310 Philips tech. Rev. 40, 310-315,1982, No. 10

The fining of glass

A Raman-spectrometric investigation into the action of arsenic oxides

H. Verweij

At Philips Research Laboratories laser-Raman speetrometry has been used to study the re-actions occurring during glass formation. Particular attention has been paid to the action ofsmall quantities of oxides of arsenic, which have traditionally been added to the melt forproducing bubble-free glass in an economically acceptable time.

What is fining?

Soda-lime glass has been made for centuries byfusing together sand, soda ash and lime, in a reactionthat can be approximated by

75 Si02 + 15 Na2C03 + 10 CaC03 --+

15Na20.lOCaO.75Si02 + 25 C02 I'.

Many other familiar types of glass are produced bysubstituting potassium for the sodium (potash glass)or by using other metals instead of the calcium (togive lead or flint glass). In the glass for televisiontubes the calcium is largely replaced by barium, togive better X-ray absorption.

The temperature of the batch materials during glassformation is usually increased up to about 1450 DCruAt about 900 DC a large amount of C02 begins toform in the melt, so that the reacting mixture initiallycontains a very large number of bubbles (fig. 1). Theformation of bubbles in the melt may be due not onlyto the release of C02 but also the presence of nitrogen,oxygen, water, etc. During the thermal treatment ofthe glass some of the bubbles rise to the surface,where they escape from the melt. This bubble forma-tion is a useful feature of the glass-forming process,since the rising movement of the bubbles helps toproduce a more homogeneous glass, but it has to becontrolled in such a way that the final product issufficiently free from bubbles.

Dr Ir H. Verweij is with Philips Research Laboratories, Eindhoven.

In the manufacture of glass today, for exampleglass for television tubes or for optical applications,the specifications relating to the concentration andsize of the bubbles remaining in the glass are verystrict. This has led to an increased interest in theremoval of bubbles from the glass melt, a process tra-ditionally known as 'fining'.

In the fining process the size of the bubbles plays animportant part: bubbles of diameter smaller than10 urn are usually unstable and quickly dissolve again

Fig. 1. Photograph of a cross-section of a glass melt that has beenheated to 1450 oe and then rapidly cooled to room temperature.The bubbles to be seen consist mainly of CO2. To remove suchbubbles rapidly, small quantities of a fining agent are added to theingredients of the melt. The purpose of the research described herewas to discover the mechanism of the fining process.

Philips tech. Rev. 40, No. 10 GLASS FINING 311

in the melt. Bubbles of diameter greater than 1 mmusually rise quickly to the surface (Table 1) and there-fore present no problems. But bubbles of diameterbetween these values require special measures forremoving them from the melt at an economically ac-ceptable rate.

Table J. The calculated mean time of rise of gas bubbles in a repre-sentative silicate glass at the melt temperature, as a function of thediameter of the bubbles.

Diameter (urn) Rise time

1.6 years/m5.8 days/m1.4 hours/m0.8 min/m

10100

100010000

The oldest and still most widely used method offining is to add small quantities of chemicals, calledfining agents, to the ingredients for the glass. In theproduction of window glass and bottle glass the chem-icals used for this purpose are sulphates combinedwith carbon, and in the production of optical glassand glass for television picture tubes, nitrates com-bined with oxides of antimony are used. A combina-tion of nitrates with oxides of arsenic is also veryeffective, but arsenic oxides are no longer used in themanufacturing process, largely because of theirtoxicity.

Most of our knowledge of the action of fining agentshas been obtained empirically, and even today little isknown with certainty about the mechanism of theiroperation. Since the production of picture tubes isof such great economic significanee to Philips, wedecided to undertake a closer study of this mechanismat the Research Laboratories. We began by lookinginto the action of arsenic oxides, as a preliminary to amore extensive investigation into the action of anti-mony oxides and the function of the nitrates [21. Al-though arsenic oxides are now little used in the manu-facture of glass, the material is highly suitable as amodel for a study of the fining action. For the analyt-ical determinations involved in our investigation wehave used laser-Raman spectrometry.

Why Raman spectrometry?

We wished to look more closely into the way the re-actions take place in the glass melt, and in particularto find out how temperature affects the ratio of theconcentrations of trivalent to pentavalent arsenic ions,[As3+]/[As5+], in the formation and fining of glasseswith an arsenic content. According to an existinghypothesis, this ratio increases with increasing tem-perature, ,,:hile oxygen is released. It appeared that

the oxygen released caused the C02 bubbles formedearlier in the glass melt to swell and rise to the surface,and that this was the main cause of the fining action ofarsenic. Unlike CO2 bubbles; any remaining oxygenbubbles should dissolve again in the melt on cooling.The change of [As3+]/[As5+] as a function of tem-

perature has usually been determined by 'wet' chemicalmethods. We wished, however, to obtain additionalinformation that would give us a complete picture ofthe various reactions that take place during the glass-formation and fining processes. The conditions underwhich measurements have to be carried out for suchan investigation - high temperature or, after suddencooling to a low temperature, the mixed crystalline/vitreous state of the specimens - greatly restrict themethods of analysis that can be used.

