GB2230855A - Thin film calorimeter - Google Patents

Abstract

An interferometric microcalorimeter for studying heats of adsorption, absorption and chemical reaction involving gases, which uses a single-mode optical fibre Mach-Zehnder Interferometer with a short length (11) of the active arm (12) coated with a film (23) of a sorbant substance. Optical pathlength changes in this arm produced by temperature, and hence refractive index, changes are compensated by stretching the reference arm (14) using a piezoelectric cylinder (18). The voltage applied to this cylinder (18) is proportional to the temperature change in the active arm (12). The system can be calibrated using heat pulses generated electrically in a silver coating on an active arm which has been substituted for the active arm (12). The interferometer is isolated in a large evacuated chamber (22) connected to a carefully-thermostatted high vacuum sample chamber (20) containing the active coated fibre (12) and an identical length of uncoated fibre (14) compensating for ambient temperature fluctuations. The small-diameter (100 mu m), low-heat-capacity (1.7 mu J/mK/10cm length) fibre (12), with 1 mK temperature changes measurable above the background noise, gives mu J sensitivity. The calorimeter has been used to study the interaction of NO2 with Pb and Cu phthalocyanines. <IMAGE>

Description

THIN FILM CALORIMETER This invention relates to a thin film calorimeter, useful in particular for the measurement of heats of interaction between gases or vapours and thin films of sorbant materials.

It is known that considerable difficulty arises in the direct calorimetric measurement of heats of adsorption of gases onto materials provided in the form of thin films. This difficulty arises because the specific surface areas of films are considerably less than those of (for example) powdered samples, and because films must be supported on substrates whose thermal masses limit the temperature rises produced on gas adsorption. An example of a known thin film calorimeter which attempts to overcome this problem employs a planar glass support structure covered with thin resistance-thermometer wires, onto which covered structure is sublimed a thin film of sorbant material. However, in order to ensure a low thermal mass the glass support structure needs to be very thin which makes it fragile.Furthermore, the complexity and cost of the structure means that for repeated use of the calorimeter, the structure has to be cleaned and recoated with fresh film material which is a difficult and time-consuming operation.

GB patent 2192710 proposes a gas sensor including an optical fibre coated with a catalyst which causes catalytic combustion of a combustible gas being sensed. This catalytic combustion is an exothermic reaction and causes heating of the fibre which may be detected by observing the change in the refractive index of the fibre. The operation of this gas sensor requires a relatively large sample of the gas under investigation in order that a steady state may be reached, the presence of oxygen is required in order that the catalytic combustion can take place and the sensor is only able to sense combustible gases.

It is an object of the present invention to provide a thin film calorimeter in which the aforementioned disadvantages are overcome or at least mitigated in part.

Accordingly, the present invention provides a thin film calorimeter comprising an optical fibre coated with a film of a sorbant material, means for introducing a fluid sample, capable of thermally interacting with the sorbant material, into contact with the film, means for detecting optical pathlength changes in the optical fibre produced by heat transferred thereto from the film and integration means for integrating the detected pathlength change over time during thermal interaction between the sorbant material and fluid sample.

The term "sorbant material" as used in this specification means any material capable of thermally interacting with a fluid sample by a process encompassing one or more of the following: physisorption (adsorption or absorption), chemisorption, or chemical reaction.

There are several advantages in using a film coated optical fibre and measuring the heat generated calorimetrically. The low -l heat capacity of optical fibres (typically less than 0.2 J mK per cm length) coupled with the sensitivity and rapid response of their optical pathlengths to only very small changes in temperature of typically 1mK or less, provides the calorimeter with the capability of detecting very small (typically micro-Joule) amounts of heat generated within the film.Secondly, only very small fluid samples are required because only a small quantity of the fluid can be sorbed by the very small film area after which the sorbtion ceases (a steady state reaction is not established as in the GB 2192710 gas sensor) furthermore sensitive measurements are possible using small film areas of typically less than 10-3m coating short lengths of fibre. Thirdly, the presence of oxygen is not required for the calorimeter to function. Fourthly, fluids other than combustible gases can be tested. Fithly, film coated optical fibres represent relatively small and inexpensive calorimeter components which once used are easily replaced with further coated fibres, thus obviating the need for cleaning and reusing the same film support.

Preferably the calorimeter further comprises a reference section of optical fibre which is also exposable to the fluid sample. This permits compensation for ambient temperature fluctuations to be achieved.

