GB2293017A

GB2293017A - Measuring dielectric properties of materials

Info

Publication number: GB2293017A
Application number: GB9503975A
Authority: GB
Inventors: Tsing Yee Amy Chan; Marianne Odlyha
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-09-09
Filing date: 1995-02-28
Publication date: 1996-03-13
Anticipated expiration: 2015-02-28
Also published as: GB9503975D0; GB2293017B

Abstract

A probe (1) is used to measure the dielectric properties of rigid or semi-rigid materials (4), for example, works of art. The probe comprises an inner conductor and an outer, coaxial, conductor separated by a layer of insulator e.g. Teflon (TM). The probe is coupled to a network analyser (11), which provides microwave frequency radiation which is transmitted by the probe on to the surface of the material where it is reflected back from the surface back down the probe to the analyser, where it is analysed to give measurements on the dielectric properties of the material. The probe typically has an outside diameter of around 0.86 mm, and because of its size allows measurements to be made, non-invasively, and in real time on small areas of material. The outside diameter can be made of the order of microns by selecting the frequency of the radiation, which allows measurements to be made in a sample of a semi-rigid material e.g. a layer of paint for depth profiling. The probe has particular application in the measuring of works of art. <IMAGE>

Description

DEVICE AND APPARATUS FOR MEASURING DIELECIRIC PROPERTIES OF MATERIALS This invention relates to apparatus for measuring the dielectric properties of materials and it is also suitable, although not exclusively so, for the use in the non-invasive monitoring of the conservation treatment of cultural material e.g. canvas paintings.

The dielectric properties of materials have been extensively used to study the structure, composition and molecular dynamics of materials. This technique is based on the fact that all matter is made of charged particles which respond to an external electric field. Conduction is defined as a finite drift of charges, while polarisation is the relative displacement of charges with respect to the applied field. The dielectric properties of a material are interpreted in terms of the electric dipole moment, both permanent and induced, of the molecules of the material.

As is well known to persons skilled in the art, a polar substance such as a water molecule, has a permanent dipole moment capable of rotating in an applied electromagnetic field. Its relative permittivity, Cr is not constant as a function of the frequency of the applied electromagnetic field. The complex relative permittivity, can be defined as = - je7 where e; is the real part of the complex relative permittivity which decreases with increasing frequency of the applied electromagnetic field, Er is the imaginary part of the complex relative permittivity, or the dielectric loss i.e. it is the absorption of energy by the substance from the applied field which accompanies the fall in permittivity, and j is the square root of -1.

Measurement of permittivity is particularly useful for measuring the presence ( or absence) of polar molecules, for example, water (which has a high static permittivity). For example, in the conservation treatment of a painting, in which high relative humidity conditions are used, the ability to measure the moisture content at the surface of the painting ( actually a sampling volume which is defined by the size of the probe) and the canvas would allow the conservator to be in control of the humidification process.

It is known for dielectric probes to be used to make measurements in solutions and in biological materials, but these are of a dimension not suitable for use on rigid or semi-rigid surfaces, where small areas are to be sampled, for example, small areas of paintings, or where it may be necessary to take measurements within a semi-rigid substance e.g. within a layer of paint on a painting to carry out depth profiling.

According to the present invention, there is provided a device for sensing the dielectric properties of a material, the device comprising an inner conductor, and an outer conductor coaxial with the inner conductor and spaced therefrom by an insulating material, the device having a first open end for contacting with the material and a second end for coupling to means for providing electromagnetic radiation to the dielectric probe for transmission thereby to the material through the first open end, and to means for analysing the electromagnetic radiation transmitted to the material, reflected therefrom and transmitted back to the analysing means via the dielectric probe, the device having an outside diameter of its first open end of less than 1 mm.This has the advantage of allowing in situ non-invasive measurement of the dielectric properties of rigid and semirigid materials to be carried out in real time, and because of the dimensions of the probe, it has the spatial resolution to distinguish between adjoining regions of a sample to be monitored, for example between different areas of a painting with differing pigmentation and media. Where the probe has small enough dimension i.e. of the order of microns, this enables the probe to be inserted into a semi-rigid materials e.g. layers of paint, to take measurements within to carry out depth profiling.

The invention will now be described, by way of example only, with reference to the accompanying Figures, of which: Figure 1 is a schematic representation of the apparatus for measuring the dielectric properties of materials; Figure 2 is simplified cross-section of the end of a probe for use in the apparatus of Figure 1 when in contact with a sample to be measured; and Figures 3a and 3b are graphs illustrating relative permittivity and dielectric loss respectively for mixtures of methanol and oil of turpentine.

A dielectric probe 1 comprises an inner conductor 2, and an outer coaxial conductor 3. The probe 1 is open ended and is configured to contact a sample 4 to be measured at the z=0 plane (in the cylindrical (p, 4), z) coordinate system), with the z axis at the centre of the probe as illustrated in Figure 2. The outer conductor 3 has bevelled outer edges 5, 6 as also illustrated in Figure 2. Typically, the inner conductor 2 has a diameter, a, of 0.20 mm, and the outer conductor 3, an inner diameter, b, of 0.66 mm.

The distance between the beginning of the outwardly facing bevelled edges 5, 6 is 0.86 mm, i.e. the outside diameter of the probe and, therefore, the diameter of the circular area contacting the surface of the sample of the material to be measured is 0.86 mm. The present invention has particular use in the measurement of rigid and semi-rigid materials. The terms rigid and semi-rigid are particularly herein with reference to cultural materials e.g. painting and other works of art which typically comprise a canvas backing with layers of paint and glue thereon. However, it will be obvious to persons skilled in the art, that other materials fall within the definition of rigid and semi-rigid materials e.g. photographic materials.