The two methods most suitable for such an investi-gation are infrared and Raman spectrometry. Bothmethods provide information about molecular struc-tures (including the valence of the ions that occur inthese structures); it does not matter whether the ionsare in a crystalline or a glassy environment, and inneither of the two methods is it necessary to destroyany part of the specimens, with the associated loss ofinformation [31.

In view of the small concentrations of fining agent(less than 1070) used in the fining, Raman spectrometry,which is intrinsically less sensitive, would seem to beless suitable than infrared speetrometry . With thelatter method, however, there is generally too muchoverlap of the bands in the spectra, and since theintroduetion of lasers in recent years, the sensitivity ofRaman speetrometry has also been substantially im-proved.

Experimental

The analytical measurements required in our ex-periments were performed with the laser-Raman spec-trometer illustrated in fig. 2. Our aim in the choice ofthe instruments and in the performance of the meas-urements was to give the method of measurement thehighest possible sensitivity and accuracy.

[I] A description of the most commonly used methods of continu-ous glass production is given in G. E. Rindone, Glass Ind. 38,489, 1957, and J. Stanëk, J. non-cryst. Solids 26, 158, 1977.

[2] A more extensive description of our research, with all the ex-perimental details, will be found in H. Verweij, Melting andfining of arsenic-containing silicate glass batches, Thesis, Eind-hoven 1980. This thesis includes H. Verweij, J. Amer. CeramicSoc. 62,450, 1979, and H. Verweij, J. Amer. Ceramic Soc. 64,493, 1981.

[3] In an earlier investigation elsewhere, X-ray diffraction wasused for the same purpose, but the information obtained wasconfined to crystalline compounds. Earlier thermogravimetriedeterminations and differential t,hermal analysis had the dis-advantage that the data obtained were all indirect. Furtherdetails are given in the articles of note [2].

312 H. VERWElJ Philips tech. Rev. 40, No. 10

One of the methods of increasing the accuracy is tomeasure each spectrum a number of times and thentake the mean. Since each separate 'scan' takes a con-siderable time, owing to the low intensity of the signalto be measured, the complete measurement may takehours. For this reason the measurement procedurehas been largely automated.

Fig. 2. View of the Raman spectrometer set up at Philips ResearchLaboratories. On the far left is a laser, which emits green light. Onthe right can be seen the lens that focuses some of the light scatteredfrom the specimen on to the entrance slit of the monochromatorsituated behind it. The Raman spectrometer is a 'Ramanor HG2S'made by Jobin Yvon, Long Jumeau, France.

The experimental arrangement is shown in jig. 3.The primary beam, generated by a 4W Ar+ laser, La,which delivers a beam of highly monochromatic lightof high energy density at 488.0 or 514.5 nm, is focusedby a lens Lel into the specimen chamber Sp. Lightscattered at right angles to the direction of the primarybeam is then focused on to the entrance slit En of themonochromator Ma.

To eliminate undesired stray light as far as possible,successive spectral filtering is carried out in the mono-chromator by means of concave gratings. The controland computing unit Corn controls the position of thegratings and of the various slits in the monochromatorby means of stepping motors. At the exit Ex of themonochromator there is a photomultiplier tube Phi,which converts photons into current pulses; the photo-multiplier has a very low dark current. After amplifi-cation and pulse-height selection in the amplifier/dis-criminator system Ami, these current pulses arecounted by a 100 MHz counter Cou.

To obtain the optimum correction for the drift inthe laser intensity during the measurements, the meas-urement time of the counter (the 'time window') isadapted to the laser intensity in the following way.Part of the primary beam is conducted along a glassfibre to a second photomultiplier tube Ph2. Aftersimilar amplification and pulse-height selection, inAm2, the current pulses from this tube are applied to

the counter as external clock pulses. The time windowof the counter is set to a fixed number of these externalclock pulses, and its 'length' is therefore inverselyproportional to the laser intensity - this has the resultthat the number of signal pulses counted does notvary with the laser intensity.