The film-coated optical fibre preferably comprises the active arm of a fibre-optic interferometer, preferably a Mach Zehnder interferometer. The active and reference optical fibre arms of the interferometer preferably extend through a sample chamber enclosing the film, with the remainder of the interferometer from its input to its output beam splitters (which are preferably directional couplers) preferably thermally isolated from the surrounding environment within an isothermal shield. Such a device can detect the very small changes in the film-coated fibres optical path length caused by the sorbtion. The use of a separate sample chamber permits a wide range of chamber pressures to be employed, especially low pressures from 10-10 to 10 mbars.

Preferably feed back means dependent on output from the interferometer is used to alter the optical path length of the reference optical arm. This is conveniently achieved by the use of a piezoelectric device. This arrangement enables the interferometer to more accurately measure the total energy produced by the sorbtion process by compensating for thermally induced optical path length changes in the active arm of the interferometer.

Preferably the film of sorbant material comprises copper phthalocyanine or lead phthalocyanine so that the calorimeter can be used for the detection of NO2.

Other preferred features of the invention are recited in the claims.

An embodiment of the present invention will now be described with reference to Figure 1, which illustrates a schematic view of a twin-isoperibol fibre-optic microcalorimeter based on an all-fibre Mach-Zehnder interferometer configured as a thermometer.

The microcalorimeter illustrated in Figure 1 consists of a semiconductor laser 2 powered by a power source 4, which directs a laser beam through an optical isolator 6 and launcher 8 and into a first directional coupler 10. The coupler 10 splits the beam into two monomodal optical fibres 12, 14 of identical length and diameter which fibres comprise the active (signal) and reference arms respectively of the interferometer. From the coupler 10 the fibres 12 and 14 extend for an identical number of turns around active and reference piezoelectric transducer cylinders 16 and 18 respectively, through a high vacuum sample chamber 20 and into a second directional coupler 21.The two couplers 10 and 21, and the two arms 12 and 14 are thermally isolated within an isothermal shield consisting of a thermos tatted low vacuum chamber 22 connected to the high vacuum chamber 20, to ensure that both arms experience the same ambient temperature conditions.

A length 1 of the active arm 12 within the chamber 20 is stripped of its protective sheath and is coated with a film 23 of a sorbant material for sorbing gaseous sample material fed into that chamber through an inlet 24 having an electrochemical valve 24a.

The thickness of the film is typically 10-lOOnm. The coated length 11 of the active arm 12 is hereinafter referred to as the active element 25. The part of fibre 14 which is situated in the chamber 20 constitutes a passive reference section 14a.

Two additional optical fibres 26 and 27 extend from the second directional coupler 21 to active and reference photodetectors 28 and 29 respectively. The electrical outputs from the photodetectors 28 and 29 are combined in a differential amplifier 30 the differential output from which passes through an integrator 32 and an output amplifier 34 to a temperature/time plotter 36 and a temperature/time integrator 38. A feedback loop 40 connects the output amplifier 34 to the reference piezoelectric transducer cylinder 18.

In use, sample gas is fed through the inlet 24 via the open valve 24a into the evacuated chamber 20. Sorption of the gas by the film 23 induces a temperature change in the film which is transferred by heat conduction to the underlying active arm 12 along the active length 11. The sorption process may involve physisorption (adsorption or absorption), chemisorption or chemical reaction or a combination of two or more such processes depending on the nature of the film material and the gas sample. Temperature changes along the active element 25 lead to changes in the refractive index hence optical pathlength of the optical fibre 12 comprising the active arm.Corresponding changes in the interference fringes at the second directional coupler 21 are detected by the two photodetectors 28 and 29, which vary in antiphase and whose differential signal processed through the differential amplifier 30, integrator 32 and output amplifier 34 is fed back through the loop 40 to the reference piezoelectric transducer cylinder 18. This signal changes the volume hence diameter of the cylinder 18 and so effects a change in the length of the reference arm 14 wound round the cylinder, which compensates for the thermally-induced optical pathlength change in the active arm 12. The change in the length of the reference arm 14 also compensates for ambient temperature fluctuations, thus providing the calorimeter with sensitivity only to the heat generated by processes occurring on or within the thin film coating 23 of the active arm 12.The outputs from the photodetectors 28 and 29 are maintained in balance at a point of maximum fringe quadrature. The voltage applied to the reference piezoelectric transducer is proportional to the temperature rise in the active element 25, and hence to the amount of heat generated in the film 23.

