Low-temperature single-step reactive sintering methods for lead magnesium niobate

Materials Letters 59 (2005) 3262 – 3266 www.elsevier.com/locate/matlet

Low-temperature single-step reactive sintering methods for lead magnesium niobate Xiaohua Fang a, Bingquan Li a, Huiming Gu b,* a

Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York 11794, United States b Matview Inc, 102 Holmes Road, Holmes, PA 19043, United States Received 13 March 2005; accepted 23 May 2005 Available online 15 June 2005

Abstract Two reactive sintering methods for Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Mg1/3Nb2/3)O3 – PbTiO3 (PMN – PT) processing were introduced. The first method started with Mg(NO3)2 and was able to decrease the sintering temperature to 1050 -C. The second method started with Mg(OH)2 and was able to decrease the sintering temperature to 1000 -C. Both methods simplified the processing and lowered the sintering temperatures of PMN/PMN – PT simultaneously. D 2005 Elsevier B.V. All rights reserved. Keywords: Electroceramics; Ferroelectrics; Piezoelectric materials; Perovskites

1. Introduction Relaxor Pb(Mg 1/3 Nb 2/3 )O 3 (PMN thereafter) or Pb(Mg1/3Nb2/3)O3 – PbTiO3 (PMN – PT thereafter) has been studied extensively due to their superior dielectric, electrostrictive, and piezoelectric properties [1]. However single-phase perovskite PMN or PMN – PT are not easy to be obtained due to the formation of the unwanted pyrochlore phase. Swartz and Shrout [2] first succeeded in eliminating the pyrochlore phase in powders by developing the columbite method that involved two calcination steps. This method was widely adopted due to its relatively less stringent requirement to the raw materials and equipment. Later, many other methods were also found to be able to eliminate the formation of pyrochlore phase in powders, which include sol– gel methods [3– 6], solution processes method [7 – 9], coprecipitation method [10], soft mechanochemical pulverization method [11], Mg(NO3)2 mixing method [12], and coating method [13].

* Corresponding author. E-mail address: [email protected] (H. Gu). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.054

Using the methods mentioned above, the pyrochlore phase free PMN or PMN –PT powder could be prepared successfully. Nevertheless, multiple steps or special raw materials/equipment were needed for these methods. For example, two steps of ball-milling and two steps of calcination were needed for the preparation of the perovskite PMN/PMN –PT powder by using the traditional two-step columbite method. For the single calcination step processing methods, either special raw materials or special equipment were needed. The sol – gel methods, solution processes method, co-precipitation methods, and thermal spray method required even more processing steps than the traditional columbite method. After the preparation of the perovskite phase PMN/PMN –PT powder, additional steps of ball-milling, pressing, and sintering were needed for obtaining the PMN/PMN – PT ceramic. Another issue related to PMN/PMN –PT processing is its relatively high sintering temperature of 1200 -C [14]. At this temperature, PbO is volatile [15], which will cause imprecise compositions control, deteriorated properties, and serious harm to the environments. In addition, at such a high sintering temperature, expensive electrode materials such as Pt or Pd instead of the cheaper Ag and Cu have to be used for multi-layer/co-firing applications.

Intensity (AU.)

X. Fang et al. / Materials Letters 59 (2005) 3262 – 3266

Mg(NO3)2 reactive sintering at 10500C

Mg(OH)2 reactive sintering at 10000C 20

25

30

35

40

45

50

2θ θ Fig. 1. XRD pattern of 90% PMN – 10% PT prepared with Mg(NO3)2 and Mg(OH)2 reactive sintering method sintered at 1050 -C and 1000 -C respectively.

