GB2237292A - Ion drift tubes - Google Patents

Abstract

Ion drift tubes which operate by ion mobility are required to possess uniform resistivity. Producing ion drift tubes possessing the necessary resistivity has been difficult to achieve. Ion drift tubes and other shaped materials required to have a predetermined nominally homogeneous electrical resistance per unit volume can be fabricated by forming a silicon compact, machining the compact to the desired shape for the ion drift tube etc and controllably converting the machined compact of silicon to silicon nitride by heating it in the presence of nitrogen to produce the desired resistance. Ion drift tubes produced by this technique can be used in ion mobility gas detectors.

Description

Ion Drift Tubes This invention relates to a novel method of fabricating shaped materials to a predefined naninally hanogeneous electrical resistance for use, but not exclusively, in an ion drift tube within an ion mobility detector for the detection of gases.

Prior to the present invention various designs of ion mobility detection devices were known. All are designed to operate by electron-molecule and ion-molecule reactions with a gas producing an ionic species and subsequent separation and detection of the ionic species by their differing rate of transport down a tube. The species is transported down the tube by a potential difference applied along the axis of travel. Its rate of transport is essentially determined by the charge on it, its polarisability and its mass. The ion drift tubes used are required to have uniform resistivity of the appropriate magnitude both radially and along the axis of travel by the ionic species so that the ionic species are subject to a uniform field at all points within the drift tube thereby ensuring optimum resolution for the separation of the ionic species.

Various designs have been proposed and used. US Patent 4,390,784 discloses a tube coated on its inside with a thick film resistor for the detection of ion transport rates. By application of a voltage potential across the thick film resistor an ion acceleration electric field gradient is produced within the tube. There are however difficulties in producing a uniform thickness of coating on the inside of the tube and therefore in the tube having the required uniform resistivity.

Perhaps the most caunonly used design is that described inUS Patent 3,626,180 in which there is an arrangement of metal discs set apart by insulating spacersr the arranganent forming a column with an aperture co-axial to and through the centre of the column. Across the discs is placed a series of resistors and a potential difference of typically lOOOV placed across the column. This arrangesnent suffers fran the disadvantages of being more difficult and expensive to produce than a simple tube design, with a less well defined drift region, a less lx gx field and is more difficult to clean and ventilate.

Consequently there is a requirement for a material suitable for an ion drift tube which overcomes or mitigates the above disadvantages.

Accordingly there is provided a method of fabricating shaped materials to a predetermined nominally homogeneous electrical resistance per unit volume which comprises forming a silicon compact, converting the compact to a desired shape where necessary and controllably converting the desired shape of silicon to silicon nitride by heating in the presence of nitrogen to produce the desired resistance.

Consequently, any particular shape of silicon may be formed as required and then its resistivity altered as necessary by nitriding. Shaping of the silicon prior to nitriding has the advantage of being relatively easy and may be achieved by standard machining techniques. Once the silicon is nitrided it is considerably harder and accordingly far more difficult to shape.

Typically commercial silicon powder will contain iron and aluminium impurities at greater than 0.18 and whilst the present invention refers to forming a compact of silicon it is not necessary that the silicon be pure and the presence of certain impurities may actually aid the nitriding process.

Therefore it is preferable that there is at least some iron present in the silicon. It is believed that iron impurities can aid the nitriding reaction process by forming a liquid iron/silicon eutectic.

Because the nitriding is by nitrogen gas and the silicon is porous the final article will be, under ideal conditions, nitrided quite uniformly throughout. Uniformity of resistance with the present invention depends to an extent on the machining tolerances of the silicon compact whereas with the prior technique resistance depends on trying to control film thickness with its attendant difficulties. Uniformity of resistance in materials produced according to the present invention is also dependent on avoiding temperature gradients in the silicon during nitridation.

Preferably the compact is formed by compacting silicon powder at a pressure of about 178 M Pa at low, i. e. room temperature, and under ambient atmospheric pressure followed by sintering. The e sintering of the riliconpowder compact is preferably undertaken in the presence of an inert gas. Preferably the compact is sintered in argon at aboutll75OC. The sintering typically lasts 2-4 hours and under appropriate conditions a low volume resistivity porous solid compact results with a typical volume resistivity of about 102 oin.an at 25or. The compacts formed can be readily machined to a desired shape.

The conversion of the siliconcompact to a material with higher resistivity is preferably achieved by partially reacting the silicon compact in an oxygen free nitrogen environment. Shapes formed fran the coMpact are preferably reaction bonded by heating in nitrogen in a staged process, so as to avoid the exothermic heat of reaction melting the silicon to form larger volume inclusions which are difficult to react. A typical staged process would be: 10 hours at 1150oC 5 hours at 1200 C 5 hours at 1250 C 10 hours at 1300 C 60 hours at 1350 C and; 10 hours at 1425 C to convert all the available silicon to silicon nitride. This should give to a weight gain of 66.7%.

