Preparation of Pb(Mg13Nb23)O3 powder by molten salt method

Ceramics International 15 (1989) 289-295

Preparation of Pb(Mg /aNb2/3.)O 3 Powder by Molten Salt Method K. Katayama, M . Abe & T. Akiba Research & Development Division, Chichibu Cement Co. Ltd, Saitama, Japan (Received 1 September 1988; accepted 3 November 1988)

Abstract: The preparation of single-phase perovskite Pb(Mgl/3Nb2/3)O3 powder or ceramics has been known to be quite difficult by means of the conventional mixed oxide method. Its high dielectric constant, however, makes Pb(Mgl/3Nb2/3)O 3 one of the most promising perovskite materials. In the present paper, a conventional mixed oxide method was reinvestigated and non-conventional ceramic processing methods such as the PbO flux method and a KCI molten salt method have been performed to prepare single-phase Pb{Mgl/aNb2/3)O 3 powder. It was found that the KC1 molten salt method is extremely successful only when excess PbO is present in the salt, because pyrochlore Pb 1.83Nb1.71Mg0.2906.39, which inevitably appears upon synthesizing Pb(Mgl/3Nb2/3)O3, can be completely transformed to perovskite.

1 INTRODUCTION

confirmed by Goo et al. 9 To the best of our knowledge, there have been no other reports on the formation of single-phase PMN ceramics or powder. In the present study, a molten salt method using KC1 was used to prepare the single-phase PMN powder and its formation process was studied. In addition, a mixed oxide method and a PbO flux method were briefly investigated.

Lead magnesium niobate Pb(Mgl/3Nb2/3)O a (designated PMN hereafter) is one of the most widely investigated relaxor ferroelectrics because of its superior dielectric properties. 1 Since Smolenski and Agranoveskaya 2 synthesized PMN, many papers concerning its preparation 3 - a have been published. The preparation of P M N powder ceramics, however, has been known to be quite difficult by the mixed oxide method. Inada 7 has studied the formation kinetics of PMN and concluded that the repetition of long-time calcination and pulverization is inevitable and that much attention has to be paid to the vaporization of PbO. Lejeune and Boilot 5 have investigated various factors affecting the formation of P M N ceramics and they have failed to obtain single-phase PMN. Swartz and Shrout s have reported a novel procedure for preparing singlephase P M N ceramics but their procedure needs a slight excess of MgO to suppress the formation of pyrochlore phase Pbl.a3Nbl.71Mgo.2906.39 (designated pyro). The inclusion of MgO in P M N was

2 EXPERIMENTAL 2. 1 Powder synthesis The starting materials were commercially available 99.99% purity oxide powders (PbO, MgO, Nb2Os) and analytical grade KC1 was used for the molten salt method. Appropriate amounts of oxide powders were thoroughly mixed in ethanol in an alumina mortar and pestle, and dried at 120°C overnight. The notation PMN(n) that denotes the mixture powder of(100 + n/lO0) moles of PbO, one-third mole MgO and one-third mole N b 2 0 s will be used hereafter. That is to say, PMN(0) means the stoichiometric 289

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290

K. Katayama, M. Abe, T. Akiba

mixture of PMN composition and PMN(10) the mixture of 1.1 mol PbO, 1/3mol MgO and 1/3mol Nb2Os. The heat treatment was done in a covered alumina crucible and the rate of heating and cooling was 250°C/h. In the mixed oxide method, mixtures were heated at 900-1200°C for 1-5h and pulverized to pass through a 60-mesh screen. In the PbO flux method, 1° the mixtures such as PMN(50), PMN(100) and PMN(200) were heated at 1000-1200°C for 2 h. The products were leached in boiling nitric acid to remove excess PbO, filtered and washed in hot deionized water at least five times. In the KCI molten salt method,1 x three grams of PMN(n) with two grams of KC1 were mixed (designated PMN(n)+KC1). The mixtures were heated at 800-1200°C for several hours. The products were leached in boiling diluted nitric acid (50wt%) to remove KC1 and unreacted PbO simultaneously, filtered and washed in hot deionized water at least five times. 2.2 Powder characterization