Another measure designed to increase the accuracyof the measurements is to subject all the measuredcounter values to a procedure for evaluating theirstatistical significance. Each time window is dividedinto twenty equal parts, and the twenty correspondingcounter values are examined to see whether they de-viate significaritly from the mean or not. If they do,they are rejected and replaced by new counter values,and the evaluation procedure is repeated.

Since the spectra are available in digital form, a largenumber of operations can easily be carried out at alater stage. Examples are the production of differencespectra, obtaining spectra of single components fromspectra of mixtures, the accurate determination ofseparate peaks, various statistical operations on thespectra, and the representation of the spectra in dif-ferent ways.

The glass composition

The glass specimens to be investigated were pre-pared from a mixture of 70 molOJoSi02 and 30 mol%K2C03, with 1 mol% of AS203 added as a finingagent. The composition was made as simple as pos-sible to minimize the number of reaction productsoccurring during the glass formation, and to avoidphase separations in the melt at the high temperaturesat which the glass forms. We used potassium instead

Fig. 3. Diagram of the laser-Raman spectrometer in fig. 2. La 4W-Ar+ laser. Lel, Le2, and Le3 lenses. Sp specimen chamber. En andEx entrance and exit slits of the monochromator Mo. Phi and Ph2

photomultiplier tubes. (Ph2 receives light from the primary beamvia a glass fibre.) Am, and Am2 amplifier/discriminator systems.Cou 100 MHz counter. (The 'time window' of the counter closesafter a fixed number of external clock pulses, which are applied viaPh2 and Am2; in this way the number of signal pulses counted viaPh I and AmI is made independent of the laser intensity.) Com con-trol and computing unit.

Philips tech. Rev. 40, No. 10 GLASS FINING 313

of sodium because the Raman spectra of potassium-silicate glass generally have somewhat sharper peaksand have been studied more extensively than those ofsodium-silicate glass. The glass of the composition weused has much the same melting and fining behaviouras glasses in normal use, and both types of glass havethe same silicon content.

Results of measurements

The Raman spectra presented in fig. 4 will now bediscussed. These were recorded for powdered speci-mens with the equipment described above, after thematerials had been heated for an hour at a tempera-

g glass

s Si02

c carbonate

d disilicate

m metasilicate

B50·Cd c

cg

I

rBOO·C c

d

s

700·C c s

c4. s

1200cm-1 800Llv-

400

Fig. 4. Raman spectra recorded with the equipment in figs 2 and 3.The spectra give the intensity I of Raman light scattered at rightangles, as a function of the difference in wave number /),.v from thatof the primary beam; they were recorded for powdered specimenswith an initial composition of 70 molOJoof Si02, 30 molOJoofK2COS and 1 molOJoof AS20s. Before the recording the specimenswere heated for an hour at 700, 800 or 850°C and then rapidlycooled to room temperature. The letters and chemical formulaeindicate the atomic groups associated with the various parts of thespectrum. s Si02. c carbonate ion in ordered lattice of K2COS.d crystalline disilicate (K20.2Si02). mg metasilicate glass. dg di-silicate glass (see also fig. 5). cg carbonate ion dissolved in the liquidglass phase. It can also be seen that, after heating to 850°C, thearsenic occurs only in the form of Asol- groups, that is to saysolely in the pentavalent state.

ture of 700, 800 or 850 oe and then rapidly cooled toroom temperature. Details of the preparation of thespecimens and of the rest of the measurement proce-dure, and also of the way in which the contributionsfrom the various reaction products in the spectrumwere identified, have been given elsewhere [2].

After it has been heated to 700 oe, the specimenstill only givesthe peaks due to Si02 (s), the carbonateion in a crystalline lattice (c) and the As04a- group,occurring in a crystalline lattice of KaAs04. This canbe seen from the lower spectrum in fig. 4. At 700 oethe arsenic, which was added in the form of AS20a,therefore only occurs in its pentavalent state, pre-sumably owing to oxidation by oxygen in the ambientatmosphere.After heating to 800 oe the specimen gives the first

peaks that indicate the occurrence of a vitreous state.The peaks mg are due to non-ordered metasilicatechains (which are represented separately in fig. 5a),while the peaks cg originate from carbonate ions dis-solved in the glass phase. There are also the peaks doriginating from crystalline disilicate (K20.2Si02).The upper spectrum shows that the development thatstarts at 800 oe continues at 850 oe.

