Aspects of effective doping and the incorporation of dopant

I~I. J. of RefractoryMetals & Hard Materials 13 (1995) 1-34 Q 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0263-4368/95/$9.50 0263-4368(95)00001-l

ELSEVIER

Aspects of Effective Doping and the Incorporation of Dopant J. Neugebauer

& L. Bartha

Research Institute for Technical Physics of the Hungarian Academy of Sciences, Budapest, Hungary

Abstract: This paper describes results of many decades of research work on non-sag (NS) tungsten. Some of the results did not solve a given technical problem but they can be used to solve other technical or theoretical questions. Besides the role of intentional additives, the effect of nitrogen (as NH,), of phosphorus (as H,PO,) and of other additives is also discussed as is their role in building tungsten bronzes and beta-tungsten. In connection with the important role of gaseous hydroxides on the incorporation of additives into the metal particles, the effect of As, MO, etc., on this volatilization process (CVT), is also treated. Numerous SEM pictures show that the role of the melting effects is emphasized in the incorporation of potassium (as silicates). SEM pictures give information also about the influence of many experimental additives on the morphology of intermediate products of metal powder production. Some suggestions are given concerning the testing of NS metal powder without precursor tests and, further, about special chemical effects influencing the performance and use of tungsten products.

1

INTRODUCTION

siderations about the second phases, important for the NS effect, were not developed until later. There is no doubt that chemical processes must play a role because no one has yet succeeded in incorporating the necessary potassium-containing secondary phase into the tungsten metal by mere physical or mechanical means. Therefore, people who are involved in NS tungsten chemistry are especially interested in the question: how do the potassium compounds get into the metal powder particles as a secondary phase when potassium is not soluble in tungsten? There are several views, each a result of very valuable work in this field. Different authors have constructed theoretical solutions to the problem based on the following fragments of processes:‘-’ 3

When people working in the field of NS tungsten met several decades ago, a major topic of their conversation was the perpetual matter of brittleness. Problems arise from the contradictory requirements for workability and good NS quality: as a rule a fine metal powder is demanded, the rods and wires will not split, and the filaments will not break during working and handling. The essence of NS quality was not fully understood. The bubble structure was not yet known; the only basis for such considerations was the importance of the chemical dopant containing Al, K and Si (AKS). Then came the era when researchers in this field often had to face ridicule when they proclaimed the theory of ‘the bubble’. Today, however, this concept is an integral part of the most advanced technology, and both the positive as well as the negative effects of the bubbles play roles in the planning of new metal powder types. Their recognition was a result of advancement in electron microscopy based on the technique of geometrical pictures. Chemical con-

-

position of the dopant in or on the base material (oxide) - the role of oxide bronzes and/or silicates as intermediates - the role of Al in: - forming KSi/AlO,,,, which has a lower vapour pressure 1

.I. Neugehauer, I,. Rartha

forming KAlSi,O,, which is hardly soluble even in HF suppressing the formation of K-oxide bronzes with high K contents stabilizing P-W at higher temperatures - the substitution of Al by Ga - the action of P-W, such as the solution of K - incorporation of the dopant during the metal powder production, i.e.: - by grain growth - by volatility of tungsten oxide-hydroxide - by sintering of small metal particles at C. 900°C - by diffusion and chemical reactions in a melt formed by the dopant during metal powder production Theories of incorporation based on one or more of these processes are supported by empirical and experimental evidence: -

-

The observations were carried out mainly by electron microscopy, the information of which appears on a black and white screen. This fact and the complexity of the process makes it difficult to recognize the dopant phases in the course of the reduction of doped oxides. Therefore, we attach great importance to earlier observations as well, made on undoped and doped oxides and their reduction products, where under an optical microscope the colours also helped in the identification. In several cases the reduction process could be followed in a reduction chamber under the microscope. Thus, the differences between doped and undoped samples helped in discerning the key morphological formations, first under the optical mcroscope and then by SEM.

In this paper we rely upon such information gathered from magnifications of up to 20000 X . Much of our information comes from experience with ‘tungstic acid’ as a basic material, in addition to the now common APT.

2

BASIC MATERIALS SUITABLE FOR PRODUCING NS TUNGSTEN

In industrial practice, the usual starting material for the production of high-quality NS tungsten (e.g. for filaments in krypton-filled incandescent

lamps) was tungstic acid, WO,H,O, and the dopant contained compounds of potassium, silicon and aluminum, in some cases with excess sodium and iron. Now the commonly used starting material is ammonium paratungstate tetrahydrate (APT): (NH,),,, H,W,,O,, 4H,O (or, in the older form, as the pentahydrate: S(NH,),O .12WO,. 5H20). W0,H20 was doped directly, while APT was first decomposed to blue oxide (BO). There is an enormous morphological difference between the almost colloidal particles of tungstic acid and the rather coarse crystals of APT having a quite different crystal structure. Therefore, it was surprising that both starting materials needed about the same amount of dopant in order to get a good-quality NS metal powder. This means that even when bonding of the dopant compounds on the surface and in the lattice of the tungsten oxide may play a role, evidently very important reactions between dopant and substrate will take place later during drying as well as at the front part of the reduction furnace. Here, reduction still cannot begin because of the low temperature, but significant solid-state chemical reactions do occur. It is known that only about 10% of the doping potassium is necessary in the tungsten metal to produce the NS effect in bubble form. Thus, after sufficient dopant has bonded during the doping process, the surplus remaining in the liquid phase could be eliminated and it should be possible to achieve the desired NS effect. This is, however, only partly true. There are processes in which the mother liquor is decanted and only the adsorbed and chemosorbed dopant remains. In such ‘adsorption doping’ or ‘doping by ionic change’, however, at the beginning the doping solution has to contain even more potassium than for the traditional doping, where the doped oxide is evaporated to dryness. This means again that the formation of effective doping compounds is still not complete even at the end of the so-called doping process. This does not mean that the physical and chemical behaviour of the starting material is unimportant. It was found that earlier when the incorporation of not more than 40 ppm of potassium was the goal, the reduction process was extremely sensitive to technological parameters when the starting material was tungstic acid. With APT as the starting material the sensitivity decreased, but at the same time it was more dif-

Aspects of effectivedoping and the incorporation of dopant

ficult to achieve the desired NS quality. The problem was finally solved, first by assuring the proper decomposition of APT and then by increasing the amount of potassium incorporated. It turned out that APT is best decomposed in a reducing atmosphere to a ‘blue oxide’, which is an ammonium-tungsten oxide bronze (ATB)14 that is formed when the decomposition temperature is kept under c. 500°C (Fig. 1). The formula of this nonstoichiometric compound is ( NH4)xHYWO, _z. By heating over 5OO”C, x and y decrease to zero and we get the ‘true’ blue oxide, W03_Z, where z = c. 0.1. The pure crystalline form is W,,O,, (Fig. 2). It was reported that the hexagonal ATB structure can in some cases be preserved when NH, is not present. 15,16We were not able to confirm that. We found instead that potassium can replace NH:, in which case the structure survives as a potassium-tungsten oxide bronze (KTB). This is always the case for the reduction of the NS-doped oxide. A tetragonal hydrogen-tungsten-oxide bronze can sometimes also be detected by X-ray phase analysis in decomposed APT (Fig. 1 and Fig. 2(c)-1). The crystal structure is the same as that of Na,.,WO, (see ASTM 5-0389). It is interesting that it can be produced in almost pure form by decomposing APT in CO gas (Table 1).’’ The specification and testing of the starting materials are difficult due to the complicated processes involved in doping and especially in reduction. Therefore, it is more convenient to test their effect on the reduction process itself. This can be done by pilot plant reduction or by laboratory

I

.

100

200

300

LOO

500

600

WC)

Fig. 1. MS diagram of the decomposition of APT in hydrogen. H: hexagonal oxide bronze (ASTM 26-1340). T: tetragonal oxide bronze (ASTM S-0389). B: /-?-oxide(ASTM 5-0386). (Made by G. Gerey.)

3

scale model reduction, as in a TG apparatus. The incorporation of potassium can be tested in the end product after washing it in hydrofluoric acid. In pilot plant tests it is often advisable to produce even sintered bars (ingots) and wires and test them. Let us first see what we can hope to achieve with doping: (1) with Al, K and Si, of course, the best NS quality; (2) with K and Si, a moderate NS quality with fewer bubbles; (3) with K and Al we cannot obtain NS quality but do get good working properties. In combining these: (1)+(2) (2)+(3)

( 1) +

undoped

properties can almost be interpolated; NS quality is not severely decreased unless Si is less than 25% of the usual concentration for AKS; NS effect is W oxide no obtained when K drops to 60% of the usual AKS concentration.