The phases in the products were identified by XRD. The perovskite PMN was synthesized with inevitable appearance of pyrochlore pyro. la The relative amounts of PMN were calculated by measuring their highest peaks ((100) for PMN, (222) for pyro), a The morphology of the powder was examined by SEM and the particle size distribution was determined by a laser diffraction method with the dispersion of powders in ethanol by ultrasonic vibration. 3 RESULTS

AND

DISCUSSION

3. 1 Mixed oxide method

As pointed out in the experimental section, XRD revealed that products were composed of perovskite phase PMN with the pyrochlore phase, pyro. Pyro could not be eliminated at all even though various sintering conditions were employed. In Table 1, the relative amounts of PMN in the resulting powders prepared using a stoichiometric mixture of PMN(0) are represented. The amounts of PMN are higher than those reported by several workers 6,s and decrease with increasing heating temperatures or duration times. The decrease might be related to the vaporization of PbO, which is a major problem when lead-containing compounds such as PMN were synthesized by this method.

Table 1.

Relative amounts (%) of P M N by conventional mixed oxide method

Duration of heat treatment (h)

Temperature of heat treatment (°C) 900

1 000

1 100

1 200

85 89 72

88 91 83

86 90 74

73 71 48

1 2 5

In order to investigate the effect of vaporization of PbO on the formation of PMN, the PMN(n) (n -- 1, 2, 5, 10 and 20) powders were heat treated at 900-1100°C for 2 h. It was found that the excess PbO did not affect the formation of PMN positively, but rather the amounts of P M N decreased with increasing amounts of excess PbO, as shown in Table 2. The XRD peak height of each PMN decreases slightly with increasing excess amounts of PbO, while the peak height of pyro remains unchanged. The higher values in Table 1 were considered to be related to the reactivity of each oxide. A preliminary experiment showed that magnesium oxide was the most important oxide affecting the formation of PMN, as reported earlier. ~ In the following, the influence of the fineness of the MgO powder is briefly described. As-purchased MgO powder (designated PURE) was calcined in air for 10h at 800, 1000 and 1200°C. The powders prepared in this fashion were roughly pulverized in an agate mortar and pestle, and designated (800), (1000) and (1200), respectively. The particle sizes of all these powders are less than 0.1 #m and are almost the same. Agglomerate sizes are, however, somewhat different and these agglomerates are rigid but cannot be pulverized easily. Therefore, the agglomerate sizes were measured by a laser diffraction method and are reported in Table 3. Table 2. Effect of excess PbO on the formation of P M N (%). Duration of heat treatment 2h Temperature of heat treatment (°C)

Composition

PMN PMN PMN PMN PMN

(1) (2) (5) (10) (20)

900

1 000

1100

91 91 89 85 87

87 78 79 81 79

85 86 80 79 79

291

Preparation of Pb(Mgl/3Nb2/3)O 3 powder Table 3. Effect of MgO agglomerate size on the formation of P M N (%). Heat treatment: 1000°C, 2 h Temperature of calcination (oc) PMN (%) Average agglomerate size (/~m)