• Sio 00K' Q

o

Fig. 5. Schematic 'planar' representation of the structures of meta-silicate (0) and disilicate glass (b). Tetrahedra should be envisagedwhose basal plane coincides with the plane of the drawing. Thetetrahedra are projected on to this plane. Four oxygen atoms (0)are located at the corners of each tetrahedron, so that there arethree oxygen atoms in the basal plane and one at the apex. At thecentre of each tetrahedron is a silicon atom (.), which in thisschematic projection coincides with the oxygen atom at the apex(@). In the linear chains of 0 (metasilicate glass) two oxygen atomsalways belong to two tetrahedra. In each tetrahedron only half ofthe oxygen atoms should therefore be taken into account, and foreach tetrahedron we therefore arrive at the formula SiOs2-. Twopotassium ions (0) should therefore be envisaged as added to eachtetrahedron to compensate for the charge. In the more condensedstructure of b (disilicate glass) three oxygen atoms always belong totwo tetrahedra, so that the formula for each tetrahedron is Si02,5-,or, for each pair of tetrahedra, Si2062-. Here only two potassiumions are required for each two tetrahedra to compensate for thecharge. Bymeans of such condensations it is possible to compensatefor the reduction of the potassium content that occurs in the liquidglass phase during the glass-forming process. Such a compensationis also the result of the condensation of the arsenic-containingAs04s- groups into AS2074- groups, and also of the reduction ofpentavalent arsenic to trivalent arsenic, in the form of As02-groups.

314

Fig. 6a illustrates schematically the various stagesin the progress of these reactions in the specimen. Asa result of reaction with K2C03, layers of crystallineK20.2Si02 (grey) form around grains of Si02 in amedium of liquid metasilicate, whose compositionapproximates to K20.Si02; carbonate ions are dis-solved in this liquid metasilicate.

On heating to above 850°C changes occur thatappear to be very important in the fining process.Fig. 7 shows Raman spectra for glass specimens thathave been heated for an hour at temperatures of 900,950, 1000 or 1100 °C. We see at 900°C the first emer-gence of peaks originating from AS2074- and As02-groups, the latter thus relating to arsenic in its trivalentstate. At this temperature we also see the first peaksoriginating from disilicate glass (dg), with the com-position K20.2Si02 (fig. Sb). Finally we see that thepeak of crystalline carbonate (c) has disappeared atthis temperature, as might be expected since themelting point of K2C03 is 891°C. At 1000 °C thepeak of Si02 (s) has also disappeared, and at 1100 °Cso has that of crystalline K20.2Si02 (d), thus com-pleting the formation of the glassy state.

Fig. 6b summarizes a number of reactions thatoccur in this phase of glass formation: growth of theK20.2Si02 layer (grey) at the expense of the Si02grain, the formation of disilicate glass in addition tometasilicate glass, the formation of AS2074- andAs02 - groups in addition to As043- groups and theformation of both C02 and 02 bubbles.

Conclusions from the investigation

All these reaction processes in the temperaturerange up to 1100 °C considered here may be describedas being connected with competition for the potas-sium ion. As more potassium ions find a place at theoutside of the Si02 grains forming the compoundK20.2Si02, and hence less positive charge is presentfor compensation in the glassy phase, so more C02has to escape from the C032- ions in the form ofbubbles:

2Si02 (s) + K2C03 (s) -+ K20.2Si02 (s) + C02 (g) /' .

The conversion of metasilicate glass into disilicateglass can also be regarded as a condensation reactionthat liberates potassium ions to compensate for thereduced content of potassium in the liquid glass phase:

2 Si02 (s) + 2 K20.Si02 (I) -+K20.2Si02 (s) + K20.2Si02 (I),

or, which amounts to the same thing, as a condensa-tion reaction that liberates potassium ions for thesolution of Si02:

H. VERWElJ Philips tech. Rev. 40, No. 10

mg.cg.Asotmg.cg.AsOZ- ~

Fig. 6. Schemàtic representation of the progress of the glass-forming and fining processes. In the first stage (a), which takesplace in the temperature range from 700 to 850°C, we see howthe outer layer of a grain of Si02 is converted into crystallineK20.2Si02 (grey). Around this a metasilicate glass forms, in whichC032- ions and AS043- groups are dissolved and CO2 bubbles (a)are generated. With the continued conversion of Si02 intoK20.2Si02 at a temperature of 900-1100 °C ib), and with the con-sequent reduction of the potassium content in the liquid glassphase, there is the formation of disilicate glass, AS2074- and AS02-groups, and in addition to CO2 bubbles, oxygen bubbles (0) arealso formed; see also fig. 7. The oxygen formation is the result ofthe reduction of pentavalent arsenic to trivalent arsenic in the formof AS02- groups.

