Journal of Electrostatics, 20 (1987) 205-218 205 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

A NEW TECHNIQUE FOR STUDYING IONS STRIKING INSULATING SURFACES

JOHN A. MILLER and DAVID W. THOMAS

Department of Electronics and Computer Science, The University, Southampton, S09 5NH (Great Britain)

(Received January 30, 1987; accepted in revised form March 28, 1987)

Summary

This paper presents a new technique that employs an ultra-violet excited phosphor for studying the interaction of air ions with insulating surfaces. This technique has an important advantage over more traditional methods as it produces dynamic images of surface electric fields in the form of changing patterns of light from the phosphor. Such fields might arise from changes in surface charge density due to the arrival of air ions, or from the redistribution of surface charge due to local breakdown phenomena. The type of light modulation exhibited by the phosphor layer is known as electromodulated photoluminescence. This phenomenon is more commonly referred to as the Gudden-Pohl effect (GPE).

Bertein [ 1 ] employed the classical point-to-plane arrangement for generating an asymmetrical electric field and produced circles on an insulating layer when dusted with a dielectric powder. These circles display the surface field intensities that remain after removal of the ion source (the point). When the new technique is employed in a similar setup to that of Bertein, expanding rings of light are observed. This paper shows that these dynamic rings are related to the circles visualised by dusting, this latter method revealing the ring at a powder circle only at its final diameter.

Notation

DCE GPE h ICE PVA RH T U Ui UV

Direct corona effect. The Gudden-Pohl effect. The vertical point-to-plane distance. Inverse corona effect. Polyvinylacetate. Chamber relative humidity. Chamber air temperature. The initial potential applied to the point-plane system. The critical potential necessary to produce an ICE. Ultra-violet light.

0304-3886/87/$03.50 © 1987 Elsevier Science Publishers B.V.

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1. Introduction

Traditional methods for studying the distribution of ions on insulating sur- faces have relied on two principal techniques: (a) the scanning probe method and (b) the dust figure method. In the former, the surface under study is scanned in a regular fashion by a very high impedance probe placed in close proximity to the surface. This technique by its very nature is slow because of the need to mechanically position the measuring probe. This limitation restricts its use to observing stationary or quasi-stationary distributions of charge. Nevertheless the technique is useful because it can provide very high spatial resolution coupled with good accuracy [ 2-4 ].

The dust figure method consists of spraying the charged surface with a fine cloud of dielectric powder. The powder is attracted to the charged regions of the surface and accumulates in accordance with the local field strength. Obviously this second technique will allow only a single observation of a par- ticular charge distribution to be made, the surface charge being modified by the charged powder particles. The technique has an advantage over the induc- tion probe in that images of two-dimensional stationary fields can be quickly obtained, but quantitative measurements are difficult.

The previous methods can only provide information about stationary fields; the technique described in this paper overcomes this limitation and is able to image dynamic surface fields in real time, allowing both spatial and temporal phenomena to be observed. This new technique is applied in a similar setup to that used by Bertein so that the results can be compared and contrasted with the dusting method.

Bertein employed the dust figure technique to reveal the pattern of charge produced by a point electrode suspended above an insulating sheet. This tech- nique displayed the surface field as a circular powder shape having axial sym- metry with the needle. Because dusting could be performed only when equilibrium conditions were attained ( with the HV either on or off), the method only revealed the circles at their final diameter.

2. Electrophotoluminescence and phosphors

The earliest study of electrophotoluminescent effects in phosphors was car- ried out by Gudden and Pohl in 1920. The effect, which is generally known by their name, takes the form of a transient increase in the light output of an excited phosphor by the application of an electric field. The phosphor is ele- vated to an excited state by the action of an external photon source ( generally ultraviolet light but can be X-rays). The photon excitation provides the pri- mary energy source in the effect, this distinguishes it from electroluminescence where the energy is supplied by the electric field. Ivey [ 5 ] gives a good account of the early work in this area. In the excited state, electrons in the phosphor

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are elevated into the conduction band where a proportion become lodged in traps. Because of thermal energy present in the crystal, a proportion of the trapped electrons spontaneously return to their ground state giving up their energy as photons. This thermal release of electrons gives rise to the phos- phor's characteristic background glow.