Sintered bars and drawn wires show many differences (when the starting material is tungstic acid) (Table 2). The data for potassium (Table 2) also suggest that this dopant alone does not determine the NS quality because not all of the potassium is in an effective form. Undoped tungsten metal may also contain potassium (but without the NS effect) which arises from original and collected impurities. It is interesting that tungsten ingots doped with Al alone cannot be worked at all. This diffusion of Al into the metal powder particles is detrimental if potassium does not moderate it.‘* Of the doping elements, silicon can be decreased to a very low level (about 100-200 ppm in the doped oxide) without losing the NS character, but the absence of silica (SiO,) during metal powder production is a disadvantage. Silica keeps the oxide bed loose and permeable first of all for H,O during the reduction. Figure 3 gives an overview of the most essential tungsten-containing compounds during doping and reduction. Of the ‘M’ radicals only NH,+ and

J. Neugebuuer, L. Bartha

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and I values from X-ray diagrams of tungsten oxides and oxide bronzes and of P-W and a-W. (See Table 3.)

Aspects of eflectivedoping and the incorporation of dopant

876

5

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d2

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Fig. 2. - contd. Table 1. X-ray diffraction co

lines of APT heated to 600°C in

Tetragonal

APT heatedin CO

W oxide bronze (ASTM 5-0389)

to 600°C

dl&

IN.

d/Al

Int.

3.87 3.705 3.215 3.108

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Table 2. Differences in the characteristics

K’ are considered, although Rb, Al and Ga can also form isomorphous compounds alone or in combination with K. Besides the compounds shown in Fig. 3 the silicates of K and Al can also be products of doping, e.g. heating the doped oxide. So far it has been possible to substitute the following doping elements: Rb for K, Ga and Al and, to a lesser extent, Na+Tl for K and Be for Si.19 With reference to Fig. 3 (in the first line): WO,H,O and APT can be produced by hydrometallurgical processes. For industrial doping WO,H,O is used without being decomposed, but for some purposes (e.g. for producing thoriated tungsten) it is heated even to 1 lOO”C, i.e. to WO, (see second line). In this case WO, has a monoclinic structure at room temperature. If, however,

of sintered bars and drawn wires

NS(K,Al,Si)

K,Si

K,AI

W undoped

Sintered bar % shrinkage Crystals on fractured surface Tendency to burst Workability

Medium Big, jagged boundary + Medium

High Small Jagged b.

Medium Small Hardly j .

High Small Equiaxial -

Good

Very good

Wire, filament heated at 2400°C crystals Brittleness Potassium (ppm (c.))

Large Low 40-100

Medium Medium 30-40

Small High 20

Good-medium

Small Very high O-20

J. Neugebauer, L. Bartha

6

Tungsten

oxtdes

and

tungstates increasing

pH )

from solution

heated on air

%-x(K@‘I~Qo)

WQ3.H20 “tungstic acid”

I

meta-tungstates

M2O,6WO3

WO3 tungsten

trioxide

M10042W,,o,,) para-tungstates

hexatungstate

M2O.LWO3 tetratungstate

I

M2W4 normal tungstate

M20 *3WO3

M20.2WO3

MZWQL

tritungstate

ditungstate

normal

tungstate

I I ----

(NH&-$ ,WOj blue hexagonal

t K0,25W03 L blue hexagonal oxidebronze

w10°L9~ w02,75 1 -oxide (violet )

ammorwm

tungsten

oxidbronze

v Ldo+pe

K0,5W03 Yiolet tetragonal oxIdebronze

I”

x G

Fig. 3.

Tungsten oxides and tungstates

NS-doped WO, (or WO,H,O) is heated, it has an orthorhombic structure at room temperature also. APT is always decomposed before doping, commonly in reducing gas (see arrow on Fig. 3 from ‘paratungstates’ to ‘blue hexagonal ATB’ and to W200sx; see also Fig. 1). It is reported that in some cases the violet oxide W,,O,, can also be used as a starting material for doping. As can be seen in Fig. 3 in the column below WO,, this is the product when WO, (or APT-blue oxide) is heated to a higher temperature in a reducing atmosphere. It is certain, however, that the lower oxide WO, cannot be doped effectively. That coincides with both the fact that no oxide bronzes are found where W3 + occurs alone as well as with the fact that during the reduction process potassium compounds are set free when WOZ appears. Oxide bronzes may play a role in doped tungstic acid before the formation of potassium bronze, because there are hydrogen-oxide

bronzes or oxide-hydroxides in the initial products of decomposition, and because of the lowtemperature reduction of tungstic acid (see Fig. 2 and Table 3). Before the role of potassium bubbles was known, there was an idea, that the role of the dopant is first of all to collect all the impurities and then help to evaporate them during sintering. Based on this conception efforts were made to start with the purest possible oxide, without dopant, and to conserve this purity during the chemical and thermomechanical production processes. Now of course it is evident that this was a dead end. Even a tungsten rod from NS metal loses this character on zone-melting and is more difficult to work, because it recrystallizes at the swaging temperature and crystal boundaries are extremely sensitive to the inevitable impurities in a technological environment. Another theory was that the dopant phases simply promote the formation of overlapped long

Aspects of flective doping and the incorporation of dopant Table 3. List of phases shown in Fig. 2(a)-(d) 2(a) Yellow phases 1 WO, monoclinic 2 WO, orthorhombic 3 WO, 4 wo, 5 WO, tetragonal 6 WO, monoclinic 7 ‘WO,’ hexagonal 8 WO,H,O heated in air to 300°C

ASTM ASTM ASTM ASTM ASTM ASTM ASTM

20-1525 20-l 524 32-1394 32-1395 5- 388 24- 747 33-1378

2(b) Blue oxides 1 WZOO,, ASTM 5- 386 2 wo2.‘) ASTM 18-1417 3 W2,0,, sublimed in vat. 4 W0,H20 reduced in H, with dew points of 25°C to 350°C 5 WO,H,O heated in argon to 700°C 6 WO,H,O heated in argon to 900°C 7 ‘C,-phase’ of Glemser-Sauer 8 ‘C,-phase’ of Gelmser-Sauer 9 W*,O,, ASTM 30-1387 2(c) Blue oxide bronzes 1 Na and H tungsten oxide bronze, tetragonal ASTM 5-0389 2 NazO/WO, .0.5H,O/,_,,, of Freedman*” 3 Rb and K tungsten oxide bronze, hexagonal ASTM 26- 1340 4 (NH,),,.,,SW0,.,,,2 .0*133H,O of Kissz7 5 K-oxide bronze, hexagonal of DechanvreP 6 K,.,,,W03 of Reau-HagenmulleP 7 KcI.X40,,.42 . W02,L)hof Neugebauer”’ 2(d) Violet phases 1 WI&49 ASTM ASTM 2 K,,.,WO, tetragonal 3 W,O,,(OH), of Ebert and Flasch3’ 4 W,203,(OH)z of Ebert and Flasch”’ WO, (brown) ‘W,O’, i.e. /3-W W

5- 392 5-0368

ASTM 32-1393 ASTM 2-l 138 ASTM 4- 806

crystals in wires and filaments, but this can also be achieved by mere thermomechanical processes, such as by enhancing crystal growth with annealing schedules. *OThis, although very useful, cannot, however, make the use of dopant unnecessary.

3

THE REDUCTION OF THE DOPED OXIDE TO METAL POWDER

Having a doped oxide already tested by pilot plant production, the actual reduction process will determine the NS quality and workability of the end product after reduction: the metal powder. We have gathered our own experience from the time when, starting with a doped tungstic acid, a ‘fine’ metal powder was the goal. This was a

7

primary prerequisite that fractured cross-sections of the sintered bars showed crystals of quite large size with jagged boundaries grown by ‘exaggerated grain growth. If the crystals were too large, and especially if they formed a crust near the surface only, the bars often swelled up or even exploded during sintering or in the subsequent working steps. This would also happen if the metal powder used was too fine or too coarse. In the first case the excess dopant was trapped because the pores closed so rapidly (percolation limit was reached before evaporation was complete). In the second case too much dopant was enclosed in the metal powder grain and it was thought that they exploded at the end of sintering. Although the optimum powder grain size between too fine and too coarse was very difficult to maintain at that time, much valuable experience was collected. It was found that water vapour in the hydrogen encountered coarsening. Arsenic, first as a contaminant and later as part of the dopant, worked in the opposite direction by allowing lower temperatures. Microscopic investigations showed that from the almost colloidal particles of doped and heated tungstic acid, metal particles of considerable size, about 2-3 pm, are formed. In very moist H, heated to 1000°C sizes of 8 pm and more were obtained. It was found that this surprising grain growth was the result of a phenomenon unknown until then: the volatility of tungsten oxides in the presence of water vapour due to the formation of gaseous tungsten oxidehydroxides.*i,** The effects of arsenic and other additives have been investigated mostly by TG (thermogravimetry) and by pilot plant investigations19 (Figs 4 and 5, Table 4). It was found that in addition to the phases in Fig. 3, many others appear during the different heating and reduction processes. Some of them can be identified only by X-ray phase analysis (see Fig. 2). As can be seen from Table 3 several of them are found in the ASTM files, while others were collected from the literature and from our unpublished results.