The output from the calorimeter is provided in the form of a plot of temperature rise against time on the plotter 36. The integrated peak area under the temperature/time curve is proportional to the heat generated in or on the film 23 and is calculated by the temperature/time integrator 38 which also converts the integrated peak area into heat output by the use of a calibration factor. Calibration of the calorimeter in order to effect this conversion is effected by using an active arm 12 coated along the same length 11 with a thin film of metal (for example silver) with electrical connections across 11 to permit generation of standard electrically-produced heat pulses of known size.

The present calorimeter was used to study the interaction of gaseous NO2 with thin films 23 of lead phthalocyanines (Pbpc) and copper phthalocyanines (Cupc). A 10cm active length 11 of a 100 micron diameter monomode optical fibre active arm 12, equivalent to a total surface area of fibre of 3 x 10-5m , was coated with the film material to a depth of between 10 and 100nm. The heat capacity of the optical fibres 12 and 14 was 1.7 J/mK per 10cm length. The film 23 so produced was exposed to 2 mbar NO admitted rapidly at a 2 given momemt in time through the inlet 24, and the heat of interaction measured. The measurements were repeated several times for both Pbpc and Cupc, using a freshly-coated active arm 12 on each occasion.

The results of these measurements are given in Tables 1 and 2 below. On each occasion, the calorimeter was found to react to mK changes in active element temperature within about 1 second from the moment of NO2 admission, indicating a fast and sensitive response.

The heat-generating interaction between the film and NO was found 2 to last for periods of typically 200-400 seconds. The results show the sensitivity of the calorimeter to be between 10-5 and 10-6 Joules.

Table 1 reflects chemical reaction following adsorption.

Table 2 shows that the film surface area is in fact greater than the geometric surface area of the optical film, accounting for the apparent high values for the heats of adsorption.

TABLE 1 Heats of Interaction of NO2 with Fresh Lead Phthalocyanine Films Film Number Heat/J Heat of Interaction/kJ mol Assuming bulk Assuming surface reaction adsorption 1 7.5 x 10-3 883 54,000 2 7.0 x 10-3 819 50,000 -3 3 8.0 x 10-3 860 56,000 4 6.0 x 10-3 650 43,000 TABLE 2 Heats of Interaction of NO with fresh Copper Phthalocyanine Films 2 Film Number Heat/J Heat of Interaction/kJ mol Assuming surface Assuming bulk adsorption reaction -4 1 2.9 x 10 2066 34 -4 2 1.2 x 10 1350 22 -4 3 2.0 x 10 1533 25 -4 4 6.0 x 10 4305 70 -4 5 4.2 x 10-4 3070 50 6 5.6 x 10-4 4025 66 -4 7 3.6 x 10 2590 42

Claims (11)

1. Claims 1. Thin film calorimeter comprising an optical fibre coated with a film of a sorbant material, means for introducing a fluid sample, capable of thermally interacting with the sorbant material, into contact with the film, means for detecting optical pathlength changes in the optical fibre produced by heat transferred thereto from the film and integration means for integrating the detected pathlength change over time during thermal interaction between the sorbant material and fluid sample.
2. 2. Thin film calorimeter according to claim 1 wherein the coated optic film extends through a sample chamber enclosing the film.
3. 3. Thin film calorimeter according to claim 1 or claim 2 further comprising a passive reference section of optical fibre which is also exposable to the fluid sample.
4. 4. Thin film calorimeter according to any one of the preceding claims wherein the coated optic fibre comprises the active arm of a fibre optic interferometer having active and reference optical fibre arms.
5. 5. Thin film calorimeter according to claim 4 wherein feed back means dependent on output from the interferometer is used to alter the optical path length of the rerference optical fibre arm.
6. 6. Thin film calorimerer according to claim 5 wherein alteration of the optical path length of the reference optical fibre arm is effected by a piezoelectric device.
7. 7. Thin film calorimeter according to any one of claims 4 to 6 wherein the fibre optic interferometer comprises a Mach-Zehnder interferometer, the active and reference fibre optic arms extending between input and output directional couplers.
8. 8. Thin film calorimeter according to claim 7 wherein the interferometer from the input to the output directional coupler is thermally isolated within an isothermal shield, preferably a thermos tatted vacuum chamber.
9. 9. Thin film calorimeter as claimed in any preceding claim wherein the film of sorbant material comprises copper phthalocyanine or lead phthalocyanine.
10. 10. Use of the thin film calorimeter as claimed in claim 9 for the detection of NO 2
11. 11. Thin film calorimeter substantially as hereinbefore described with particular reference to the drawing.