Because the silicon compacts are porous and the process is one of gaseous nitridation, the nitriding is nominally homogeneous throughout the bulk structure as will be the resistivity per unit volume.

The e reaction bonded silicon nitride produced by the present inventions process is, when practically maximally nitrided, an insulator with a volume resistivity of greater than 108 ohm an at 25 C. It is however possible to produce various levels of resistivity by altering the proportion of silicon converted to silicon nitride.

The large part of the reaction takes place in the early part of the 1350 C stage so that by arresting the reaction prior to the 60 hours indicated above a partially nitrided compact is produced.

Where the resistivity of an article formed is not as desired it is possible because of the nature of the silicon nitride formed to either increase or decrease the proportion of silicon nitride present and alter the article's resistivity. Preferably where the proportion of silicon nitride in an article produced is too low it may be increased by heating the article and exposing it to nitrogen gas. This will increase the article' s resistivity.

Preferably where the proportion of silicon nitride in an article is too high it may be reduced by heating in a low nitrogen or nitrogen free environment.

This will reduce the article's resistivity.

Because of the relative ease with which the above technique mqy be used to produce shaped materials of silicon nitride of nominally hcmogeneous resistivity there is according to a further embodiment of the present invention provided an ion drift tube wherein the tube is formed fran silicon nitride.

The e inventors have found that silicon nitride may conveniently be used in ion mobility detectors and can be produced by nitriding silicon as described above.

Producing ion drift tubes of silicon nitride by the above method allows the desired shape of tube to be readily formed and the resistivity altered to a desired value which yields a tube with nominally good resistance homogeneity and comparable if not better peak resolution than prior known devices. The process of nitriding also improves the strength of the tube.

Whilst commercial silicon has a volume resistivity of about 102 ohmcm an at 25OC which is inappropriate for ion drift detection its formation into uniform shapes by machining or other techniques makes it a useful material to form the basis for an ion drift tube. When a required tube is produced it can then be nitrided by heating in the presence of nitrogen, and appropriate conditions are preferably heating at from 1150or up to 1350or in a nitrogen environment for approximately 11 hours. Ideally the nitriding should be carried out as a staged heating process as mentioned above. By controlling the extent of reaction the percentage of nitrogen in the silicon nitride product can be controlled.It is possible to form materials with required resistances as a bulk as opposed to surface property because the porous nature of silicon allows the nitridation reaction to permeate the silicon. The nitrided tube thus produced is of uniform resistance not only across its surface but also throughout its bulk structure. Silicon nitride has the further advantage of being very hard. Preferably the nitrogen content is in the range 47 to 51%.

Ideally the nitrogen content is 49% i 0.5 which gives a material volume resistivity of about 106 ohm cm at 25or. This value of resistance is desirable for an ion drift tube to enable reduced power consumption so that a system can be battery operated but with the resistance also being low enough not to present current leakage problems.

Whilst the above described method for producing materials of uniform resistivity is of utility in the preparation of ion drift tubes it may not be totally successful in producing the desired product every time and in cases where it is found that the nitrogen content of the silicon nitride is too low the nitrogen content may be altered by further controlled heating under appropriate conditions to the preferred level as mentioned above. It is also possible to reduce the conductivity by heating in a vacuum at 1450or. A further advantage of using silicon nitride is the cheapness of production of ion drift tubes by the above method.

A still further advantage of ion drift tubes made according to the invention is that they can be used in ion mobility detectors aver a broad operating temperature range, the normal maximum of 350OC being limited by the ionisation source rather than the ion drift tube.

Because the ion drift tubes of the present invention are formed fran a silicon/ silicon nitride composite which is a semiconductor material the resistance of the tube is voltage and temperature dependent. The resistance changes in an exponential manner decreasing by a factor of about 3 from0 volts to 1000 volts. This is not a problan since ion drift tubes are normally used at constant voltage and allowances can be made in the manufacturing process for this resistance change. Temperature of operation is also not a great problem since the ion drift tubes are used under isothermal conditions and devices can be calibrated if necessary to take account of usage at distinctly different temperatures.

The invention will now be described by way of exasale only with reference to the accompanying Drawings of which: Figure 1 shows a sectional view along an ion drift tube; and Figure 2 shows a graph of % weight gain of silicon on nitridation against resistance; Figure 3 shows a graph of response for a sulrppn concentration of methyl salicylate detected in an ion mobility detector using a drift tube formed of silicon nitride; and, Figure 4 shows a graph of response for a sub,ppm concentration of methyl salicylate detected in an ion mobility detector using a drift tube formed with a conventional resistive coating.