4

INTERMEDIATE AND END PRODUCTS IN THE INDUSTRIAL REDUCTION PROCESS

Figure 6 shows the intermediate products in a one-step reduction process. (In a two-step reduction the first reduction goes as far as the arrow.) In

J. Neugebauer,

8 Table 4. Evaluation

of Fig. 5

Doping agent

Beginning of reduction (“C,

NaCl KCI KOH AuCI, HgCl, H,BO, AICl, Ce (NO,), C,H,COOH Si (OC2HJ, GeO, SnCI, PbCG Th (NOh H,PO, As,O, (0.1% As) SbCl, (0.1% Sb) BiCI, NH,VO, Ta + HF+ HNOj S H$O,

Step I (“Ci

510 500 490 295 430 490 480 490 45s 45s 470 470 475 475 480 450 440 315 465 350 460 465 465 4.50 420 49s 475 470 4 10 285 350 290 430

H$eO, Cr203

NH,. molibdate MNClz FeCl, CoC& NiCI, PdCI&l%Pd) oso, PtCI, U ndoped

560 570 560 530 550 5x0 580 550 540 540 540 530 560 580 660 530 5 10 570 540 5 IO 570 570 550 540 530 530 570 560 530 510 530 520 530 ___-

the lower layer of the oxide bed the reduction processes remain behind the upper ones because of the higher concentration of H20. The concentration of water vapour is also not uniform along the reduction furnace tube because the Hz0

-92‘

I

PW

C

100

200

300

LOO

500

600

700

800

TPCI

200T/hour

Fig. 4.

L. Bartha

TG diagram of a AKS-doped

tungsten oxide.

Step II (“0 680 660 655 650 685-695 71s 690 730 670-695 695 665-705 645 685 725-740 670 670 645 655-695 670 6X0-725 675-690 680 635-690 590-660 640-680 670-685 660-670 625 650 665 660 660

Composition in Step II (x in WOx)

End of reduction PC)

2.03 2.12 2.12 I.19 1.49 I.34 I .87 I.12 1.41 1.46 I .95 1.07 0~95 0.5 1

800 77s 8 I0 780

0.15 0.3 1 I.87 2.02 0.87 I.38 1.28 I .o 1 2.02 2.03 2.02 I .96 2.03 1.87 0.39 0.78 0.54 1.74

815 X65 900 X40 8 10

815 815 760 X05 830 750 760 730 X15 X65 835 820

815 810 885 820 850 830 830 805 770 79s 780 x00

evolved during reduction accumulates and therefore increases in the direction of streaming. There is a considerable concentration of H,O at the arrow in Fig. 6. In the first step of a two-step reduction, however, pure dry hydrogen is present, making a significant difference between the two technological processes. In Fig. 6 the ranges at which the intermediate oxides, oxide bronzes and melts form are shown. In the first part of a two-step reduction process the most essential features are those shown in Fig. 7. Equilibrium conditions are given in the upper right corner. It can be seen that we are working in the range where a-W is in equilibrium (with the exception of the deeper layers in the boats). The lower curve represents the dew points above the boats and the upper curve (with maximum) at the bottoms of the boats. Let us now consider the single phases appearing in Fig. 6. (See also SEM pictures in Fig. 8.)