Aspurchased

91

2.35

800

83

2.72

1 000

1 200

80

78

3.30

4.20

The preparation of P M N was conducted using these four kinds of MgO powder in the same manner as described in the experimental section while the mixture composition was fixed to be stoichiometric and the sintering condition was 1000°C for 2 h. The amounts of P M N increase as the average agglomerate sizes decrease. This drastic increase leads to speculation that single-phase ceramics might be prepared even in a mixed oxide method if MgO powders, of which agglomerate sizes are less than 2/~m, were used. In any event, it is clear that the fineness of the MgO powders largely influences the formation of PMN. 3.2 PbO flux method Crystals of lead-containing ferroelectric relaxor materials have been commonly grown by the PbO flux m e t h o d and crystals of P M N were first synthesized by Myl'nikova and Bokov. 14 Gururaja et al.l° succeeded in preparing perovskite Pb(Znl/3Nb2/3)O3-PbTiO 3 powder in a similar method. The preparation of P M N powder was performed according to these methods. The products were tightly solidified and adhered to the alumina crucible interior and it was rather difficult to obtain the resulting powders by leaching, even in boiling nitric acid. The difficulty of leaching increases with the increase of excess PbO. The amounts of P M N in the resulting powders did not exceed 90% and were scarcely affected by amounts of excess PbO. 3.3 Molten salt method Perovskite materials such as Pb(Zr, Ti)O311,15 and BaTiO312 have been prepared by the molten salt method. Chlorides such as KC1, NaC1 and KC1-NaC1 have commonly been used as molten salts and in this study, KC1 was employed. Since the fineness of the MgO powder affects the

formation of P M N in the mixed oxide method, the effect of the fineness of MgO in the molten salt method was pre-examined using MgO powders such as P U R E and (1200). With these MgO powders, two kinds of PMN(0) mixture powders were prepared. Three grams of each PMN(0) with two grams of KC1 were mixed and heat-treated in a covered alumina crucible at 800-1100°C for 2 h. The relative amounts of P M N in the resulting powders were almost the same within experimental error in the case of the same heating temperatures. F r o m these results, the agglomerate size of MgO powders was found not to affect the formation of P M N in the molten salt method. Consequently, P U R E was used as the MgO source for further use of the molten salt method. The relative amounts of P M N prepared by the molten salt method are shown in Table 4. The values are considerably higher than those prepared by the former methods. In most cases, more than 95% of P M N was attained and the single-phase P M N powders were obtained in three conditions. These three kinds of P M N powders are exhibited in Fig. 1 and the particle size distributions are shown in Fig. 2. The average particle sizes calculated from the particle size distributions are 5.6 #m in (a), 6-5 #m in (b) and 7.8 pm in (c). With increasing heat treatment temperatures or the amounts of PbO, the particle sizes increase and the cubic structure of its inherent crystal habits becomes more remarkable, as is clearly shown in Fig. I. The heat treatment dependence on the formation of P M N powder was investigated using PMN(200) + KC1 and the results are shown in Table 5. When the heating temperature exceeds 800°C, the relative amounts of P M N increase abruptly. This is attributed to the melting of KC1, which will be discussed further in the following Table 4. Relative amounts of P M N (%) by molten salt method. Duration of heat treatment: 2 h Composition

PMN(O) + KCI PMN(1 ) + KCI PMN(2) + KCI PMN(5) + KCI PMN(IO) + KCI PMN(20) + KCI PMN(50) + KCI PMN(IO0) + KCI PMN(200) + KCI

Temperature of heat treatment (°C) 800

900

1 000

1100

94 94 93 91 93 94 94 94 96

96 96 95 94 94 96 98 97 100

98 97 96 94 97 98 94 100 100

98 97 97 95 96 96 89 94 83

K. Katayama, M. Abe, T. Akiba

292

(a)

(¢) Fig. 1. (c) PMN(200) + KC1 heated at 1000°C for 2 h.

Table 5. Heat treatment dependence on the formation of P M N powder (%) by the use of PMN(200) + KCI Temp. of heat treatment (°C)

Duration of heat treatment (h)

700 750 800 850 900

1

2

5

0 25 94 98 100

0 28 95 99 100

0 35 96 100 100

Table 6. Time dependence of the formation of P M N powder by use of P M N ( 2 0 0 ) + KCI. Temperature of heat treatment: 900°C

(b) Fig. 1. SEM photographs of PMN powders: (a) PMN(200) + KCI heated at 900°C for 2 h; (b) PMN(100) + KCI heated at 1000°C for 2 h.

section. The duration 1 h or longer rarely affects the amounts of P M N but the shorter duration appreciably affects the formation of PMN. Table 6 shows the relative amounts of P M N and the average particle sizes of P M N powders prepared at 900°C