Insulation (13), e.g. Teflon (TM) is inserted in the region between the outer and inner conductors.

At the other end 7 of the probe 1, the probe 1 is coupled, via known types of connectors e.g. SMA and K connectors, 8 and 9, and a known type of flexible 3.5 mm cable 10 to a network analyser 11 (for example an HP 8720C as manufactured by the Hewlett Packard Corporation). The use and operation of network analysers is in itself, well known to persons skilled in the art, and need not be described in any further detail herein. The network analyser 11 is controlled by a controller 12.

The network analyser 11 generates a swept signal in the range 50 MHz to 20 GHz which is transmitted via the flexible cable 10 to the probe 1, and, subsequently penetrates, to some extent, the sample 4 and is then reflected away from the sample 4 back to the outer probe 1, where the signal is transmitted back up the flexible cable 10 to the network analyser 11. The use of a swept signal in this frequency range has the advantage that the measurement are fast and can follow the changes taking place in the sample 4 in real time. It is also sensitive to the presence of small polar molecules e.g. water which rotate in an applied electromagnetic field at a frequency of 17 GHz at 20 C.

The reflection characteristics at the interface between the sample 4 and the open end of the probe 1 are measured using the network analyser 11, which then uses these and to calculate the dielectric properties of the sample 4, e.g. the relative permittivity and the dielectric loss, for the given frequency of the signal generated by the network analyser 12. The complex relative permittivity of the sample 4 is related to the reflection coefficient by matching the fields at the boundary of the probe 1 and the sample 4 as

The left hand side of this equation represents the difference of the transverse electromagnetic (TEM) wave incident on the sample 4 and the portion which is reflected back along the outer conductor 3. The first term on the right hand side is the magnetic field intensity radiated into the sample 4. The second term on the right hand side represents the higher order modes which are produced at the discontinuity between the probe and the sample and travel back along the outer conductor 3. The primed co-ordinates correspond to simulated source points to predict the magnetic field in the sample region, R is the distance from the source point to a corresponding field point at z = 0 and R2 = p2 +p'2 - 2pp'cos('- ), Ep(p) is the electric field at the open end of the probe, Y the admittance, 4)' (p) is the derivative of the potential function n(p), with respect to p where n is the order of the TOM,, mode, B is the relative amplitude of the corresponding order mode propagating in the -z direction, k2 is the wavenumber in the sample 4, and w is the angular frequency. This equation can only be solved numerically.

The apparatus was tested using a number of standards such as air, water, methanol, ethanol, ethanediol and methanamide to provide a simplified model suitable for accurate real time measurement.

The probe can be used in a variety of applications as illustrated by the examples given below: 1. Methanol, used for the non-aqueous deacidification of canvas, is applied to piece of canvas. The probe can be used to measure how rapidly the methanol evaporates from the canvas; 2. The probe can be used to monitor the dielectric response of a painting undergoing humidity treatment.

Other examples include using the apparatus to monitor moisture uptake in cultural materials, to monitor the cleaning of surfaces of cultural surfaces using both organic and aqueous based preparations, to monitor the effect of exposure of materials to variations in relative humidity, to monitor changes in materials undergoing localised moisture treatment, and to distinguish between the behaviour of acrylic, oil, and glue based primed canvases.

Figures 3a and 3b illustrate how the relative permittivity and dielectric loss vary as function of frequency for a variety of mixtures of methanol and oil of turpentine.

It will be obvious to a person skilled in the art, that modifications are possible within the scope of the present invention. For example, by selecting the appropriate frequency of the radiation, other probe outside diameters are possible, with a range of sizes from a diameter of the order of microns to the order of millimetres (mm) e.g. from 5 microns up to 5 mm. Where the probe has an outside diameter of the order of microns, the probe can actually be inserted into semi-rigid materials e.g. layers of paint, to carry out depth profiling. It can be used to detect the variations in other polar molecules, not only those discussed above. Other suitable insulators can be used between the inner and outer conductors.

Claims

1. A device for sensing the dielectric properties of a material comprising an inner conductor, and an outer conductor coaxial with the inner conductor and spaced therefrom by an insulating material, the device having a first open end for contacting with the material and a second end for coupling to means for providing electromagnetic waves for transmission thereby to the material through the first open end, and to means for analysing the electromagnetic radiation transmitted to the material, reflected therefrom and transmitted back to the analysing means, the device having an outside diameter of its first open end of less than 1 mm.

2. A device according to claim 1 wherein the outside diameter is of the order of microns.

3. An apparatus for measuring the dielectric properties of rigid and semi-rigid materials, the apparatus comprising a probe having an inner conductor, and an outer conductor coaxial with the inner conductor and spaced therefrom by an insulating material, the probe having a first open end for contacting with the material and a second end coupled to means for providing electromagnetic waves having a frequency in the microwave range of the electromagnetic spectrum to the device according to claim 1 for transmission thereby to the material through the first open end, and to means for analysing the electromagnetic wave transmitted to the material, reflected therefrom and transmitted back to the analysing means to provide a measurement of the dielectric properties of the material therefrom.

4. An apparatus according to claim 3, wherein the probe has an outside diameter of the order of millimetres or less.

5. An apparatus according to claim 3 or claim 4, wherein the rigid or semi-rigid materials are cultural materials.

6. An apparatus according to any of daims 3 to 5, wherein the probe is operable to make measurements in real time.

7. An apparatus according to any of daims 3 to 6, wherein the probe is operable after calibration to determine the moisture content of materials.

8. A device as herein before described, with reference to the accompanying drawings.

9. An apparatus as herein before described with reference to the accompanying drawings.