9

Aspects of effectivedoping and the incorporation of dopant

~~~~~~~~~~~

!iv

Fig. 5.

1

c

Hexagonal

d21tiv

TG diagrams of various doped tungsten oxides

boats -

oxide-bronze)

Wl SOL9 W02(“cauIiflowers”) Tetragonal Separate

K (Al)-oxide

bronze

melt particles

Melt on the surface

of metal

grains

Beta-tungsten \ .;. .

Alpha-tungsten Dopant

inclusions

X

Fig. 6.

Ranges in which various phases appear in a one-step reduction.

h

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J. Neugebauer, L. Bartha

‘HZ0 dew

pomt

-2

temperature

‘Ogc’ H2 +l

0

+azoc

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set

Pw

4 -23

-3

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-5 500

560

625

727

838

977

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12

Fig. 7.

Composition

of the gas phase in the first part of a two-step reduction. (See also Table 8.)

The doped BO retains the blue colour of decomposed APT also after drying (at lower temperatures) in air. In the first period of the reduction the structure changes very little and the colour takes on a violet hue. When the drying (heating) of doped BO was followed at higher temperature (e.g. 400°C) in air, then the colour turned greenish or even yellow but without considerably changing the hexagonal ATB crystal Higher drying temperatures (e.g. structure. 6OO”C), before doping, change the structure towards the monoclinic WO, . (In reducing atmosphere the P-oxide WZoOs8 is formed at these temperatures.) Our scheme in Fig. 6 starts with doped hexagonal ATB. It loses NH, on reduction and reacts with potassium. In this way it is possible that part

of the doped material is reduced immediately to P-tungsten, avoiding the intermediate WO,. The bulk of the material (about 90%) goes, of course, from WO, to W. Therefore, in practice, at c’. 700°C a considerable amount of P-tungsten is present in the oxide, a fact which was ignored in earlier years. We know from laboratory observations that when an oxide bronze and WO, are placed close to each other in a reduction chamber of a microscope, the oxide bronze is reduced to metal at a temperature 30°C lower than for WO,. Thus, there is a possibility that the metallic particle nuclei are built mainly of potassium (and K-Al-) oxide bronzes. In the stage marked by an arrow in Fig. 6 the dopant phases are in different conditions, which are not precisely known. It was possible, however,

Aspects of effectivedoping and the incorporation of dopant

11

Fig. 8. Twenty-nine SEM pictures of morphological formations in a reduction process similar to that of Fig. 6. Using the numbers of the boats in Fig. 6: Boats 1-4, SEM 8(a), (b). The blue oxide (tungsten oxide bronze) becomes violet; in the upper layer reduction to WO, begins, with dopant particles sometimes serving as nuclei. Boats 5-6, SEM 8(c), (d). Reduction to WO, is going to be complete. Residue after dissolving WO, in H,O, contains a reddish and a white phase. Boats 7-8, SEM 8(e)-(i). Reduction to W begins in the top layer. The metal particles are growing in the center of WO, formations and at their boundaries.

12

J. Neugebauer, L. Bartha

Fig. 8. - contd. Boats 9-10, SEM 8(j)-(w). A s more and more crystalline W particles appear, solidified melt and spherical particles of 02-0.3 pm can be seen.

Aspects of gective doping and the incorporation of dopant

Fig. 8. - contd.

13

14

J. Neugebauer, L. Bartha

Fig. 8. - con&. Boats 1 I- 12, SEM 8(x)-(z), (a), (/?). WO, disappears, solidified melt can be seen everywhere. There are many spherical particles and holes on the surfaces of the metal grains. The melt begins to creep on to the surfaces of the metal grains. The residue after dissolving WOz and W no longer contains the reddish oxide bronze, only the white phase remains.

Aspects of eective doping and the incorporation of dopant

15

Fig. 8. - con&. Boats 13-14, SEM (a), (j3). T races of melt can be seen only on the surfaces of the metal grains. There are cracks on the edges. The residue after dissolution of the W shows that the grains were covered with a crust. There are enclosed dopant particles of c. 0.3 firn 4.

to collect considerable data about them. Even under an optical microscope it was possible to find the above-mentioned blue and reddish-violet crystals of oxide bronzes. (Under a layer of water they have a brilliant colour and can thus be distinguished from violet and blue oxides.) Their form is characteristic: with the highest potassium content they are long needles or columns of a reddish colour; with less potassium they form smaller, stubby blue crystals. The bulk of the WO, + W can be dissolved by H,O, so that the dopant phases can be better observed. We have then in the residue heavy, coloured oxide bronzes, easily separated by standard sedimentary procedures together with white phases difficult to sediment. In the intermediate products big flakes of silica can also be observed by an optical microscope. It is interesting to consider how the said intermediate product (see, e.g. Fig. 8(e)-(i)) behaves on

washing with HF and how it is affected by the ‘chlorination residue’ test. Although 90% of the doping phases can be eluted from the metal powder, little can be removed from the brown oxide, even with hydrofluoric acid. Similarly, in the chlorination test (heated in a Ccl, stream), the greatest part of the dopant is so closely bound to the tungsten oxide that it retains considerable WO, in the residue on chlorination, which therefore cannot volatilize as WOCl,. The end product, the metal powder, however, is easily volatilized by chlorination, even if not previously washed by HF, and only the doping compounds remain in the residue (more precisely: the K as KCl, the Si as SiO,, but only part of Al as A&O, because of the volatility of AlCl,). This very essential intermediate product of the reduction process was, in earlier two-step reductions, the end product of the first step and we

16

J. Neugebauer,

were therefore able to test it. First of all it was possible to check by gas permeability tests and by microscopic and SEM observations whether or not the second reduction stage was in danger of producing too fine a powder, for example due to Ascontamination or to an improper temperature schedule for the first part. It was also possible to homogenize or even to grind the product of the first step (the ‘brown oxide’ BrO) and thus influence the quality of the final metal. Let us see what we can learn from the SEM pictures: Fig. 8(e)-(i). At a magnification of 10 000 x the amoeba-like formations in a solidified melt and the small spherical particles can always be found in the oxide bed wher-e WO, begins to form. They are visible as long as WOZ is still present. The literature data and our own observations are sometimes contradictory, which is no wonder since these small particles are in a very confusing environment. This situation can be simplified by dissolving WO, + W with H,O,. In the residue the coloured, heavier oxide bronzes can be separated by sedimentation and the remaining white residue tested, as by X-ray analysis. It turned out that in the said intermediate products (in Fig. 6 at the arrow and in Fig. 8(e)-(i)) hardly any or no Al was found; besides potassium the bulk was an Si compound. This fact together with the microscopic observations indicate that the white phase consists of solidified glassy potassium silicate melt particles and of silica. In the reduced metal powder, however, we always found K, Si and Al in the residue after the treatment with H,O,. The bulk here is also Si. In the crust covering the metal grains the concentration of Si is higher than in separate melt particles. Considering that here (Fig. 6, arrow) the temperature is 700°C and that the creeping of the melt onto the metal grains begins at 750°C it is probable that first potassium silicates and finally K-Al-silicates of the orthoclase type are formed.

5

THE EFFECT OF THE DOPANT FROM THE CHEMICAL POINT OF VIEW

When approaching the most burning questionthe incorporation of dopant phases into the metal grains-let us start from the facts: at the moment of the formation of the first metal particles (with greatest probability from K- or IS/J-oxide bronze) we have solid and melted silicates and

L. Bartha

WO,, which can, at that temperature, to a small extent volatilize as WO,(OH),. Finally we have the often disputed highly reactive /?-tungsten (‘W,O’) with the capacity to sinter together when transforming into a-tungsten. Further, it is a fact that the inclusions found in the metal grains are very similar in size to the small spherical particles seen, for example, in Fig. 8( 0). The NS effect of the dopant can be achieved only when a certain amount of potassium-containing phase is incorporated into the metal and is optimally dispersed. Therefore, every sieving, milling or grinding operation made before or during reduction has an influence on the NS quality, as characterized, for example, by recrystallization temperature and crystalline texture thereafter (see Fig. 9). It was found, however: that the possibility of the formation of potassium-tungsten oxide bronzes hinders the uniform distribution of potassium because of the great thermodynamical stability of bronzes with high potassium contents (K,WO, with x = 05 or O-33). This can be proved by a spectacular test: a pressed bar of doped, unwashed NS metal powder heated to 750°C for 60 min in hydrogen with a dew point of 68°C shows spots consisting of potassium oxide bronze. (This experiment is of course only for the sake of demonstration since the effective potassium compound is in the industrial process already enclosed in the reduced metal powder particles and, thus, does not get out to the surface.) The experiment can also be done with intermediate brown oxide as well. Also, in this case coloured spots of c. 50-70 ,um $ will appear on the surface of a pressed specimen. These spots consist in both cases (in metal powder and in brown oxide, respectively) of K,.,,WOR or K,.,W03. This means that a c. 30-50-fold agglomeration of potassium occurred inhomogeneously, because if all the initial dopant were present in the form of evenly distributed KTB that would correspond to the formula Ko.oIWO,. Therefore, the intensive mixing, even a simple sieving after the earlier first part of the two-step reduction, had a beneficial effect. This is actual practical experience and is still true even if we were to suppose that the oxide bronzes are not the doping phases which are incorporated directly into the growing metal powder particles. The potassium and aluminum forming the necessary silicate phases are supplied namely to a great extent by the oxide bronzes