The action of the external electric field is to ionise these traps, providing a temporary increase in the population of electrons in the conduction band. As these "extra" electrons return to their ground state, an increase in light output from the phosphor (the GPE) is observed.

The idea that electron trapping plays an essential role in the Gudden-Pohl effect, seems to be agreed upon by most researchers [6-10]. However, the exact mechanism or mechanisms by which electrons are liberated into the con- duction band is unclear. Kiveris, Kudzhmauskas and Pipinis [ 11 ] performed experiments with ZnS:Pb phosphors. They at tempted to obtain evidence for one of three theories of electron release. Their results suggested that no single mechanism may be responsible for the observed effects over all ranges of fields and temperatures.

Thus it would seem that although the general mechanism of the GPE is understood, the precise details concerning the action of fields on electrons have not been fully explained.

In the present research, a thin layer of polycrystalline phosphor is deposited onto the conducting (tin oxide coated) surface of a glass plate. The phosphor (of the zinc sulphide type) displays a strong GPE when excited by an ultra- violet source and placed in a changing electric field. Any changes in electric field intensity on the plate's surface are displayed by the phosphor layer as a pattern of light variation.

3. Methodology and apparatus

A common method for injecting charge into a gas (in this case air) is by means of a pointed electrode. If a sufficient potential is applied between the electrode and a conducting plane, a corona will be created in the high field region at the tip of the pointed electrode. The field between the point and the plane will then sweep the ions (created in the corona region) towards the conducting plane. If an insulating layer is placed over the conducting plane, the ions will be intercepted and will accumulate on the insulating surface. This situation is described in some detail by Bertein.

The apparatus employed in the current research, consists of a vertical sewing needle of nickel-plated steel, about 3.5 cm long and having a tip radius of approximately 50 zm. The needle is attached coaxially to the end of a steel tube (1.7 mm diameter, 5 cm length). This is suspended from the top of the apparatus but insulated from it, and provides the electrical connection to the needle. Placed below the needle is a glass plate about~i.6 mm thick, coated on

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PHOSPHOR LAYER EPOXY COATING CONDUCTING -r- \ \ / / ,, /LAYER

. . . . . . . . . .

1500)J GLASS SUBSTRATE ,,,

Fig. 1. A cross-sectional view through the plane electrode showing the various layers (not to scale). The thickness of the conducting layer is negligible compared to the epoxy coating.

its upper side with a layer of tin oxide. This provides a transparent conducting layer of approximately 15 cm square and serves as the plane electrode. A trans- parent plane is employed so that the phenomenacan be observed from below by means of a 45 ° mirror. It also allows UV light (from a source mounted below the plane) to pass through and excite a phosphor layer. This phosphor layer is deposited directly onto the conducting surface of the plane electrode. This layer consists of an even distribution of polycrystalline phosphor particles ( about 50/zm in depth) bound to the plate's surface by an epoxy coating. The overall thickness of the phosphor-epoxy coating is 100/zm. Figure 1 illustrates a cross-sectional view through the plane electrode with its phosphor-epoxy coating. The phosphor covers an area 10 cm square at the centre of the plane electrode. The remaining uncoated surface is covered by black electrical mask- ing tape to reduce corona discharge from the exposed conducting layer, and particularly from the edges of the plate. The sandwich of glass, conducting layer and phosphor will be referred to from now on as the "imaging plate".

The imaging plate and needle electrode are mounted inside a grounded metal chamber, providing a stable electrical environment. Figure 2 shows a photo- graph of the apparatus with the front access door open. Hot air is blown into the chamber as required via a humidification system. Thus, a stable tempera- ture and humidity could be established independently of the laboratory environment.

All experiments were carried out using 100 #m polyvinylacetate (PVA) film approximately 20 cm by 29 cm, placed over the surface of the imaging plate. The needle electrode is positioned over the centre of the imaging plate at about 5 mm above the surface of the film. A unipolar voltage (from a Brandenburg HV regulated supply) of between 2 and 20 kV (with respect to the needle) was applied to the plane electrode. The needle was connected to ground potential via a microammeter enabling the needle current to be directly measured. A low light TV camera was then employed to view the underside of the imaging plate via a 45 ° mirror. A graticule, drawn on transparent film, was placed below the imaging plate to provide a reference grid so that the scale and topology of the phenomena could be measured. Figure 4a shows the graticule illuminated by

209

Fig. 2. The point-to-plane apparatus shown with the front access door open to the environmental chamber.

