Sen, B K

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37VOLUME RECTIFICATION OF CRYSTALSBy B. K. s e n . M.Sc.f o r  p H b l i c a i i o n ,  M a y  z b,  jy^j)ABSTRACT. Experiments have been perforitieil tn .show that the volume rectifica­tion effect is not confined to crystals like carborundum, zincite and silicon alone, which have n:t centres of symmetry, but that it is also exhibited by crystals like galena, iron- pyrites and pyrolusite pos.sessing centres of symmetry, hrom the results of the^e experiments if is .suggested that the so called volume rectification may not lake place \vithin the body of the crystal, but may be due to the differential effect of the surface rcclification occurring at the two large contacts of the crystal with the electrodes.I N T R 0  D U C T I  0  NTife rectifying action of crystals lias usually been divided into two classes—(a) one is associated with a point contact at one cud of the crystal and a large electrode at tlie other, (6J the other is obtained u'hen the crystal has large contacts at both the ends, e.g., when the crystal is placed between two mercury electrodes. The former kind of rectification has been called point or surface rectification and the latter kind, volume icctification.It has been thought that this volume rectification takes place within the body of the crystal, and following Krouig,’ this effect has been attributed to the asymmetry in the crystal structure. The expcrimeB|s of Khastgir and Dasgupta* have shown tliat crystals like carborundum ,SiC), zincite (ZnO) and silicon, wliich have no centres of symmetry, give volume rectification when placed between mercury electrodes, whereas with symmetrical crystals, t.g., galena (PbS'i iron-pyrites {FeS*), pyrolusite (Mnt)a), etc., no trace of such recti­fying effect was obtained by passing low-frequency alternating current through the crystals similarly placed between two mercury electrodes oflarge contact areas.Natural Cu,0  crystals, which show the Dember effect, exhibit rectifying properties when illuminated. In this case, however, it has been shown* that the action does not occur at the electrodes but lakes place in the body of the crystal. It is to be noted here that artificial Cu,0 crystal, in which the Dember effect is absent, does not show any rectifying effect.In -course of the investigation on the effects of heat and ultra-violet light on crystal rectification* the present author noticed that besides3^0 B. K . Sencarborimcluni, zincite and silicon, crystals of iron-pyrites, galena andl^yrolnsite exhibited pronounced rectifying properties when placed between mercury electrodes. It was, therefore, felt desirable to further study this phenomenon of volume rectification aiid the preliminary results obtained were refiorted in a short note/’ The details of some experiments ate, however, given in the present paper.1C X JC K 1 M rc N T A I,The nietliod of mounting the crystals was )jractically the same as that adopted by Kliastgir and Dasgupta in their study of volume rectification/' with an additional arrangement for slowly moving the glass tube, carrying the crystal vertically up and down. For this purpose the stand of a travelling micio.scope was utilised. The glass tube with the crystal was kept vertical by passing it tluough a hole in a cork which was fitted to the microscope stand in the position of the microscope, so that ‘ the crystal lioidcr could be moved up and down as required with the associated slow-uiotion attachments. With this arrangement the contact-area between the crystal and llie mercury at the lower end could be conveniently altered, that at the upper electrode remaining constant, and the efi'ect of the changeof , contact area at one of the electrodes could be studied. The source of ^ 'low-frequency alteniatiiig voltage was a looo-cycle audio-oscillator. J''oi the iiieusurenient of. the rectified current a niilli-arnnieter was generally used which vvias, however, sometimes replaced by a shunted galvanometer^The following symmetrical crystals were tested, both with low- frequency and high-frequency alternating cuirents, for volume rectification and in each case prominent rectified current was obtained, the nature uf which is given below in detail. Oharacteiistic curves have also been obtained in all eases by applying direct voltage in the usual way.Low-frequency Test.(j) Galena ; When the alternating current is jjassed through a gooil crystal of galena put between ineicury electrodes, there is a pionoiuiced rectified cm lent as indicated by the shunted galvaiiomelei- ploc'ed in the circuit. Next, keeping the upper contact-area constant, as ther Ibwci coulact-arca is gradually altered the following changes, as regards .diregtion, and magnitude, are noticed in the rectified current : WMieu the lower end of the crystal just touches the itiercuty surface, the rectified current flows upwards, the magnitude being nearly i milli-ampere. But this current is rather unsteady and occasionally goes downwards. When the lower contact area is made sufficiently large, the rectified current remains steady. Tin current now flows upwards and its strength is 1.25 milli-anips. Qn furtbtrVolume Rectification of Crystals SSIincreasing the contact-area gradually the lectificd current first increases a little, then diminishes, hecouies zero and finally chiiuges direction and llows downwards. I his result has been reproduced several limes dining the experimentsCharacteristic curve is next drawn with both ends of the crystal well within mercury. The data given in Table 1 and the curve in Fig. i clearly show that under the given arrangeineut the crystal produce.s good rectificition.(2) Iron-pyrites: Alternating voltage.is applied to the crystal placedIjctvveen mercury electrodes. A  steady rectified current Hows up when there are large contact-areas at the electrodes at both ends of the crystal. The upper contact-area is kept constant and the lower one is changed when the following results arc obtained : (a) For a small aica of contactthe rectified current flows upwards, (b) for increasingly huge contact-area the rectified current diminishes gradually, reaches a niiuiiimiii value and then changes direction, ’.c., flows downwards through the crystal. The experimental results obtained with direct voltage applied to the crystal are given in Table I  and in F ig . 