Time

PMN (%)

Average particle size (/zm)

5 min 15 30 1h 2 5 10

53 97 99 100 100 100 100

---3,84 5,60 5.74 5.75

Preparation of Pb(Mg ~/aNb2/3)O3 powder o--o (a) ~--, (b) 30" ~.-----~(c)

293 using PMN(200)+ KCI. The amounts of P M N gradually increase with the increase of time and at least one-hour duration is necessary to obtain the single-phase P M N powder. The average particle size of P M N powder prepared by two-hour heat treatment is larger than that prepared by one-hour treatment and the longer duration also does not affect the average particle size, as well as the particle size distribution, as shown in Fig. 3. These observations lead to the conclusion that the particle sizes of single-phase P M N powders prepared by the KCI molten salt method are fairly strongly affected by treatment times up to 2 h.

.bt /"~'~ o/~/../~,,

I/I

li , ~

//

I'i

/ ,~

~,,~

Iii

?/Ili

10

/ l i C i t

0.5

2;0

1.0

5.0

10.0

20.0

3.4 Effect o f excess PbO and KCl salt on the formation o f P M N powder

50.0

P a r t i c L e size(/~m) Fig. 2. Particle size distribution of P M N powder: PMN(200) + KCI heat-treated (a) at 900°C for 2 h and (c) at 1000°C for 2h and PMN(100) (b) at 1000°C for 2h.

ao

o---o (a) ~---o (b)

/.~

,,--~ (c)

..,~7~,',

e-..-.--e (d)

13 ~

20

~',

f

10

0

0.5

1.0

2.0

5.0

10.0

20.0

P a r t i c L e size(/~m)

Fig. 3. Particlesizedistribution of PMN powders prepared by beat-treating PMN(200) + KCI at 900°Cfor (a) 1h, (b) 2 h, (c) 5 h and (d) 10h. Table 7.

As described in the previous section, the single-phase P M N powder can be prepared by the KC1 molten salt method only when there is excess PbO. In order to investigate the effect of excess PbO and KC1 salt on the formation of P M N powder, heat treatments were done at 750-900°C for 1 h using four kinds of mixture powders: PMN(0), PMN(200), PMN(0) + KC1 and PMN(200) + KCI, and the relative amounts of P M N in the resulting powders were calculated, as shown in Table 7. The values in the last column of this table are the same as those in Table 5. The effect of KC1 salt on the formation of P M N is clearly shown from the values prepared at 800°C. The values in the third and the last columns are much larger than those in the first and second columns. This is attributed to the melting of KC1 (772°C). Even in the case of 750°C heat treatment, the amounts of P M N are affected slightly by the presence of KC1. This might be caused by the lowering of the melting point of KCI, since other compounds coexist such as PbO, MgO and N b 2 0 5 in the KCI salt. The effect of excess PbO is recognized in the amounts of P M N prepared at 800°C. When there is 200mo1% excess PbO present in mixture powders, the amount of P M N increases from 3% to 20%. This also might

T e m p e r a t u r e dependence of excess PbO and KCI salt on t h e f o r m a t i o n of P M N (%). D u r a t i o n of heat t r e a t m e n t : 1 h

Temp. of heat treatment (°C)

PMN(0)

PMN(200)

PMN(0) + KCI

PMN(200) + KCI

750 800 900

0 3 85

1 20 88

17 94 95

25 94 100

K. Katayama, M. Abe, T. Akiba

294 T a b l e 8.

T i m e d e p e n d e n c e of excess P b O a n d KCI salt on t h e f o r m a t i o n T e m p e r a t u r e of h e a t t r e a t m e n t : 9 0 0 ° C

Duration

PMN(O)

2 min 5

15 30 1h 2h

PMN(200)

0 64

0 53

34 78 85 89

57 75 88 90

93 95 96 96

97 99

In a mixed oxide method, the single-phase P M N powder could not be prepared because pyro is more easily formed at lower temperatures and rather stable at higher temperatures. In the KC1 molten salt method, however, pyro completely disappears and the single-phase P M N powder can be synthesized if appropriate amounts of PbO exist in the salt and the heat treatment is done under certain conditions. In order to investigate the stability and the elimination of pyro, the following experiment was conducted.