In the past decade, several methods have been found to be able to either lower the sintering temperature or simplify the processing for PMN/PMN – PT processing. For example, it was found that 5– 21 wt.% excess of PbO could decrease the sintering temperature to 950 -C [16]. 1 – 4 at.% of SrO doping would result in the low sintering temperature of 800 –900 -C [17]. Using the PMN powder made from Mg(NO3)2 mixing method, the sintering temperature was decreased to 900 -C [12]. Directly pressing the columbite phase (MgNb2O6) and PbO into green bodies and then reactively sintering the green bodies reduced the total processing steps of PMN/PMN – PT ceramics into two steps of ball-millings, one step of calcination, and one step of sintering [18]. But the sintering temperature was higher at 1250 -C. Later, using the same method, Kwon et al. [19] decreased the sintering temperature to 1000 -C by using nanosize TiO2, more reactive (PbCO3)2Pb(OH)2 instead of PbO, and O2 sintering atmosphere. In the very recent years, a few single-step reactive sintering methods have been found to be able to simplify the processing and/or lower the sintering temperature of PMN/PMN –PT simultaneously. For example, it was found that the ball-milled powder mixture of Mg(NO3)2, Nb2O5, and PbO can be reactively sintered into perovskite PMN ceramics at the temperature of ¨ 1200 -C [20,21]. Spraydried product of Pb2+, Mg2+, and Nb5+ cations containing aqueous nitrate solution was reactively sintered into perovskite PMN ceramics at 950 – 1050 -C after a precalcination at 350 -C [22]. The ball-milled mixture of Mg(OH)2 coated Nb2O5 and PbO was reactively sintered to perovskite PMN/PMN – PT ceramics at 1000 -C [23]. These methods were believed to be a very significant improvement to the PMN/PMN – PT processing and possibly to the processing of other pervoskite phase ceramics. They enabled the production of PMN/PMN – PT to be more cost-efficient, more environmental friendly, and less problematic. These methods could also possibly be used to produce PMN/PMN – PT with slightly better and more

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stable property due to the lower evaporation loss of PbO at lower sintering temperature. However, there is still space of improvements. For example, the sintering temperature of the Mg(NO3)2, Nb2O5, and PbO reactive sintering method is still high at ¨ 1200 -C. The preparation of Pb2+, Mg2+, and Nb5+ cations solutions is a quite complex process. The Mg(OH)2 coating method still need an extra step of coating. In this article, two simpler single-step reactive sintering methods of PMN/PMN –PT ceramics preparation were introduced. The first method was similar to the Mg(NO3)2, Nb2O5, and PbO reactive sintering methods [21,22] with improvement on the ball-milling and the sintering curve. With the improvements, the sintering temperature was decreased to 1050 -C. In the second method, Mg(OH)2 was used instead of Mg(NO3)2 as reactant. This could further decrease the sintering temperature to 1000 -C.

2. Experimental procedure The starting materials used in this study were Nb2O5 (99.9% Aldrich), PbTiO3 (99.+%, Aldrich), PbO (99.+%, Aldrich), Mg(NO3)2I6H2O (99%, Aldrich), Mg(OH)2 (99%, Aldrich), and MgO(99%, Aldrich). The majority of the experiments in this article were carried out based on the composition of 90% PMN – 10% PT. PMN and 65% PMN – 35% PT were also studied due to their importance in the ferroelectric ceramics industry. Same as many other methods, we started with slight excess of Mg2+ (5 at.%) and Pb2+ (1 at.%). First, Mg(NO3)2I6H2O, Nb2O5, PbO, and PbTiO3 (PT) at the mole ratio of 1.05:1:3.03:0.333 were ball-milled together in ethanol alcohol with yttrium stabilized zirconia (YSZ) balls for 24 h. Then the mixture was evaporation dried and pressed at 120 MPa into pellets of ¨ 2 mm thick and 12.5 mm wide in diameter. In pressing, no binder was used since

Fig. 2. Relative density as a function of sintering temperatures for 90% PMN – 10% PT. The error bars are the standard deviation of three independent samples.