With reference to Figure 1 powdered silicon containing 0.38 iron as iron disilicide particles was formed into a compact by application of a pressure of 178 M Pa under ambient atmospheric pressure and at room temperature. The compacts were subsequently sintered at 1175 C for 2 to 4 hours in an argon atmosphere. Fran this silicon coanact and others produced by the same method were machined five identical tubes, (1) of dimensions 37nin long by 15mm outer diameter by 12mn inner diameter.These tubes (1) were nitrided according to the following schedule of times and temperatures; 10 hours at 11500C 5 hours at 1200oC 5 hours at 1250 C 10 hours at 1300 C and; 10.5 hours at 1350 C in the presence of nitrogen gas (BOC white spot grade) at atmospheric pressure. Their weight gains and resistances were: Tube Wt gain (%) Resistance (ohms) 1 49.72 5.5x106 2 51.69 > 20xl06 3 51.96 > 20x106 4 53.56 > 20x106 5 52.88 > 20x106 (Mean weight gain 51.96%) Figure 2 shows a graph of the effect of nitriding upon resistance.

Three of the tubes with appropriate resistance (tube 1 fran the above batch and two other tubes from individual nitriding processes) were tested in an ion mobility detector and gave drift tube resolutions, as defined by the ratio of the drift time to the peak width at half peak height, of 20-30 for reactant and/or product ion peaks, methyl salicylate being the test species as shown in Figure 3. Ihe drift rate in figure 3 was 169 ml/min with a sample rate of 41.9 namin. Measurementswere taken after the tube had been purged with cylinder air for three hours. A trap was used to collect contaminants.

An analogous response for the prior known resistively coated ion drift tube is shown in Figure 4. In this case a Welwyn 8M05 tube was used with a drift rate of 158.9 /min and a sample rate of 39.0 il/min. As before measurements were taken after the tube had been purged for three hours with cylinder air, with a trap used to collect contaminants.

Claims (16)

1. A method of fabricating shaped materials to a predetermined nominally hanogeneous electrical resistance per unit volume which comprises forming a silicon compact, converting the compact to a desired shape where necessary and controllably converting the desired shape of silicon to silicon nitride by heating in the presence of nitrogen to produce the desired resistance.

2. A method of fabricating shaped materials to a predetermined nominally hanogeneous electrical resistance per unit volume as claimed in claim 1 wherein scme iron is present in the silicon.

3. A method of fabricating shaped materials to a predetermined naninally homogeneous electrical resistance per unit volume as claimed in claim 2 wherein the silicon contains greater than 0.1% iron.

4. A method of fabricating shaped materials to a predetermined nominally hanogeneous electrical resistance per unit volume as claimed in any one of the preceding claims wherein the compact is formed by compacting silicon powder at a pressure of about 178 MPa at room temperature and under ancient atmospheric pressure followed by sintering.

5. A method of fabricating shaped materials to a predetermined nominally hanogeneous electrical resistance per unit volume as claimed in any one of the preceding claims wherein the sintering of the silicon powder compact is preferably undertaken in the presence of an inert gas.

6. A method of fabricating shaped materials to a predetermined naninally hanogeneous electrical resistance per unit volume as claimed in one of the preceding claims wherein the compact sintered is in argon at about 1175oC.

7. A method of fabricating shaped materials to a predetermined naninally hanogeneous electrical resistance per unit volume as claimed in any one of the preceding claims wherein the sintering lasts for 2-4 hours.

8. A method of fabricating shaped materials to a predetermined nominally hanogeneous electrical resistance per unit volume as claimed in any one of the preceding claims wherein the conversion of the silicon compact to a material with higher resistivity is preferably achieved by partially reacting the silicon compact in an oxygen free nitrogen environment.

9. A method of fabricating shaped materials to a predetermined nominally homogeneous electrical resistance per unit volume as claimed in any one of the preceding claims wherein the shapes formed fran the compact are preferably reaction bonded by heating in nitrogen in a staged process.

10. A method of fabricating shaped materials to a predetermined naninally homogeneous electrical resistance per unit volume as claimed in any one of the preceding claims wherein various levels of resistivity are produced by altering the proportion of silicon converted to silicon nitride.

11. A method of fabricating shaped materials to a predetermined naninally homogeneous electrical resistance per unit volume as claimed in any one of the preceding claims wherein if the resistivity of an article formed is too low it is increased by heating the article and exposing it to nitrogen gas.

12. A method of fabricating shaped materials to a predetermined nominally homogeneous electrical resistance per unit volume as claimed in any one of claims 1 tolO wherein if the resisitivityof an particle formed is too high it is decreased by by heating in a low nitrogen or nitrogen free environment.

13. An ion drift tube produced according to the method of any one of claims 1 to 12.

14. An ion drift tube as claimed in claim 13 wherein the nitrogen content is in the range 47 to 518.

15. An ion drift tube as claimed in claim 14 wherein the nitrogen content is 49% + 0.5.

16. An ion drift tube as claimed in either of claims 14 or 15 wherein the ion drift tube is used in a battery operated ion drift detection system.