I dg

c carbonate

d disdicate

m metasilicate

1100°C dg

1200cm-1 800.1v-

400 o

Fig. 7. Raman spectra recorded for specimens heated for an hour at900,950,1000 or 1100 °C. Other data as in fig. 4. The occurrence ofthe different structures in this temperature range corresponds toconversions as represented schematically in fig. 6b. The conclusionto be drawn is that during 'the glass-formation process the potas-sium content in the liquid glass phase is reduced, leading to allkinds of condensation reactions and to the reduction of the penta-valent arsenic.

Philips tech. Rev. 40, No. 10 GLASS FINING 315

The condensation of the ions containing arsenic canalso be related to this:

2 Si02 (s) + 2 KaAs04 (1) ~K4As207 (1) + K20.2Si02 (1).

The reduction of pentavalent arsenic can occur withsimilar results, accompanied by the formation of 02bubbles:

2 Si02 (s) + K4As207 (1) ~2KAS02 (1) + K20.2Si02 (1) + 02 (g) » .

Our investigation thus indicates that changes in theconcentration of the potassium ion during the glass-formation process lead to the reduction of arsenic,which is a prerequisite for the release of oxygen, andthat this in turn is the most important element of thefining action.

Further research has confirmed that the shift in theAsa+ /As5+ equilibrium during glass formation ismainly chemically induced, owing to the changingcontent of potassium ions (or, in general, cations),and is not due only to temperature changes, as usuallyassumed previously.This chemical induction of the shift in equilibrium

does suggest that the oxygen not only forms later thanthe C02, but also forms at a different location, closerto the nucleus of the Si02 grains (fig. 6). The forma-tion of 02, separated from that of C02, could providean explanation for the exceptional effectiveness of the

fining action of AS20a: any C02 produced would notnecessarily appear as bubbles that expand under theinfluence of oxygen formation and disappear in thisway from.the melt. The C02 would also be displacedby the evolving oxygen. According to this hypothesis,this displacement of C02 would happen at theformation stage of the process, when open pores arestill present in the reaction mixture, not yet com-pletely liquid, and gas can still escape through thesepores.Although the simple composition of the system in-

vestigated permits no definite practical conclusion asyet, it does seem likely that the different light that ourinvestigation casts on the fining mechanism will proveuseful in attempts to find the most effective heat-treat-ment programme for the production of bubble-freeglass.

Summary. At Philips Research Laboratories a study has been madeof the fining action of arsenic oxides in glass formation. Specimenscomposed of 30 mol% K2Cûs, 70 mol% Si02 and 1 mol% AS20swere exposed for an hour to temperatures of 700, 800, 850, 900,950, 1000 or 1100 °C. After cooling the specimens rapidly to roomtemperature, spectra were recorded with a laser-Raman spectrom-eter. Above 900 °C there is reduction from pentavalent to trivalentarsenic, accompanied by the release of oxygen. This oxygen has afining action, either by causing the C02 bubbles produced earlier inthe melt to expand, or by displacing the C02. Any oxygen bubblesstill remaining dissolve again in the melt on cooling. The Ramanspectra also suggest that the reduction of As5+ to Ass+ is chemicallyinduced, by a reduction of the potassium content in the liquid glassphase during the glass-forming process.

316 Philips tech. Rev. 40, No. 10

Scientific publicationsThese publications are contributed by staff of laboratories and plants that form part ofor cooperate With enterprises of the Philips group of companies, particularly by staff ofthe following research laboratories:

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D. E. Aspnes (BellLaboratories, Murray Hill, N.J.) &J. B. Theeten: Optical properties of the interface be-tween Si and its thermally grown oxide.Phys. Rev. Letters 43, 1046-1050, 1979 (No. 14). L

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J. Bloem: Nucleation of silicon on amorphous andcrystalline substrates.Proc. 7th Int. Conf. on Chemical vapor deposition,Los Angeles 1979(Electrochem. Soc. Proc. 79-3), pp.41-58. E

P. W. J. M. Boumans: Inductively coupled plasma-atomic emission spectroscopy: its present and futureposition in analytical chemistry.Fresenius' Z. anal. Chem. 299,337-361, 1979 (No. 5).