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X. Fang et al. / Materials Letters 59 (2005) 3262 – 3266

Fig. 3. Peak dielectric constant measured at 1 kHz as a function of sintering temperatures for 90% PMN – 10% PT. The error bars are the standard deviation of three independent samples.

the green bodies were capable of maintaining a very good mechanical property even without binder. Then the green bodies were stacked and sintered in a closed alumina crucible with some perovskite PMN powders placed in the crucible to ensure a PbO atmosphere in sintering. In sintering, the temperature was first raised to 500 -C at a heating rate of 3 -C/min. After the samples were kept at 500 -C for 2 h to ensure a complete decomposition of Mg(NO3)2, they were raised to the sintering temperature at a rate of 5 -C/min. Finally, the samples were held at the sintering temperature for 2 h and cooled to room temperature at 5 -C/min. We investigated the sintering temperature of 900 -C, 950 -C, 1000 -C, 1050 -C, and 1100 -C, respectively, and characterized the corresponding properties and microstructures to determine the sintering temperature. This method was similar to the Mg(NO3)2, Nb2O5, and PbO reactive sintering methods published by Liou [20] and Liou and Tseng [21]. We improved in that heavier yttrium stabilized zirconia balls instead of alumina balls were used in ball-milling to ensure a better milling. A more environmental friendly media of ethanol alcohol instead of acetone was used as milling media. A slower heating rate of 3 -C/

min or 5 -C/min together with a 2 h of dwell at 500 -C were applied for the reactive sintering instead of a 10 -C/min of straight temperature raising. The other method we tried was the same as the procedure described above except that Mg(NO3)2 was replaced with Mg(OH)2. During the sintering, the green bodies from both methods were stacked and sintered in the same crucible to ensure an exact same sintering environment. We named these two methods Mg(NO3)2 reactive sintering and Mg(OH)2 reactive sintering, respectively, in the rest of this article. After sintering, the samples were polished and characterized. The dielectric properties were measured using a HP 4192 Impedance Analyzer. A scanning electron microscopy (SEM) was used to examine the microstructure. The X-ray diffractometry (XRD) was used to evaluate the phases in the samples. The sintered density was measured with the Archimedes method in kerosene.

3. Results and discussion XRD was used to check the phases in all the samples. Two examples of the XRD patterns were shown in Fig. 1. Based on the XRD patterns, the percentages of perovskite phase were calculated with the following equation. Content of compound i ð%Þ ¼

major peak intensity of the compound i ~i major peak intensity of the compound i

ð1Þ

The calculation results revealed that > 98% perovskite phase formed in all the samples. This indicated that both the Mg(NO3)2 reactive sintering method and the Mg(OH)2 reactive sintering method would generate the desired perovskite phase at the sintering temperatures we tested. Fig. 2 was the relative densities of the sintered 90% PMN – 10% PT samples at different sintering temperatures based on the theoretical density of 8.11 g/cm3. The green body densities were around 55% of the theoretical density for both methods. From Fig.

Fig. 4. SEM pictures of a 1000 -C sintered sample from the Mg(OH)2 reactive sintering method (a) and a 1050 -C sintered sample from the Mg(NO3)2 reactive sintering method (b). Both are 90% PMN – 10% PT samples thermal etched at 900 -C for 30 min in an open crucible.

X. Fang et al. / Materials Letters 59 (2005) 3262 – 3266 Table 1 Properties of the PMN and 65% PMN – 35% PT samples prepared from Mg(NO3)2 reactive sintering method and the Mg(OH)2 reactive sintering method

PMN from Mg(NO3)2 reactive sintering method sintered at 1050 -C 65% PMN – 35% PT from Mg(NO3)2 reactive sintering method sintered at 1050 -C PMN from Mg(OH)2 reactive sintering method sintered at 1000 -C 65% PMN – 35% PT from Mg(OH)2 reactive sintering method sintered at 1050 -C

Relative density (%)