Aspects of flective doping and the incorporation of dopant

Fig. 9.

Surface crystals on various doped tungsten wires of O-60 mm 4, recrystallized reagent (see Table 9).

17

at 2400°C and etched with Murakami

18

J. Neugebauer, L. Bartha

Fig. 9. - contd.

Aspects of eflectivedoping and the incorporation of dopant

when they are no longer stable and release the bronze-building components. It is, therefore, interesting to know how we can suppress the formation of large oxide bronze crystals (see Fig. 8(u)). Such large crystals are rarely seen in the plant material but very often in experimental reductions from an oxide doped with potassium alone. In an industrial reduction the bronzes can often be detected only by X-ray phase analysis (based on lines d= 2.979 A (tetragonal bronze) and d = 2.296 A (hexagonal bronze)). Also, in the test mentioned above where the inhomogeneous bronzes are made visible on pressed samples of metal powder, large needles and columns are formed only when the dopant is potassium alone. In the case of AKS doping, smaller, stubby crystals are always formed. This means that in NS-doped oxides something, apparently Si and Al, hinders the formation of K,WO, oxide bronzes with high x values being responsible for inhomogeneity. These bronzes exist namely with variable x values: the reddish bronze with x= 0.4-0.5 and the blue one with n= 0.22-0.33. With smaller values of x the colour tends to be blue and the shape is more compact and shorter. The decreasing x value means, in a chemical sense, decreasing alkalinity, so it is understandable that the acidic silica and alumina react in this direction. This can be confirmed by experiments. By doping an oxide (containing potassium in an amount corresponding to &.,WO,, 10 X the usual AKS concentration) additionally with AlCl, or silica or H,PO,, we can completely suppress the formation of the reddish bronze and in some cases the formation of the blue bronze as well. Silica-alumina-phosphoric acid are, in this order, increasingly effective. &.,WO,, the reddish bronze, requires nucleation, whereas (semiconducting) bronzes with low values of x do not. Therefore the effect of suppressing the formation of bronzes with higher x values is also a consequence of suppressing the nucleation. As we will see below this is a very pronounced effect of phosphorus. Therefore we will devote several pages to the effect of phosphoric acid in this paper even though it is not used as a dopant. In the next part of this paper we will consider the following items: -

the role of nitrogen (NH,) because APT is a starting material;

-

-

-

6

19

the role of phosphorus (H,PO,) because of its interesting effect on the nucleation of WO,; the role of H,O because of CVT by gaseous WO,(OH), and of MO, As influencing this CVT, the role of different additives on the formation of /I-tungsten and of oxide bronzes, because of many disputed questions about them.

THE ROLE OF NITROGEN (NH,) IN THE PRODUCTION OF NS METAL FROM APT

The thermal decomposition and the reduction of APT go through intermediate products to the ATB. When reduction is followed by NH,, a /3tungsten nitride is first formed after oxygen has been removed. This is reduced to W metal at c. 900°C. We found that /?-tungsten reacts with NH, at as low as 400°C and forms a WN phase but with a different crystal lattice than that known for the 6- and B-nitrides (Fig. 10). NH, is present in the reducing atmosphere of the reduction because the decomposed APT always contains NH, which is removed only gradually. Therefore, we investigated what happens when the hydrogen in the reduction process contained 0*02-l% NH,. We found that even 0.02% NH, in H, is enough to form tungsten oxide-nitride beneath WO,, consequently severely disturbing the CVT by WO,(OH), as well as the formation of /3- and atungsten particles. It can be observed in the reduction, for example of an oxide doped with K and Si,

n-l

“2

NH3

c

Fig. 10. Formation of @ungsten in hydrogen and of WN from &tungsten in NH,.

20

J. Neugebauer, L. Bartha

that the cauliflower-like WO, formations have two concentric boundaries. As seen with an optical microscope, within the inner boundary the spot has a silvery colour and only the outer ring has the usual brownish gold WO, colour. Depending on the concentration of NH, in H, the inner part can have a dendritic structure or one of small/tiny rods in a dendritic arrangement. The oxide-nitride may cover the entire surface of WO,, in which case the intermediate oxide resembles a metal. The consequence is that at values of 7‘ and p(H,O) in hydrogen which would otherwise allow the formation of individual metal crystals, this does not occur if NH, is present. This is due to the said formation of oxide-nitride and nitride. The volatility of WO, in a non-reducing gas (such as oxygen) is not influenced when 1% NH, is present. N, does not react with the phases occurring in the production of NS metal powder. It was, however, observed in some gas-filled lamps that a brown deposit on the bulb consisted of P-tungsten nitride. (In other cases a black deposit consisted partly of /?-tungsten and the nitride probably forms through this at the moment of deposition.)

7 THE ROLE OF PHOSPHORUS (AS H,PO,) IN THE REDUCTION OF WO, AND APT IN HYDROGEN It is known that in some cases the reduction does not go through WO, but instead P-tungsten is formed immediately. This is the case when the initial oxide is doped with > 10.5% potassium/ tungsten. We assumed that p-tungsten could in every case be a product of a reduction guided by a foreign substance, like the potassium, directing the way of reduction to the tungsten oxide bronzes. We found that doping with H,PO, is even more efficient for avoiding the WO, phase and going directly to P-tungsten (p-tungsten, discovered in 193 1, was prepared by electrolysis of a phosphate-melt). This was also an inspiration for further research because phosphorus, like arsenic, is an ever-present contaminant in the tungstic acids and oxides and in spite of its small amount may play a role in several problems which have been difficult to solve. (According to LEEDS measurements

phosphorus is often concentrated on the fracturesurfaces of tungsten wires.) It was found that with a dopant of O-5-1.4% P/W (as H,PO,) and p(H,O):p(H,)< 0.15 the reduction of WO, results first in the BO W,,05, and then proceeds directly to /I-W, which is stable until c. 8 10°C where it is transformed into a-W. This is a relatively easy way of preparing P-W with a particle size of l-5-2 pm. When the reduction followed in hydrogen with p(H,O):p(H,) = 0.15-0.65 (dew point 32-75°C) the observed phases were: from WO, without H3P0, doped with H,PO, from ammonium-tungstenoxide bronze without dopant doped with H,PO,

W,,O,, WO, W,“O,,, and P-W

and WO,

W, 8O49and W02 hexagonal oxide bronze, W2005x, WO, and P-W

In these cases WO, appeared, but only after the temperature exceeded 700°C. According to our interpretation,’ in a heterogeneous reaction with hydrogen WO, cannot be formed below 700°C. At such low temperatures the WO, can only be formed in a solid-state reaction from W20058 + /3-W. This is, however, hindered by phosphorus and can occur only at higher temperatures. The form of the phases is characteristic: W,,O,, always forms needle-shaped crystals of pale violet colour. When formed from ATB the crystals of this phase are transformed to an maintaining the agglomeration of needles, pseudomorphic hexagonal form of the ATB as an outer shape. WO? always grows from a nucleus, forming seemingly amorphous clumps or starlike clumps consisting of small pyramidal crystals with outside apexes. When formed from ATB without P, the irregular golden-brown spots stand out from the pseudomorphic surfaces only when the reaction temperature is high. From ATB doped with P the WO, crystallites never grow in the pseudomorphic tabular blocks, but always as separate crystals on top of the surface. W,,O, particles are < 0.5 pm. From ATB the product is pseudomorphic, from WO, separate particles are formed. Under the optical micro-

21

Aspects of flective doping and the incorporation of dopant Table 5. Evaluation of X-ray diagrams of the hexagonal ammonium-tungsten-oxide bronze, prepared by different methods (1) Prepared by decomposition of APT in H, at 400°C

(2) Prepared from I doped withH,PO, by reduction in H, at 750°C

(3) Prepared by heating APT in NH, at 560°C

d (A)

Lll

d (A)

U

d (A,

Ul

hkl

6.36 3.77 3.684 3.162 2.632 2.431 2.296 lw32 1.843

53 64 40 100 18 60 8

6.4 3.77

<5 100

3.157 2.631 2.421 2.303 lG386

30 17 25 <5 25

6.348 3.798 3.599 3.175 2.640 2.438 1.899 1.835

36 58 16 100 6 45 6 16

100 002 110 200 112 202 211 004 220

a = 7.36 A c=7.53A

:4

a = 7,40 A c= 7.544 A