C.RD.

video recorder

TV mon.

imaging ptate~

,222, h2 to.v. r~

HV supply r r n ~

Fig. 3. A block diagram of the point.to-plane apparatus used in observing the light ring phenomena.

the excited phosphor and viewed by the low-light camera. The numbers refer to centimeter scale marks. Figure 3 shows a diagram of the apparatus.

The current flowing in the needle circuit was measured using a digital

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microammeter (Keithley DMM 177) which also provided a buffered analogue output. This enabled the current waveform to be simultaneously displayed on a storage oscilloscope. The source of ultraviolet excitation was a d.c. operated fluorescent tube mounted inside a light-tight cylinder. The light from the tube passed through a single exit hole via an optical filter having a peak transmis- sion at 350 nm. This then removed the visible light emission from the source.

The presence of a corona at the point electrode also gives rise to the produc- tion of ultraviolet light, but we have found that the intensity of this light was so low as to have no effect upon the excitation state of the phosphor. The sole source of the phosphor excitation can then be attributed to the ultraviolet source positioned below the phosphor screen.

In order to obtain consistent results, the following conditions were kept con- stant throughout the observations (except where indicated).

Identical needles were used throughout, the tip radius being 50/~m. All experiments were conducted inside the screened chamber. The chamber environment was maintained at 30°C + 2°C with a relative humidity of 30% + 2%. Small variations in humidity did not affect the results; however, maintaining a constant temperature was necessary because of the temperature dependence of the GPE. All voltages applied to the plane electrode were negative with respect o the needle. Each time a voltage was applied, the surface was cleared as far as possible of any remaining charge. This was generally accomplished by selecting a fresh sheet of PVA film from the centre of a pack. The time during which the voltage was applied was kept constant at 5 min before reducing the potential to zero.

4. Results and observa t ions

The concern of this paper is to compare the results of the new technique with those of the traditional dusting method. The setup employed is similar to Bertein's point-plane arrangement. The effects studied are those that occur when a unipolar potential is applied to the point-plane system and also when it is removed. These effects will be related to the "circles" reported by Bertein.

Under stimulation from the UV light, the phosphor screen glows with an average surface brightness of about 0.04 lux. Initially when a potential of - 20 kV is applied to the transparent plane electrode (effectively making the needle probe positive ) a brief flash of light is observed from the phosphor screen. The spatial characteristics of this flash are difficult to observe since the effect lasts less than 50 ms. This makes it impossible to capture using the standard TV recording system employed, due to its relatively slow scan rate. Because of this difficulty the work reported in this paper will be concerned mainly with the

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inverse corona effect (ICE) which occurs when the potential is reduced or relaxed.

4.1 The potential relaxation phase When the high voltage is switched off at the generator, the electric field

established between the probe and screen begins to decay in a manner dictated in some way by the turn-off characteristics of the supply. Observation of the phosphor screen during this phase shows an increase in brightness from the phosphor confined to a small circular region just beneath the needle point. This bright spot expands into a circle of higher luminosity than the back- ground, increasing its radius with time. The rate of radius increase slows as it expands until a point is reached where the circle ceases to expand and fades, becoming unobservable. Figure 4 shows a sequence of time frames captured during the expansion of such a luminous circle.

If the maximum potential that is initially applied to the system is reduced, the radius which the circle reaches before vanishing is also reduced. If the potential is lowered still further, a point is reached where the circle fails to materialise when the potential is returned to zero. In the present setup this occurred between - 5 and - 6 kV. This critical potential will be designated Ui, following Bertein's convention. Figure 5 shows how the circle diameter is affected by the initially applied potential U. The circle diameter was measured by noting its position against the graticule marks at the point when it faded from view. Confirmation of these measurements were obtained by the dusting method, details of which are given in section 6.