% of perovskite phase

Peak dielectric constant

95.3

99

16,700

95.5

98

29,800

96.1

99

18,300

95.4

98

28,200

2, it was clear that, after the sintering at 1000 -C for the Mg(OH)2 reactive sintering method and at 1050 -C for the Mg(NO3)2 reactive sintering method, the samples achieved the relative densities of 95+%, which is a good number for PMN/PMN – PT or other perovskite phase ferroelectric ceramics. Fig. 3 was the peak dielectric constants of the sintered 90% PMN – 10% PT samples versus the sintering temperatures. The peak position of the dielectric constants properties for all of our 90% PMN – 10% PT samples located between 40 -C and 45-C. The dielectric constants were measured at 1 kHz in a temperature span of 0 and 100 -C with a temperature raising rate of 1 -C/min. Fig. 3 revealed that both the 1000 -C sintered samples from the Mg(OH)2 reactive sintering method and the 1050 -C sintered samples from the Mg(NO3)2 reactive sintering method achieved a good dielectric properties of 20,000 or higher. Higher sintering temperatures generated a slightly better dielectric property for both methods. Fig. 4 were the SEM micrograph of a 1000 -C sintered sample from the Mg(OH)2 reactive sintering method and a 1050 -C sintered sample from the Mg(NO3)2 reactive sintering method, respectively. The samples for SEM observation were prepared with a thermal etch at 900 -C for 30 min in an open crucible. From the SEM micrograph, it can be seemed that the grains were welldeveloped with the average size values of ¨ 5 Am. To check the universality of our methods, PMN and 65% PMN – 35% PT samples were prepared with the two reactive sintering methods sintering at 1050 -C and 1000 -C, respectively. The properties of the samples were listed in Table 1. The dielectric constants peak positions of the PMN and 65% PMN – 35% PT samples were near  10 -C and 155 -C, respectively. The data in Table 1 revealed that PMN and 65% PMN – 35% PT achieved good properties with the two reactive sintering methods as well. With the phase, density, dielectric constant, and SEM micrographs data above, it can be concluded that the processing of PMN or PMN – PT can be simplified and the sintering temperature can be decreased to 1050 -C by the Mg(NO3)2 reactive sintering method and to 1000 -C by the Mg(OH)2 reactive sintering method.

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The reasons why the two reactive sintering methods prevented the formation of the pyrochlore phase were believed to be related to the improved reactivity of Mg(OH)2 and Mg(NO3)2 over the MgO. Studies showed that the Mg2+ reactivity and distribution was critical to the perovskite phase formation [24, 25]. With this principle, the PMN pervoskite powder was prepared successfully via a single-step calcination by replacing MgO with Mg(NO3)2 [12]. The reasons why the two reactive sintering methods lower the sintering temperature were not completely clear yet. Gu [23] had a very good study on this topic for the reactive sintering of the mixture of Mg(OH)2 coated Nb2O5 and PbO, which is a very close system with ours. In his study, a large volume expansion and particle size decrease to 180 nm was identified accompanying the reactions at 500 -C. The small particle size of 180 nm helped the decrease of the sintering temperature. He also identified a significant reaction and sintering temperature overlap during the reactive sintering. His experimental data showed that the overlap of reaction and sintering had a significantly effect in decreasing the sintering temperature.

4. Conclusion A Mg(NO3)2 reactive sintering method and a Mg(OH)2 reactive sintering method for the processing of PMN and PMN – PT were developed. The Mg(NO3)2 reactive sintering method decreased the sintering temperature of PMN/PMN – PT to 1050 -C. The Mg(OH)2 reactive sintering method decreased the sintering temperature of PMN/PMN/PT to 1000 -C. Both methods simplified the processing of PMN/ PMN – PT to one step of ball-milling plus one step of sintering. These two methods are believed to be a significant improvement to the processing of PMN and PMN –PT ceramics. They have very good potential to be applied to other perovskite ferroelectric ceramics as well.

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