scope their colour appears partly deep blue and partly grayish violet, but this is only an optical effect since by X-ray analysis only the known W,,O,, can be detected. ATB’s tabular crystals change their colour towards violet, keeping their form while altering the crystal structure slightly (see Table 5). /3-W cubes can reach a size of 9 ,um; the surfaces are often concave, holes are also observed. Besides the cubes tetrakishexahedrons and rhombdodekahedrons also occur. From these observations it can be confirmed that WO, does not appear below 700°C. It is assumed that above that temperature the volatility of the oxides (in the form of WOJOH,) makes a homogeneous gas reaction possible. The ATB is stable with P up to c. 780°C with a slight change in the structure (Table 5). It was, however, not possible to produce oxide bronze from WO, doped with H,PO,. Thus, we cannot state that a P-W-oxide bronze exists. The same applies to B-W prepared from WO, doped with H3P0,. There is no proof that it contains P in its structure. When it is grown on the surface of the oxide bronze crystals, sometimes the (100) or (110) direction of its cubic crystallites coincide with an edge of the hexagonal oxide bronze crystal, but a consequent epitaxy was not observed. In the samples doped with H,PO,, W,,O,, never appeared. This is because we did not use sufficiently high p(H,O)s as would be necessary to reach the equilibrium range for the violet oxide. Thus, the violet oxide also cannot be formed in

a = 7.33 A c=7+60A

the solid state as an intermediate oxide when P is present. The problem remains as to why the /l-W can be formed as separate crystallites at temperatures where a-W would be the stable phase. This indicates that P plays a role either in the structure of /3-W or in the gas phase. P in a dopant has an effect on the surrounding undoped particles in a mixture, but it is a very short-range effect.

8 THE FORMATION OF GASEOUS HYDROXIDES FROM TUNGSTEN OXIDES AND THE EFFECTS OF As AND OF MO ON THIS TYPE OF CVT Tungsten and molybdenum oxides exhibit a surprising increase in volatility in the presence of water vapour. 21 It was found22 that the volatile compounds are WO,( OH), and MoO,(OH),, respectively, and exist only in the gas phase. The volatility is very evident from 1000°C upwards but even at 800°C considerable CVT activity can be observed which is an important factor in the grain growth of tungsten metal particles. In H,-H,O mixtures which are in equilibrium with the intermediate oxides, for instance with WO,, they all exhibit this volatility. (According to Glemser and Haeseler22 this is only true for W02 if it first transforms to WO, .) In the case of molybdenum oxides we never found this type of volatility of MOO, and even low concentrations of MO in WO, decrease the CVT.

22

.I. Neugebauer. L. Bartha

(@5% MO in WO, decreases the g h- ’ evaporation rate by as much as 20%.) Arsenic in the WO, also reduces the grain growth in HZ-H20 because the catalytic effect of As is enhanced in the presence of H,O. Thus, arsenic makes it appear as if the concentration of HZ0 is lower. In reductions where small grain size is the goal and H,O, developed during reduction. cannot be avoided, As is a useful agent. Otherwise it can result in heavy failures in the production of NS metal powder. Sb has a similar effect. Both As and Sb catalysts were used for some time in the industrial production of NS powder in small amounts (e.g. 50 ppm). This provided the possibility of counteracting a variable contamination coming from the ore by variable arsenic doping according to the analytical data. It is interesting to take note of the fact that the volatile compound WO,(OH), is quite analogous to the oxyhalides used -in halide lamps. From a theoretical point of view it would not be impossible to use H,O without halogens here, only the high temperature required for the bulb and the extremely narrow ranges of other parameters make the practical use doubtful. Concerning the mechanism of metal powder production, CVT has two more effects arising from the volatility of tungsten oxides. It increases the possibility of sintering together the small metal particles to form grains, thus enclosing dopant phases. On the other hand it was also observed that undoped WO, can collect a considerable amount of dopant by being reduced between boats filled with doped WO, (e.g. increasing Al from 10 ppm to 75 ppm). This can be demonstrated by putting a clean small porcelain boat into an empty boat during reduction in the plant. The small boat comes out black, covered with a tungsten deposit. This deposit always has the composition of the compound that is stable at the partial pressure of oxygen at the given place. As already mentioned the deposit in commercial incandescent lamps is sometimes P-tungsten on

the bulb wall. The product of the ‘water vapour cycle’ in the general purpose lights is a deposit of tungsten on colder sites of the coils. Whether these processes require gaseous WO,(OH), or if they are also possible by the sublimation of the oxides is not entirely clear.

9 THE EFFECTS OF DIFFERENT ADDITIVES ON THE MORPHOLOGY OF WO, AND ON THE FORMATION OF OXIDE BRONZES AND B-TUNGSTEN The intermediate product during the reduction of NS-doped tungsten oxides (where the bulk is WO, but some lo-20% of the metal is already present) is very important from the point of view of the incorporation of dopants. This is the intermediate formed-as we have seen-when the temperature in the furnace is 680-750°C depending on other parameters. With the technology of earlier years this was the case at the end of the first part of a two-step reduction. Using this possibility we put small boats of quartz embedded in undoped blue oxide (ATB) in the row of regular boats in a plant reduction. The experimental samples of 5 g in the small boats were doped as shown in Tables 6 and 7. A number in front of the symbol means that a multiple of the above amount was added (e.g. 5KCl means 5 x 0.15 = 0.75% KCl). The experiment is, of course, not free from the error caused by small amounts of contamination from the NS-doped boats in the plant reduction (by CVT as said above). However, due to the low temperature of 700°C this carryover is not significant. The contents of the small boats coming out of the furnace were brown in colour, and in some boats were covered with a thin gray layer. Besides the observations made with the optical microscope, Table 7 shows the X-ray data which, how-

Table 6. Data on doping for Table 7 and for Fig. 1 I The chemical NaCl NaOH KC1 KOH Rb

symbols represent the following Al @085”/0 Fe 0*06%> 0.15% Ga 0~125% TI KSi 0.25%, RbCl NaSi KSiAl

amounts: AICI: equivalent with 0425% of A120, FeCI, equivalent with 0315% of Fe,O, Ga/NO,/, equivalent with 0.025% of GaZO, equivalent with 0.05% of TICI, 0.135% KOH + 0.20% SiO, 0+)6% NaOH + 0.20% SiOz 0.135% KOH + 0.20% SiO, + 0.025% A&O,

23

Aspects of effectivedoping and the incorporation of dopant TaMe 7. Oxide bronzes and other phases observed after reduction of various doped oxides reduced simultaneously

Doping agent

No.

4 2 7 8 9 10 11 12 13 14 15 16 17 18 19

22 23 24 25 26 27

1 KC1 2 KC1 3 KC1 5 KC1 5 KClHCl 1 KOH 2 KOH 3 KOH 5 KOH 5 KOHHCl 1KSi 2 KSi 3 KSi 5 KSi 5 KSiHCl 1 KSiAl 2 KSiAl 3 KSiAl 5 KSiAl APT undoped WO,H,O undoped 2 NaSiTlGa 2 NaSiRb 2 NaSiTl 2 NaSi 2 KSi (s&o-tungstate) 2 KSi + HCl

Oxide bronze

wt

B-W a-W According X-ray analysis

SEM picture Fig. 11

f S m t m med 1 1 med f S 1 med 1 med med med med 1 very f S med med med med f med also bulk f f f med 1 med med I f S very f very f

Violet, few Violet Violet, few Violet, many Violet, very few Violet, few Violet, very few Violet, few Violet, very few -

10 10

f med very f

Golden and blue, few Many golden needles Many golden needles, greenish-blue grains Lie 23 Violet, very few

11 13 10

14 8

4 10

5 19

WT

9

21

Y

very

f

med 1 S

f

S S

f

med

Violet, very few

9 9 8 6 ; 8 7 7 7 9 11 10 11

2; 27 ;:: 12 30 26 :: :: 24 22

-

Fi :

e f g h i j

k 1 m,n,o P q

:: 11 0 0

r,s t U

V

tin the column ‘w’ the first letters denote the relative amount (few, medium, many), the second ones the size (small, medium, large).

ever, refers only to a- and /3-W because for the bronzes present in amounts of only O-4-15% the data are not accurate enough. The form in which W02 appears is very characteristic both for the dopant and for the reduction parameters. In the normal production WO, forms, as a rule cauliflower-like agglomerates of various sizes. Figure 1 l(i) shows the most frequently seen arrangement on a pseudomorphous crystal surface of about lo-30 pm. In undoped samples they are small (see Fig. 11(p)), while sometimes the entire surface is covered by one formation with or without cracks. The results may be summarized as follows:

(1) With

only KC1 and at concentrations below 0.75% the violet bronze does not appear. (2) With KOH alone this bronze is formed above 0.3%. (3) With potassium silicate alone, slightly less KTB is formed while with the addition of

(4)

(5)

(6)

(7) (8)

(9)

HCl at the doping step, definitely less KTB is formed. With potassium silicate + aluminum chloride, less KTB is formed than with K-silicate alone. Al-chloride thus suppresses the building of bronze, as does gallium nitrate. Sodium silicate as the dopant without potassium results in bronzes of dull yellow, turquoise blue and dull blue colour. A dopant used several decades ago composed of NaCl, K-silicate, Al-chloride and Fe-chloride (AKSNF) is very favorable for the formation of the violet tetragonal KTB. The effect of potassium silicotungstate is about the same as that of potassium silicate. Repeated reduction decreases the amount of the violet bronze in the case of AKS doping. In the cases of Table 7 for TlCI, RbCl and Be-nitrate used in addition to Na silicate had no effect on the formation of bronzes.

24

J. Neugebauer, L. Bartha

Fig. 11.

SEM pictures of the products resulting with the dopants mentioned

in Tables 6 and 7.