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P. W. J. M. Boumans: ICP: d.c. are in a new jacket?Spectrochim. Acta 35B, 57-71, 1980 (No. 2). E

J. C. Brice: The lattice constants of a-quartz.J. Mat. Sci.15, 161-167,1980 (No. I). R

F. J. A. den Broeder & J. van der Borst: Magnetizationreversal in FeaoB15Si5metallic glass with large uniaxialmagnetorestrictive anisotropy.J. appl. Phys. 50, 7116-7121,1979 (No.11, Part I). E

A. Broese van Groenou: Some results on the wear of abronze-bonded grinding wheel.The science of ceramic machining and surface finishing11,eds B. J. Hockey & R. W. Rice, N.B.S. special pub-lication 562, Washington, D.C., 1979, pp. 147-156. E

A. Broese van Groenou: Optimization of multistagegrinding operations: the choice of consecutive removalrates.The science of ceramic machining and surface finishing11,eds B. J. Hockey & R. W. Rice, N.B.S. special pub-lication 562, Washington, D.C., 1979, pp. 191-200. E

A. Broese van Groenou & R. Brehm: Experiments onhigh-speed grinding of ferrites and glasses.The science of ceramic machining and surface finishing11,eds B. J. Hockey & R. W. Rice, N.B.S. special pub-lication 562, Washington, D.C., 1979, pp. 61-74. E

A. Broese van Groenou, N. Maan & J. B. D. Veld-kamp: Single-point scratches as a basis for under-standing grinding and lapping.The science of ceramic machining and surface finishing11,eds B. J. Hockey & R. W. Rice, N.B.S. special pub-lication 562, Washington, D.C., 1979, pp. 43-60. E

H. H. Brongersma, G. C. J. van der Ligt& G. RouweIer:The reaction of bromine and oxygen with a tungstensurface studied by means of low-energy ion scattering.Philips J. Res. 36,1-14,1981 (No. I). E

M. Brouha & J. van der Borst: The effect of annealingconditions on the magneto-mechanical properties ofFe-B-Si amorphous ribbons.J. appl. Phys. 50, 7594-7596, 1979 (No. 11, Part 11). E

K. H. J. Buschow &P. F. de Chêtel (University of Am-sterdam); Hydrogen absorption and magnetic proper-ties of intermetallic compounds based on 3d elements.Pure and appl. Chem. 52, 135-146, 1980 (No. I). E

K. H. J. Buschow &A. G. Dirks: On the crystallisationbehaviour of amorphous alloys of rare-earths and 3dtransition metals.J. Phys. D 13, 251-258,1980 (No. 2). E

K. H. J. Buschow & R. M. van Essen: Loss of ferro-magnetism in YNis after H2 absorption.Solid State Comm. 32,1241-1242,1979 (No. 12). E

S. Colak, B. Singer &E. Stupp: Lateral DMOS powertransistor design.IEEE Electron Device Letters EDL-I, 51-53, 1980(No.4). N

L. E. Cross (Pennsylvania State Univ.) & K. H.Härdtl: Ferroelectrics.Kirk-Othmer, Encyclopedia of chemical technology,3rd ed., vol. 10, Wiley, New York 1980,pp. 1-30. A

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P. Delsarte: A generalization of the Legendre symbolfor finite Abelian groups.Discrete Math. 27, 187-192, 1979 (No. 2). B

P. Delsarte: Regular schemes over a finite abeliangroup.Geom. Dedicata 8,477-490, 1979 (No. 4). B

P. Delsarte, Y. Genin & Y. Kamp: Two-variable stabil-ity criteria.Proc. 1979Int. Symp. on Circuits and systems (ISCAS),Tokyo, pp. 495-498. B

P. P. J. van Engelen: Some ENDOR studies of 3d tran-sition metal ions in semiconductors.Thesis, Utrecht 1980. E

K. Enke, H. Dimigen & H. Hübsch: Frictional proper-ties of diamondlike carbon layers.Appl. Phys. Letters 36,291-292,1980 (No. 4). H

J. van Esdonk: Verbindingstechnieken voor ultra-hoog-vacuumsystemen.Revue de la Soudure/Lastijdschrift 35, 123-131, 1979(No.3). E

J. A. Geurst: Zilsel's variational principle derived from.Lin's principle in superfluid hydrodynamics of 4He.Physics Letters 74A, 225-228, 1979 (No. 3,4). E

B. H. A. Goddijn: Static performance of a hybrid step-ping motor with ring coils.Thesis, Eindhoven 1980. E

W. van Haeringen: On the choice of tube lengths anddiameters in a HeS-Hé dilution refrigeration system.Cryogenics 20, 153-157, 1980 (No. 3). E

J. Hallais & D. Boccon-Gibod: Applications deshétérostructures GaAs-(Ga,Al)As aux transistors àeffet de champ.Acta Electronica 23, 339-345, 1980 (No. 4). L

K. H. Härdtl: New aspects in ferroelectric ceramics.Ferroelectrics 24,75-80, 1980 (No. 1/4). A

P. Harrop: Gallium arsenide field effect transistormixers: theory and applications.Acta Electronica 23, 291-297,1980 (No. 4). L