2. In this case characterhstic curves have been obtained under two different adjustments. For the first, the rectified current flows upwards through the crystal and for the second it flows downwards. The curves reveal marked asyiiinietry and it is evident that the crystal should give good rectification effect under both the adjustments.(3) Pyrolusite : In the case of the pyrohisite crystal the rectified current is highly prominent and as in the previous Iw’o cases it also changes direction underB. k .  Sert' suitable adjustirieiits of the contact-areas at the electrodes. The results of D. C. test for this crystal are given in Table I and F ig . 3. The character-/^ 0/v -*pymT£S ( deZ-i^ een^Mercury «/ec/ro^ «5 0/  co^ /^ c^  ^rroijF kjurk 20 -0 —0—0 for rectified current flowinn uiiwardsX >t — X— X — ,, flowing downwardsistic curve here also indicates the existence of a strong rectification effect with the crystal placed l)etween mercury electrodes.v^p<?Lc/j/T/r Mercuryd/fc/ro</eJ <y* l^rac- contact urQfSii JF io c r e  3Volume Rectification of CryntahTAM.E I333VoltageCurrent(niilli-anips)Voltage(volt)Ciirrent(inill^ anips)(volt) ---- - -...Up Down Up l>)wnIron-pyritesfGalena ia).10 .64 .62 .10 •^5 •»3• 15 1.00 .92 ,20 .40 .30.20 ^•50 1.20 •3 <^ t.io .7024 1.80 1-49 .62 2.50 1 6029 2.50 2 00 .82 4.00 3.0U•37 3.754,802.85w•43 3.70.10 .05 •os '?3Tolusite .24 .16 20.2 .18 •15 .38 .30 38•4 •50 .30 .45 .60.6 •50 .62 JO I 00.8 3-00 .90 .8a T.30 2.50■9 4 3'-^ 1 .10(o) I'*or recti‘6ed current flowing upwards.(6) For rectified current flowing downwards.High-frequency A. C, test.A ll the crystals are next tested with high-frequency alternating voltage obtained from a valve oscillator. In every case there is a pronounced rectified current so that it is definite that volume rectification is exhibited by these crystals, also with liigh-frcquency current, when placed betwteii mercury electrodes.Crystals tested for the Demher effect. „To be sure that the volume rectification, so far obtained with the crystals in the above experiments, is not due to any Dember-effect produced by the diffused sun-light, experiments are repeated after cutting off the light from th e . crystal surfaces. Good rettification effect was, under tbjs334 B. K. Senconditjon, obtained in all the cases by passing alternating current the crystals.througliD I S C U vS S 1 o NIn point-rectification it is an admitted fact that different parts of the surface of a crystal possess rectifying properties to quite different degiees. It may also l>e noted here that parts which have good lectifying properties exhibit very poor conductivity, w'hereas parts having good conductivity show very little rectifying action. Moreover, the rectification effects at different parts arc also of different sense, i.e., at some points the rectified current flows from crystal to whisker while at others it flows from whisker to crystal. The results of the experiments reported in the present paper, combined with the facts stated above, point to the conclusion that the volume rectification in crystals is due to the dilTerential or the average effect of point or surface rectification occurring at the two electrodes. For, the surface of a crystal may be divided into a large numlrcr of rectifying regions of varying efficiency and giving rectification in opposite directions. At any particular electrode these regions, having varying degrees of rectification and revealing the effect in opposite directions, will pro­duce a net lectiCed current whose magnitude and direction at that electrode will be determined by the predominance of the total effect in one direction (i.c., of one sign) over that in the other, When we take into account the effects at both the electrodes, the final resultant direction of the rectified current through the crystal, for any particular adjustment of the contact areas of the electrodes, will be determined by the combined effects at the two surfaces of the crystal in contact with the electrodes. For instance, if the net rectification effect at one electrode A produces current from, say, ciystal to electrode and that at the other contact B gives current from electrode to crystal, then the combined effect will be a very strong rectified current flowing through the crystal in the direction B-A . If, on the other hand, rectification effect at the end A  produces current from crystal to electrode and that at the other end B also sends the current from the crystal to the electrcKle, then the resultant direction and magnitude of the rectified .current will be determined solely by llie strength or rather the.differena; of the strengths of the rectified cunents produced at the two eleCTrodes. Thus there is no distinction between iioint-rectification and the so-called volume- rectification of crystals^ as regards the mechanism by which the effects take place. Both miust be due to the same cause. The only difference between the two lies in the contact area at one of the electrodes. As a matter of fact the end of the whisker, in point-rectification, has never such an extremely small area as to lie.ou a,single rectifying ttgion but it would possibly rest on a number of rectifyiug legions generally of varying efficiency and different signs. It is thus quite possible for .all crystals, exhibiting point-rectification, to give the effect—Volume Rectification of Crystals 335the so-called volume-rectification— when placed l-elween laige electrodes on bothP h y s ic s  L ahokatok\',R ajsh a h i C o u >:(U'1,R a js h a h i.K !•: F  K R H N  C F  SKronig, N a t u r e ^  123, Maich 3 (11)29).K liastgir and Das Gupta, I m 1 .  J ,  P h y s .  9, 259-60 (1935). Groetzinger and Liclitschini, P h y s .  Zeit, 38, zQZ (1937). Sen, J n d .  J .  P h y s .  10 91-102 (19361- Sen, N a t u r e ,  IM, 1102 (1937).K hastgir and Das Gupta, J n d  J .  P h y s . ,  0, 259 (1935)9— 1423P— VERRATAIn the paper 'On effect of resistance cowponeni in reave filter elements and performance of non-ideal filter sections' by Waquar Ahmed (August, p. 231 j, in equation (2<), instead of ”  = a  pure resistance," read “ a reactive impedance "  and in equation (5), instead of RR in first turn under radical sign, read RR'.

Volume Rectification of Crystals

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Volume Rectification of Crystals

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