Aspects of flective doping and the incorporation of dopant

Fig. 11. - coned.

25

26

J. Neugebauer, L. Bartha

Fig. 11. - contd.

Aspects of flective doping and the incorporation of dopant

Fig. Il. - coned.

27

28

J. Neugebauer, L. Bartha

Fig. 11.

10

TESTING OF NS TUNGSTEN POWDER ‘IN ITSELF’

METAL

It was always a major problem to test the metal powder without time-consuming pilot-plant tests (‘lot tests’). The metal powder is the key to the good NS quality of the resulting wires and filaments, but it is difficult to test the metal powder itself for these properties. In earlier years the sintering test was used with small (70 g) bars by measuring electrical parameters during the sintering. It was thought that the doping substances manifested themselves by higher internal stresses and thus by higher electrical resistances. It turned out however, that the higher resistance (caused mainly by potassium-containing dopants) was due to the micropores which were still not known at that time. (See Ref. 1, p. 26.) The dopants as chemical substances were tested by the ‘residue on chlorination’ method, by evaporating the tungsten as oxychloride. It was possible with careful handling to make an SEM picture first of the original powder grain, and then after the chlorination test of the residue from the same grain (Fig. 12).25 The composition of such residues was always SiO, and KC1 ( + NaCl if the dopant contained sodium compounds) because of the chlorination. As can be seen from Fig. 12 the residue from one grain consists often of several parts. The determination of the potassium contents in metal powders after washing in hydrofluoric acid

- contd.

is now a common method and gives a good characteristic value for the potassium incorporation, but no information about the local distribution. Therefore, the evaluation of the inclusions measured on polished cross-sections of embedded powder samples is also necessary. Some information can be gained by chemically etching the metal powder with Murakami reagent. Powder grains containing only one particle exhibit an octahedral form, while grains containing several single particles exhibited more rounded forms. The octahedrons very seldom have holes while the rounded ones often do. The best etching solution for opening these holes and thus making them visible is 4% H,02 applied at 50°C. The metal powder may contain undoped grains or, in rare cases, B-tungsten. Undoped metal grains are usually regular in shape and do not exhibit holes even after etching. The particles of p-tungsten are cubic. When they are very small, then we see hexagonal agglomerates in the metal powder containing submicron particles of what was originally #?-W but was transformed to a-W (Fig. 13). As already mentioned, sometimes it is necessary to test the NS quality of the powder in pilotplant lot tests by testing the sintered ingots or wires made from said metal powder. Figure 9 shows crystals of various doped experimental tungsten wires to show how sensitive the crystalline texture is to the chemical nature of dopant.

Aspects of effectivedoping and the incorporation of dopant

Fig. 12.

29

SEM pictures of metal grains before, (a) and (c), and after, (b) and (d), chlorination.

11 SPECIFIC CHEMICAL EFFECTS INFLUENCING THE PERFORMANCE AND USE OF NS TUNGSTEN PRODUCTS In the Metallurgy of Doped/Non-Sag

Tungsten’ the

last chapter deals with the chemical behavior of tungsten. Here we will further discuss the chemical effects which are characteristic first of all or exclusively for NS tungsten.

The chemical factors influencing the metal powder and the intermediate phases during reduction have already been mentioned. The metal powder is sensitive to carbon and to oxygen. Contamination by carbon may arise, for example, from newly painted pipes or new rubber pipes. Even a very thin layer of carbide can prevent the onset of sintering. Carbide can be detected by applying Murakarni etching reagent which causes

J. Neugebauer, L. Bartha

Fig. 13. Agglomeration of very sn j metal particles (originally /?-W particles) which have retained the pseudomorphic ATB form.

the evolution of bubbles of methane gas in the presence of carbide. Regarding the oxidation of the finished metal powder, the temperature range in the cooling zone in the reduction furnace of between 500 and 650°C is critical, if the partial pressure of oxygen (water vapour) is not very low. Even more sensitive is tungsten powder produced from KTB which is therefore mostly a faint brassy colour. Washed metal powders are less sensitive. In tungsten wires and filaments, carbon and oxygen are ‘rival’ contaminants. When dissolved carbon or traces of carbide prevail, the wire or filament is less sensitive to water vapour or oxygen in the hydrogen during annealing or in the filler gases of bulbs. Therefore the ‘tolerance’ limits of different wire charges are not the same

Table 8. Phases during the first reduction step (see Fig. 7) as observed microscope. The forms in the column on the right are not usual ones ATB

1 Blue-violetish pseudomorphous with APT (B)

W,*%

B-W a-W

K,WO,

6 Caulinower-like agglomerates (C) of max.l5pm+inblueB 7 Greater C in violetish B 8 Cs in golden-brown B 10 B is filled with golden spots of 1 flrn $ 11 B is homogeneously transformed into golden W02 13 Dark streaks around the Cs consisting of particles of < 100 nm 4 15 Metal particles of c. 1 pm between Cs and in the middle of C, also above the B 17 B is filled with metal particles 18 Separate metal particles 19 Stubby blue or small reddish crystals

W2N

SiO, Silicates

2 New, stubby, dark blue crystals 3 Blue needles 4 Short needles in ‘B 5 Separate longer needles

W20%

wo2

by optical

24 Big flakes 25 Solidified melt of amoeba-like spherical form size of 0.22-0.5

particles and of of the iurn

9 Concentric double Cs in in golden B

12 As 11 but granular

14 Dark areas of needle-like pattern between Cs 16 Steel gray shiny domains iIlB

20 Reddish columnar crystals 2 1 Steel gray shiny domain on Cs 22 Gray dendritic domains on Cs 23 The same, broken up into rodlets

31

Aspects of gective doping and the incorpomtion of dopant Table 9. Doping plan for the samples in Fig. 9 Fig. 9(a)

1 : 4

Fig. 9(b)

2 7 1 2 3 4

Fig. 9(c)

5 6 1

Fig. 9(d)

1

Fig. 9(e)

: 4 5 6 7 1 2 3 4 5

Fig. 9(f)

Fig. 9(g)

6 1 2 3 4 5 6 7 1 2 : 5 6

AKSNF+ NH,F AK NF (without Si) AKSNF metal powder mixed with K, Al metal powder 1: 4 AKS + Fe basic material APT AKSNF basic material APT Na, K, Si metal powder washed with HF AKSNF and K, Al brown oxides mixed 1: 9 AKSNF metal powder and K, Al metal powders mixed 1: 1 K, Al brown oxide sieved intensively AKSNF metal powder washed with HF AKSNF metal powder mixed with K, Al metal powder AKS but 650 ppm K, 350 ppm Si Undoped, from very pure sublimed WO, AKSNF but 300 ppm Na, 500 ppm Si, 60 uom Fe K, ki with longer sintering schedule AKSNF K alone K, Al AKSNF with longer sintering schedule AKSNF from contaminated WO,H,O _ _ (burst on sintering) 170 ppm Al, 3000 ppm K, 2500 ppm Si, 3000 ppm Na Na alone K, Al like1 Na, K, Si K, Al K, Al K, Al; the brown oxide intensively sieved AKS but only 500 ppm Si + 300 ppm Na + 60 ppm Fe As 2, but sintering at higher T (temperature) AKSNa Na, K, Si; the metal powder was washed with H,O K alone K, Si K, Si and the metal powder doped with Al Fe alone K, Al, starting material extremely pure (Na-free) WO,HrO AKSNF AKS Commercial AKSNF wire Commercial Na, K, Si doped wire Without dope from extremely pure sublimed WC3 Al, K, Na, Fe AKSNF+ 0.5% MO From undoped commercial WO,H,O Si only (added as silicotungstic acid)

The starting material was in all cases tungstic acid W0,H20 unless noted otherwise. ‘AKS’means a standard doping with 250 ppm Al, 2500 ppm K and 2500 ppm Si. ‘AKSNF’ means AKS + 300 ppm Fe + 1500 ppm Na. Single symbols, e.g. ‘K’, mean that dopant alone, e.g. 2500 ppm K. The metal powders were not washed unless so noted.

with respect to the inevitable contaminants in the bulbs. There is an effect of oxygen during annealing at 1300- 16Oo”C, which does not become apparent until after the second recrystallization (e.g. at 2400°C). In these cases anomalous recrystallization occurs which destroys the NS property. Annealing of filaments in alumina-tube furnaces can cause damage if volatile SiO can be formed from Si contaminants in the tube. During the many working operations the various pieces of equipment used can be sources of contaminants. These local impurities are very difficult to detect. The global level of impurities is more convenient to measure. Local impurities can sometimes be very successfully detected based on the electrode potential when the sample is used as an electrode against a pure tungsten electrode. Table 5 gives qualitative information about the electrode potentials of different metals using a simple galvanometer. By using different electrolytes the single metals can also be identified (see more precise measurements made by Kiss). The most detrimental spot impurities are Ni, Cr, Fe and Al when they are alloyed during a subsequent annealing without first being dissolved, for example, by HCl. They always cause local destruction of the NS property and brittleness. There is also one other very interesting effect of impurities and even of doping phases which occurs the first time a light with an NS tungsten filament is switched on. The avoidance of sagging is the main goal of NS tungsten but some sagging

Sag @IS,

I

f

100

200

Fig. 14.

I

300

) Voltage

Saggingwhen first switched on.

[V]

32

.I.Neugebauer, L. Bartha

