 to 100 ym. The rough crucibles thus made were then subjected to a second coating process during which a sealing pyrographite coating 10 to 40 ym thick was applied.

Claims (3)

1. A pyrolytic graphite crucible for flameless atomic absorption spectroscopy, in which the crucible consists of a hollow preform of pyrolytic graphite, the outer surfaces of which are mechanically machined, at least in places, and at least one coating of pyrolytic graphite is additionally deposited on the outer and inner surfaces of the hollow perform.

2. A method of making the crucible of Claim 1 by the reactive deposition of a coating of pyrolytic graphite from the gas phase onto a spindle of a high-melting-point material, with the shape and dimensions of the spindle being such as to correspond to the shape and size of the inner surfaces of the desired crucible, taking into account the heat expansion which occurs during the pyrolytic reactive deposition, in which the gas pressure, temperature and deposition time are controlled during the reactive deposition of pyrolytic graphite in such a way that the thickness of the pyrolytic graphite coating becomes slightly greater than that required for the desired outside dimensions and for the wall thickness of the crucible, the hollow pyrolytic perform thus produced is brought to the desired outside dimensions and wall thickness by the mechanical machining of its outer surfaces, and at least one coating of pyrolytic graphite is then additionally deposited on the inner and outer surfaces of the rough crucible thus obtained.

3. A pyrolytic graphite crucible substantially as hereinbefore described with reference to the figure of the accompanying drawings.

-4. A method of making a pyrolytic graphite crucible as described by Claim 1 or Claim 3 substantially as hereinbefore described.