J. B. Hughes, J. B. Coughlin, R. G. Harbott, T. H. J.van den Hurk* & B. J. van den Bergh" (* PhilipsElcoma Division, Eindhoven): A versatile ECL multi-plexer IC for the Gbit/s range.IEEE J. SC-14, 812-817,1979 (No. 5). R

F. M. Klaassen, H. J. Wilting &W. C. J. de Groot: AUHF MOS tetrode with polysilicon gate.Solid-State Electronics 23, 23-30, 1980 (No. I). E

E. K1otz, U. Tiemens & H. Weiss: X-ray tomographyby grid coding.Appl. Optics 19,474-476, 1980 (No. 3). H

G. Kowalski, R. Rieckeheer &W.Wagner: New meansfor picture formation in computer tomography.Optik 55,67-86, 1980 (No. I). H

M. H. Kuhn, H. Ney, R. Geppert & R. Gierloff: Auto-matische Zugangskontrolle mit Hilfe der "akustischenUnterschrift" .VDI-Z 122, 125-130, 1980 (No. 4). H

F. P. J. Kuijpers & G. F. M. Beenker: The exact solu-tion of the Stefan problem describing the growth rateofbinary Ill-V compounds for LPE with linear cooling.J. Crystal Growth 48,411-415,1980 (No. 3). E

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J. Lohstoh: The punchthrough device as a passive ex-ponentialload in fast static bipolar RAM cells.IEEE J. SC-14, 840-844, 1979 (No. 5). E

F. Meyer, J. H. J. M. Buster, B. G. Bagley & D. E.Aspnes (Bell Laboratories, Murray Hill, N.J.): Opticalproperties of the metallic glass PdO.775CUO.06Sio.165over the energy range 0.67 to 5.6 eV.J. non-cryst. Solids 34,441-444, 1979 (No. 3). E

R. F. Milsom: Three-dimensional variational analysisof small crystal resonators.Proc. 33rd Annual Symp. on Frequency control1979,Atlantic City, pp. 263-270. R

A. Mitonneau, A. Mircea, G. M. Martin & D. Pons:Electron and hole capture cross-sections at deep centersin gallium arsenide.Rev. Phys. appl. 14, 853-861,1979 (No. 10): L

J. Nicolosi & J. LadelI: X-ray topographic analysis ofdislocation line defects in solution grown deuteratedtriglycine fluoberyllate.J. Crystal Growth 49, 120-124, 1980 (No. I). N

J. M. van Nieuwland & C. Weber: Eigenrnodes in non-rectangular reverberation rooms.Noise Control Engng 13, 112-121, 1979 (No. 3). E

V. Pauker: Amplificateur équilibré large bande hyper-fréquences à transistor à effet de champ en GaAs.Acta Electronica 23,299-315, 1980 (No. 4). L

L. J. van der Pauw: A rigorous set of shell equationsderived from the law of conservation of energy.Philips J. Res. 36, 31-39,1981 (No. I). E

J. G. J. Peelen: Transparent hot-pressed alumina;I: hot pressing of alumina.Ceramurgia Int. 5, 70-75,1979 (No. 2). E

J. G. J. Peelen: Transparent hot-pressed alumina;II: transparent versus translucent alumina.Ceramurgia Int. 5, 115-119, 1979 (No. 3). E

A. Pirotte: Fundamental and secondary issues in thedesign of non-procedural relationallanguages. .5th Int. Conf. on Very large data bases, Rio de Janeiro1979, pp. 239-250. B

A. Rabier: Conception de circuits linéaires assistée parordinateur.Acta Electronica 23,277-289,1980 (No. 4). L

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318 SCIENTIFIC PUBLICATIONS Philips tech. R~v. 40, No. 10

H. Rau: Estimation of the homogeneity range of MoS2.J. Phys. Chem. Solids 41,765-767,1980 (No. 7). A

J. L. Robert, B. Pistoulet, F. M. Roche, P. Girard,J. M. Dusseau, A. Raymond (all with Université desSciences et Techniques du Languedoc, Montpellier) &G. Martin: Model of conduction in highly compen-sated semiconductors. Application to semi-insulatingGaAs.Physics of semiconductors, 1978 (14th Int. Conf.,Edinburgh; Inst. Phys. Conf. Ser. No. 43), pp.569-572; 1979. L

J. M. Robertson & H. A. Algra: Inhomogeneities inbubble films measured by~spin wave resonance.J. appl. Phys, 50,7810-7814.,1979 (No. 11, PartIl). E

J. M. Robertson, M. W. van Tol, W. H. Smits &J. P. H. Heynen: Colourshift of the Ce3+ emission inmonocrystalline epitaxially grown garnet layers.Philips J. Res. 36,15-30,1981 (No. 1). E

C. Schiepers (Insitute for Perception Research, Eind-hoven): Response latency and accuracy in visual word.recognition.Perception & Psychophysics 27,71-81, 1980 (No. 1).