is of course inevitable. When the movement of the (coiled) filament is represented as a function of temperature, for a gradual switching on we get curves like in Fig. 14. Much larger deviations follow when the filament is in a gas atmosphere containing, for example, ethanol, CCL,, or similar. In these cases very big holes are formed between the anomalous crystal&es in the wire, which can produce reversible movements of the coil by cycling the temperature. These phenomena are in connection with oxidation-reduction or other chemical reactions inside the metal and/or inside the micropores.23,24 This can serve as a good model for demonstrating how the said ‘tolerance’ is influenced by: (a) the dispersion of the micropore system; (b) the doping phases (i.e. the NS quality of the metal); (c) the environment during heating.

11. Gahn, A., Importance

12.

13.

14.

15.

16.

17.

18.

REFERENCES 19. 1. Neugebauer, J., The technology of efficient doping and sintering. Yamazaki & Shu, Controlling the doping effect in the tungsten manufacturing process. Lassner, E., Schubert, W. D., Haubner, R. & Lux, B., Mechanisms of tungsten powder reduction. In The MetallurgyofDoped/ Non-Sag Tungsten, ed. E. Pink & L. Bartha. Elsevier, London, 1989, pp. 15-30,47-59, 119-40, respectively. 2. Walter, J. L. & Lou, K. A., J. Mater. Sci., 24 (1989) 3577. 3. van Put, J. W. & Zegers, T. W., Hydrogen reduction of ammonium paratungstate into tungsten blue oxide-Part I: Literature review. lnr. J. Refr. Metals & Hard Mater., 10 (1991) 115-22. 4. Lunk, H.-J. & Schade, P., Zum Verhalten der Dopingelementverbindungen bei der Reduktion von Wolframoxiden. Neue Hiitte, 27 (10) (1982) 382-6. 5. Neugebauer, J., Oxidations- und Reduktionsvorgtige im System K-W-O. Planseeber. jiir Pulvermetallurgie, 23 (1975) 77-85. 6. Hegediis, A. J., Millner, T., Neugebauer, J. & Sasviri, K., Thermo- und riintgenanalytischer Beitrag zur Reduktion des wolframoxyds. Z. Anorg. A& Chernie, 281 (l-2)(1955)63-82. 7. Millner, T., Hegediis, A. J., Sasv&i, K. & Neugebauer, J., Weiterer Beitrag zur Reduktion des Solframoxyds. Z. Anorg. Allg. Chemie, 289 (S-6) (1957) 287-312. 8. Neugebauer, J., Hegediis, A. J. & Millner, T., Zum problem des p-Wolframs. Z. Anorg. Allg. Chernie, 293 (5-6)( 1958) 241-50. 9. ZeiIer, B., Schubert, W. D. & Lux, B., On the reduction of NS-doped tungsten blue oxide. 13th Plansee Seminar, RM6(1993). zur 10. Gahn, A., ‘Bubbles’ Chemische Untersuchungen Herstellung kaliumdotierter Wolframpulver fiir die Gliihlampenproduktion. Dissertation. Fakultlt Chernie Ludwig-Maximilians-Universittit, Pharmazie, und Miinchen, 1986.

20. 21.

22.

23. 24.

25.

26.

27.

28. 29.

30.

31.

of B-tungsten for the production of potassium-doped tungsten powder. Proc. 5th Znt. Tungsten Symp., Budapest, 150-6 (1990). Neugebauer, J., Bartha, L., M&z&OS, I. & MtszBros, M., Separation of dopant compounds from NS tungsten metal powder. About the origin of dopant inclusions. PlanseeProc., 1(1993)312-20. C&k& J., Tbth, Cs. L., MCsziros, I., Neugebauer, J. & Horacsek, O., Powdermetallurgical problems in the manufacturing of lampwires. Proc. 5th Int. Tungsten Symp., Budapest, 132-9 (1990). Neugebauer, J., Hegediis, A. J. & MiIIner, T., iiber die Red&ion des Ammoniumwolframates und Wolframtrioxyds mittels Ammo&k, Beitrag zur Kenntnis des Systems W-N. Z. Anorg. Allg. Chemie, 302 (l-2) (1959) 49-59. Cheng, K. K., Jacobson, A. J. & Whittingham, M. S., Hexagonal tungsten trioxide and its intercalation chemistry. SolidStatelonics, 5 (1981) 355-8. Gerand, B., Nowogrocki, G., Guenot, J. & Figlarz, M., Structural study of a new hexagonal form of tungsten trioxide. J. Solid State Chem., 29 ( 1979) 429-34. Hegediis, A. J., Sasvbri, K. & Neugebauer, J., ijber die thermo- und rijntgenanalytische Untersuchung der Reaktion von Wolframtrioxid und Kohlenoxid. Acta Chim. Hung.,26(1-4)(1961) 113-27. Haubner, R., Schubert, W. D., Lassner, E. & Lux, B., EinfluB von Aluminium auf die Reduktion von Wolframoxid zu Wolfram. J. Ref Hard Metals, 6 (1987) 161-7. Millner, T., Ten years of research into tungsten at the Hungarian Academy of Sciences. Acta Technica Academiae Scientianrm Hungaricae, 70 (3-4) ( 197 1) 269-89. Dutkay, G., Dissertation, Budapest, TU ( 197 1). Millner, T. & Neugebauer, J., Volatility of the oxides of tungsten and molybdenum in the presence of water vapour. Nature, 163 (1949) 491-2. Glemser, 0. & Haeseler, R., Uber gasfiirmige Hydroxide des Molybdtis und Wolframs. Z. Anorg. Allg. Chemie, 316 (3-4)( 1962) 167-81. GaGl, I., Neugebauer, J. M. & Uray, L., Acta Techn. AC. SC.Hung., 78 (1974) 123. Ga$, I., Harmat, P., Kele, A., Lipt&, L., Major, J. & Uray, L., Conf on Less-Common Metals (VIII. Ritkaf&m Konf.) Budapest, Vol. 3, 1982, pp. 621-36. Neugebauer, J., Geszti, T. nC & Marczink6, M., K&Al adalekok elhelyezkedese egyedi volfram fkmpor szemcskkben. M TA MFKI Kiizlemknyei, O-l 5 (Publications of MTA MFKI Budapest) (1975) 9-22, Freedman, M. L. & Leber, S., Identification of tungstic and molybdic ‘acids’ by X-ray powder diffraction. J. Less-Common Metakr,7 (1964) 427-32. Kiss, B. A., Amm&ium-volfrtibronz porkohtiszati alapanyag reduktiv termikus born&a. Magyar Kimiai Foly&rut,93(3)(1987)97-106. Deschanvres, A. et al., Bull. Sot. Chemi. Fr. ( 1967) 4537; (1968) 3519. Reau, J.-M., Fouassier, C., Le Flem, G., Barraud, J. Y., Doumerc, J.-P. & Hagenmuller, P., Rev. Chim. Miner., 7 (1970) 975. Neugebauer, J., A new intermediate phase in the reduction of potassium tungstates. Acta Techn. Acad. SC. Hung., 78 (1974) 267-78. Ebert, F. & Flasch, H., Z. Anorg. Allg. Chemie, 217,95; 226,65.

33

Aspects of Gective doping and the incorporation of dopant

APPENDIX Table Al. Electrode potential of W againstmetals in different electrolytes Metal

10% NH,OH

2% HCI -4

Ml? Al Fe Ni

-3 -2 -1 -10 0 +2 +1 0 0 +1

z! MO Ta W-ReS% Graphite

5% FeCI,

1:

1:

+2 +1 +3 +4 +3 +2 0 0 +2

-4

S%H,O,

K,Fe(CN),

-3 -2 +1 +1 +1 +2 +2 +2 0 +2 +1

1; -4 +1 -4 0 -2 -1

K,Fe(CN), + NaOH -3+3 -3 +4 +4 +4 +4 +4 -3 +3 +4 +4

-2 +2 1; +2 Not stable 0 -1 +3

Table A2. As in Table Al, quantitativemeasurementsusing calomel electrode Metal

Ge Go

In Sn Pb Mg Al Zn Bi Ti Cu Ni Fe C Ta Ag Au ; WRe5 W MO

KMnO,

(NHddSO&

-190 - 300 + 140 + 200 - 320 - 1250 - 580 - 730 - 120 +110 +180 + 290 + 150 + 340 +40 +180 + 400 + 350 + 490 +50 +50 -120

+ 230 - 280 - 690 - 550 - 530 - 2050 - 460 - 1050 +50 - 150 - 150 -170 - 570 + 320 -100 -50 + 390 +410 + 420 +70 -50

-8:

+40

FeC1,

K,[Fe(CN),J

20% Potassium metatungstate

- 280 - 290 - 160 - 520 - 550 -420 - 780 - 1130 -70 -60 -170 - 200 - 580 +80 +70 - 140 + 220 + 220 + 200 + 140 +170

+ 130 - 140 + 140 +60 - 260 - 1470 - 300 - 230 + 170 + 260 +70 + 270 +310 + 290 + 280 +120 + 330 + 330 + 390 + 280 + 270

- 440 - 680 -620 -430 -610 - 1350 - 990 - 1130 -110 +60 -170 - 230 - 540 +180 +50 f0 + 260 +210 +30 -150 - 160

+ 230

- 140

+40

H,O

%$KMnO, HCl(Oln)

(NHJ,SiO, HCl(Oln)

- 390 - 440 - 320 -50 - 280 - 1430 - 400 - 550 +0 + 200 + 350 -250 +50 + 150 + 150 +190 + 340 + 330 + 290 +90 +60

+100 - 660 - 580 - 240 -350 - 1540 - 300 - 1000 +120 + 780 f0 - 100 - 540 + 970 + 630 + 100 + 890 + 1010 +850 + 1000 + 1550

270 200 700 530 550 - 1650 - 800 - 800 -130 + 340 - 110 -180 - 550 + 360 +350 f0 + 790 + 880 + 380 + 220 + 240

+120

+ 1300

+ 120

-

FeCI, HCl(Oln) f0 - 690

- 1150 - 530 - 530 - 1350 - 1250 - 1300 - 140 + 320 - 140 -180 - 570 + 340 + 330 - 120 + 330 + 320 + 340 + 320 + 340 +150

34

J. Neugebauer, L. Bartha

Table A3. As in Table Al, quantitative measurements using calomel electrode Metal

Ge Ga In Sn Pb Mg Al Zn Bi Ti cu Ni Fe C Ta AS Au Pt w WRe, W MO

M. tungstate 10% citric acid -

260 800 720 500 580 - 2020 - 850 -1130 - 260 -2:: -280 - 540 +130 +120 -250 +210 + 100 +230 - 100 -70 f0

H@, HCl(O.ln)

KBrO_, HCl(O.ln)

- 220 -630 - 540 -150 - 440 - 1800 - 700 - 840 k0 + 280 + 200 -60 - 150 + 450 +350 +80 +450 + 500 + 460 + 230 + 330

+ 500 -50 - 620 - 360 - 490 -750 - 560 - 900 +0 + 700 -120 +30 + 120

+ 150

Kl HCl(O.ln)

HCI(O.ln)

HCI(O*ln)

HCI O*In

+850 + 400 f0 + 800 + 1030 + 580 + 500 + 540

270 700 700 540 550 - 1450 - 1300 - 1280 - 130 + 240 - 340 - 170 -510 + 210 + 140 - 320 + 170 +180 + 230 + 200 + 220

+50 + 140 - 700 - 460 - 500 - 1850 - 700 - 980 +0 + 780 -110 -150 - 530 + 800 + 740 -80 + 870 + 870 + 580 + 580 + 540

+170 - 390 - 600 - 230 - 380 +170 - 560 - 1700 + 130 + 320 - 100 + 420 - 400 + 630 + 300 +0 +640 + 680 + 560 + 360 +480

-430 - 830 - 730 - 550 - 520 - 2000 - 950 - 1090 - 160 - 440 - 140 - 350 - 560 +180 -410 +20 +610 + 100 + 300 - 490 - 100

+ 200

+ 50

+ 230

+ 300

-

Br, water

K&O,

HNO,

HW, O.ln 450 750 700 560 540 - 2070 - 580 - 1070 -70 - 460 -60 - 200 - 590 + 190 - 140 -50 + 600 + 100 + 390 -50 -120

-80

-

O.ln

-110

-420 -450 -450 - 480 - 520 - 1650 - 480 - 990 -80 - 320 -60 - 160 - 470 +190 - 440 +25 +410 +70 + 395 +50 - 120 -80

Table A4. As in Table Al, quantitative measurements using calomel electrode Metal

Ge Ga In Sn Pb Mg Al Zn Bi Ti cu Ni Fe

C Ta Ag Au EI WRe, W MO

HCIO, O.ln

HCOOH O.ln

(COOH)? O,fn

- 390 -630 - 650 - 520 - 490 - 1950 -650 - 1040 -70 - 300 -50 -150 - 530

430 790 670 540 500 - 2020 - 690 - 1070 - 100 -60 -50 - 170 -610

350 630 770 630 560 - 1940 -650 - 1080 - 150 - 130 - 120 - 220 - 580

+ 230 - 360 -50 + 380 - 250 +430 +30 -100 -60

+ 150 - 350 -60 + 330 +100 + 370 -50 -130 -110

+ 150 - 150 -90 + 230 +40 + 360 +0 -150 -100

-

-

Ascorbic acid O.ln

NaOH-0 O.In

NHJOH O.ln

Pyridine O.ln

Morpholine Oeln

- 490 -630 - 500 - 520 - 520 - 1950 - 700 - 1070 -120 f0 -70 - 200 changing - 100 -130 - 140 -70 + 100 +0

- 830 - 1540 - 820 - 160 - 790 -680 - 1550 - 890 - 570 - 680 - 360 - 500 - 620

-830 - 1350 - 900 - 880 -710 - 1300 - 1550 -750 - 480 - 470 - 330 - 420 - 470

- 740 - 730 - 730 - 730 -610 - 1590 - 1440 - 850 - 350 - 320 - 220 - 280 - 350

- 800 - 850 - 820 - 760 -660 - 1780 - 1020 - 700 -430 - 460 210 - 350 - 440

- 160 - 330 -180 - 120 -110 + 240 - 390 -430 - 400

-170 - 440 -210 -60 - 160 +120 - 480 - 500 - 530

_-$I: -110 -110

- 350 - 550 - 350 + 270 -310 -180 -650 690 - 660

-

220 480 240 240 300 180 560 580 570