5. Interpretation of the observations

During the application of the potential U to the point-plane system, ions created in the high field region at the point tip will be attracted to the plane electrode. These ions are intercepted by the insulating layer covering the con- ducting plane and accumulate on its surface. As the ions build up on the insu- lating surface they create a field in the air which opposes the field due to the applied voltage U. This process reduces the ion current supplied by the probe and thus the rate of accumulation of surface charge. Thus, the probe current is observed to diminish in an exponential fashion until an equilibrium condi- tion is established. This condition is characterised by a constant average cur- rent. In the present setup, if the probe height above the insulating surface (typically 0.5 cm) was small compared to the dimensions of the surface, and the applied potential was less than about 10 kV, the probe current diminished to less than 5 nA from an initial peak of about 700 nA. At greater probe heights or potentials, the effect of the field from the edges of the conducting plane ( fringe fields) were sufficient to maintain a relatively large probe current of 1 or 2/IA which could have an initial peak value of about 6/~A at the application

'0

213

of U. Although the tendency was for the current to achieve an equilibrium of zero amperes, this was not possible because of the finite extent of the insulating layer.

Since the potential applied to the plane electrode was - U volts, a positive corona was induced in the point electrode which deposited a layer of positive ions on the insulating surface. This layer was allowed to build up until equilib- rium was attained (constant mean current). The potential applied to the plane electrode was then reduced from - U to 0 V by switching off the output stage of the EHT generator. Since the potential on the plane electrode was previ- ously negative, this had the effect of making the plane appear more positive to the point electrode. The positive ions trapped on the surface of the insulating layer would then induce a negative corona in the point giving rise to the inverse

'C u

v

E E

21 ~ ~ 8~ 10~ 121 14J

In; f ia l po ten t ia l U ( k V )

Fig. 5. The final diameter of the circle produced during the ICE phase as a function of applied potential U.

Fig. 4. A sequence of TV frames showing the expansion of the luminous ring produced by the phosphor screen during the inverse corona phase. In frame (a) the bright region is just appearing at the centre of the graticule. The point is positioned 5 mm above the central cross on the graticule. The time sequence is as follows: (a) t=0 ms, (b) t=40 ms, (c) t=80 ms, (d) t=120 ms, (e) t= 160 ms, (f) t= 400 ms, (g} t= 800 ms, (h) t= 1.6 s. The experimental conditions were: T=30°C, RH =30%, h=5 ram, U=20 kV.

214

corona effect (ICE) observed by Bertein. The ICE supplies ions of opposite polarity to those trapped on the insulating surface. The arrival of these ions at the surface will then start a process of charge neutralisation which will con- tinue until the resultant surface charge is insufficient to induce a corona at the needle tip. The expanding light ring displayed by the present technique appears to reveal the expanding edge of this area of neutralisation.

In the point-to-plane situation employed by Bertein, it was observed that if the applied potential exceeded a critical value Ui, a secondary circle appeared in the powder patterns. Bertein deduced that this was the result of the inverse corona effect ( ICE ) established by the trapped surface charge on the insulat- ing plane. The technique described in this paper can be used to display the ICE process as it takes place, revealing how the neutralised region expands to reach its final diameter.

Experiments originally carried out by Warburg in 1899 [ 12 ] demonstrated the existence of a finite boundary to the space-charge region between a point and a conducting plane. In the present situation, the conducting plane is cov- ered by an insulating layer which intercepts the ion flow from the point to the plane. However, it is reasonable to postulate that a finite space-charge bound- ary will also exist under these conditions.

As ions begin to arrive at the air-dielectric boundary, the surface potential as seen by the point begins to change. These ions have their maximum density in the region immediately below the point. This region is "connected" to the point by the shortest field line and therefore will rceive the greatest initial ion flow.

~~ION ISATION ~ ~ REGION outer boundary~ ~

of space ~ / ~

~nrHn ru /// ___~//I . . . . .

surface ,; I I I PVA film cha ., . . /

c onduc t i ng re layer layer changing J etectric field

Fig. 6. An approximate representation of the point-to-plane field lines during the expansion of the light ring. The phosphor increases its light output in the region of changing electric field.

215

The arrival of these neutralising ions causes changes in the local field strength between the dielectric surface and the conducting layer. The phosphor responds to these changes by varying its light emission. Because there will be a changing electric field only at the surface of the plate where the ion flow region termi- nates, the phosphor displays an expanding bright circle which tracks the expanding edge of the space-charge boundary. As the bright region expands it develops from a disc into a ring. The outside of this region is defined by the space-charge boundary, and the inside by the expanding edge of the neutral region. In the neutral region ion flow will have ceased, allowing the phosphor to return to its normal brightness.