H. J. Schmitt: Sehen mit Mikrowellen.Umschau in Wiss. u. Technik 80,82-87, 1980 (No. 3). H

A. Schnell: Nonlinear charge release of piezoelectricceramics under uniaxial pressure.Ferroelectrics 28,351-353, 1980 (No. 1/4). A

P. C. Scholten & D. L. A. Tjaden: Mutual attractionof superparamagnetic particles.J. Colloid Interface Sci. 73,254-255, 1980 (No. 1). E

P. J. Severin & H. van Esveld: On the decompositionof totalloss into absorption and scattering loss in com-pound glass fibres.Optica Acta 26, 1415-1426, 1979 (No. 11). E

B. M. Singer, W. G. Steneek, E. H. Stupp & R. V.Kurczewski (Magnavox, Mahwah, N.J.): Suppressionof pedesta noise in a pyroelectric vidicon.IEEE Trans. ED-27, 193-198, 1980 (No. 1). N

B. Smets: Atom formation and dissipation in electro-thermal atomization.Spectrochim. Acta 35B, 33-41, 1980 (No. I). E

J. Snel: The effect of donors or acceptors on theSi-Si02 interface.Insulating films on semiconductors, 1979 (Conf. Dur-ham; Inst. Phys, Conf. Ser. No. 50), pp. 119-123; 1980. E

J. P. Stagg & M. R. Boudry: Lateral diffusion of Na+ion at the Si-Si02 interface and Na+ neutralisation inthe presence of chlorine.Insulating films on semiconductors, 1979 (Conf. Dur-ham; Inst. Phys. Conf. Ser. No. 50), pp. 75-80; 1980. R

D. R. TerreIl & U. Killat: (N-vinylcarbazole contain-ing) polymers, Il. Sensitization to argon laser for use insingle-layer photothermoplastic devices.Photogr. Sci. Engng. 24, 25-31, 1980 (No. 1).

A. Thayse: Discrete function expansions in integerpowers.Discrete appl. Math. 1,127-136,1979 (No. 1,2). B

A. Thayse: Programmable and hardwired synthesis ofdiscrete functions, Part I: One level addressing net-works.Philips J. Res. 36, 40-73,1981 (No. 1). B

C. Tsironis: GaAs dual gate MESFET's and their ap-plications in microwave circuits.Acta Electronica 23, 317-324, 1980 (No. 4). L

C. Tsironis: 12 GHz receiver with a self-oscillating dualgate MESFET mixer.Acta Electronica 23, 325-329, 1980 (No. 4). L

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M. J. Underhill & R. I. H. Scott: The effect ofthe sam-pling action of phase comparators on frequency syn-thesizer performance .Proc. 33rd Annual Symp, on Frequency control1979,Atlantic City, pp. 449-457. R

J. A. T. Verhoeven: Auger surface studies of barium onsilicon oxide.Applns Surface Sci. 4, 242-246, 1980 (No. 2). E

G. Verspui: CVD of silicon carbide and silicon nitrideon tools for electrochemical machining.Proc. 7th Int. Conf. on Chemical vapor deposition,Los Angeles 1979 (Electrochem. Soc. Proc. 79-3), pp.463-475. E

J. F. Verwey: Mobility and trapping of ions in Si02.Insulating films on semiconductors, 1979 (Conf. Dur-ham; Inst. Phys. Conf. Ser. No. 50), pp. 62-74; 1980. E

R. P. Vincent: Multipath problems in aircraft approachaids - a solution.lEE Coll, on Modern techniques for combating multi-path interference in radio, radar and sonar systems,London 1979,4 pp. R

J. Vos: Design characteristics of an advanced Stirlingengine concept.Proc. 14th Intersoc. Energy Conversion Engng Conf.,Boston 1979, Vol. I, pp. 1191-1196. E

K. R. Whight: Synthesis and analysis of d.Lt.s. spectrafrom m.o.s. surface states.Electronics Letters 15, 744-745, 1979 (No. 23). R

R. V. Winkle & C. H. Warner* (* Kerry UltrasonicsLimited, Hitchin, England): A new method of qualitycontrol for ultrasonic wire bonding.Ultrasonics International 79, Proc. Conf. Graz 1979,pp.62-68. R

D. L. Wolters: The role of water in the oxidation ofsilicon.Insulating films on semiconductors, 1979 (Conf. Dur-

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Volume 40, 1982,No.10 Published 15th April 1983pages 287-318