T a b l e 9.

No.

T r a n s f o r m a t i o n of p y r o c h l o r e p e r o v s k i t e phase

1 2 3 4 5 6

2'0 2"0 2'0 2'0 2.0 2'0

-2"0 -2-0 -2"0

7

2-0

--

8

2'0

2'0

0"2 0"2 --0"2 0'2

phase to

100 100

Pyro powder was synthesized via a mixed oxide method according to Strout and Swartz. la The synthesized powder was identified as single-phase pyro by X R D and pulverized for further treatment (designated py-powder). The py-powder was mixed with appropriate amounts ofPbO, MgO and KC1, as shown in a weight column of Table 9. The amounts of PbO and MgO are greater than those required for the formation of PMN, if P M N could be prepared by direct reaction ofpy-powder with PbO and MgO. The mixtures were heat treated at 800-900°C for I h and the relative amounts of P M N were calculated. The amounts of P M N at 900°C are lower than those at 800°C in runs 2 and 6. This is because the absolute intensities of the P M N peak in each run are almost the same in spite of differences in the heat treatment temperatures and the absolute intensities of the pyro peak at 900°C are somewhat higher than those at 800°C. MgO and KCI does not act as an accelerator for the transformation even though both exist alone or together (runs 3, 5 and 7). The presence of PbO, however, accelerates the transformation (runs 2, 4, 6 and 8) and in particular, the transformation is strongly promoted only when sufficient amounts of PbO and MgO are supplied to make up for the deficient composition of pyro in comparison with the stoichiometric composition of P M N (runs 4 and 8). The complete transformation of pyro to P M N occurs at 900°C in run 8.

P M N (%)

Weight (g) Pyro PbO M g O

PMN(200) + KCI

0 0

Transformation o f pyro to P M N

(%).

PMN(O) + KCI

0 0

be attributed to the lowering of the melting point of PbO caused by the coexistence of MgO or N b 2 0 5 in the mixture powders. In Table 8, the time dependence of the formation of P M N is represented. The values in the last column are the same as those in Table 6. In all cases, the amount of P M N increases while time increases, and the single-phase P M N powder could only be prepared using PMN(200)+ KC1. It is considered that since PbO is rather soluble in KC1 molten salt, excess PbO is essential for preparing the single-phase P M N powder.15 3.5

of P M N

KCI

--2"0 2'0 2'0 2"0

800°C (1 h) 0 28 0 28 0 27

900°C (1 h) 0 6 0 86 0 12

0

0

93

100

4 CONCLUSIONS

Three kinds of powder preparation methods were performed to prepare the single-phase P M N powder. In the mixed oxide method, the relative amounts of P M N did not eventually exceed 91% even though there was an excess of PbO present in the mixture powders. It is also shown that the fineness of the

Preparation o f eb(Mgl/3Nb2/3)03 powder

MgO powder greatly affects the formation of PMN and it is presumed that the single-phase PMN could be prepared if the agglomerate sizes of MgO powders were less than 2 #m. The PbO flux method was moderately successful although the leaching of PbO was a major obstacle. The relative amounts of P M N in the resulting powders did not exceed 90%. The KCI molten salt method was completely successful and the single-phase PMN powder was obtained under certain conditions. This occurs because pyro, which inevitably appears in the preparation of PMN, easily transforms to PMN in the presence of PbO and the complete transformation of pyro to PMN takes place only when there are sufficient amounts of PbO and MgO, and KC1 salt. A C K N O W L E D G E M ENTS The authors thank H. Osawa and H. Tsunatori for their help in much of the experimental work.

295

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