Figure 6 shows an approximate representation of the field lines at an instant of time during the expansion of a typical light ring.

As the ring expands, the changing size of the space-charge region must be accompanied by alterations in the field pattern at the corona glow boundary. This in turn may modify the distribution of the glow region as suggested in Fig. 6. So far this has not been experimentally confirmed.

The leading edge of the ring is marked by a sharply defined increase in phos- phor brightness corresponding to the outer edge of the space-charge region. The inner boundary is less well defined probably because ion flow reduces grad- ually as the surface becomes neutralised. This effect is clearly shown in Fig. 7a. Because of mechanical difficulties in positioning the UV light source, it was not possible to illuminate the phosphor evenly. In the current setup, the phos- phor is more strongly illuminated at the centre. Thus the annular structure of the light ring is not clear in Fig. 4. Figure 7a is a digitised image of the light ring effect captured at about the same relative time as Fig. 4f. The raw digitised image was then processed by subtracting a similar image containing only back- ground information. This technique removes brightness variations leaving only the variations due to field modulation.

Figure 7a clearly shows a dark central region to the ring where the phosphor has returned to the background brightness level. This is also displayed in Fig. 7b which represents a graphical profile of the brightness across the image. The graph was obtained by plotting the grey-level intensity along one horizontal scan line of the digitised TV image. The line chosen was just above the centre of the ring to avoid the horizontal graticule line.

6. Comparison with the dust figure method

Figure 8 was obtained by using a dusting technique to reveal the surface charge remaining on the PVA film. The setup used was identical to that for obtaining the light ring pictures except that the film was not disturbed for 5 minutes following the reduction of U to 0 volts. The needle electrode was then withdrawn along its axis to a distance of about 10 cm from the film. The film, complete with the imaging plate assembly, was then carefully removed from

216

(b)

eR£Y t ~

I~I '~I IY

248

2 ~

188

148

129

188

88e 010 O2O

i i i i i l l

030 848 ~ 060 e'~ 888 8~8

PIXB. ~ I T I ~ )

Fig. 7. (a) A digitised image of the light ring processed using a background subtraction technique. This clearly shows the dark central region of the phosphor where charge neutralisation has taken place; (b) a graphical profile of the brightness across the image in (a).

the experimental chamber and dusted with the black toner resin employed in photocopying machines. This produced sharply defined circles corresponding to the last observed location of the expanding light ring. To improve the con- trast, a sheet of white paper was laid over the dusted surface and taped to the edge of the film. This method, if performed carefully, allowed high-contrast pictures of the dust patterns to be obtained without disturbing the powder distribution.

The sharply defined central circle in Fig. 8 corresponds to the last-observed location of the expanding light ring during the ICE. The large faint outer circle shows the extent of the charge deposited by the direct corona effect (DCE). The striations observable outside the large circle, are most probably due to residual charge patterns caused by contact charging as the film was placed over the imaging plate. These do not extend across the outer circle boundary because of the neutralising action of the DCE.

217

Fig. 8. An image of the charge remaining on a sheet of 100 ttm PVA film using a dusting technique. The central circle corresponded to the last observed position of the light ring produced by the phosphor technique.

7. Conclusions

It has been shown that a plate coated with a phosphor layer can be employed to image time-varying electric fields at a dielectric-air interface. When such an imaging plate is employed in a point-to-plane situation, the technique reveals the dynamic behaviour of the space-charge region where it terminates at the air-plane interface. This behaviour takes the form of expanding rings of light produced by the phosphor as it responds to the changing electric field on the dielectric surface. These light rings are related to the dust circles reported by Bertein, where the dust images represent the final diameter attained by the expanding space-charge region.

The change in brightness of the phosphor is related to the change in surface charge density. Research is currently in progress to quantify this relationship.

The phosphor technique is in its very early stages of research and develop- ment. However, it is expected that the technique will provide a valuable research tool, enabling the study of the dynamics of surface fields resulting from static electrification.

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Acknowledgements

The au thors grateful ly acknowledge suppor t f rom the Royal Socie ty ' s Paul I n s t r u m e n t F u n d for fund ing this research p rog ramme.

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