NTCR Behavior of La-Doped BaBiO₃ Ceramics

Abstract

The influence of the composition on the microstructure and electrical properties of BaBi_1−𝑥La_𝑥O₃(0≤𝑥≤0.1) NTC thermistors was investigated. A single phase of BaBi_1−𝑥La_𝑥O₃ with a monoclinic structure was prepared by solid state reaction method. The relationship between the resistivity and the reciprocal of absolute temperature of the prepared NTC thermistors was linear, indicative of NTC characteristics. The resistivity and the coefficient of temperature sensitivity first decreased with increasing 𝑥 in the range of 𝑥<0.03 and then increased with further increase in 𝑥. However the alpha parameter value increased to a maximum value and then decreased again.

1. Introduction

Negative temperature coefficient (NTC) thermistors are thermally sensitive resistors whose resistance decreases with increasing temperature. They are mainly used in electronics for the suppression of in-rush current, for temperature measurement and control, and for compensation for other circuit elements [1, 2]. There is a large choice of NTC materials, but those most used in practice are based on solid solutions of transition metal oxides, such as Mn₃O₄, Co₃O₄, and NiO, with the spinel structure of the general formula AB₂O₄ [3].

BaBiO₃ has a monoclinic crystal structure with a breathing- and a tilting-mode lattice distortion of BiO₆ octahedra [4], and it shows semiconducting behavior despite the theoretical calculation predicting metallic behavior [5]. The distortion produces inequivalent Bi sites and is believed to cause a possible valence disproportionation (or charge density wave) of Bi³⁺ and Bi⁵⁺. BaBiO₃ is semiconducting material and shows a good NTC effect [6].

The selection of a given metal oxide material for applications of NTC thermistors is mainly determined by the required electrical properties and low cost of manufacturing of the thermistors. And those most used in practice are based on solid solutions of transition metal oxides with high sintering temperature. The sintering temperatures of BaBi_1−𝑥La_xO₃ ceramics are only about 750^∘C. At the same time in our research it has been found that BaBi_1−𝑥La_xO₃ ceramics show a good NTC effect. It is interesting to study the crystalline structure and the electronic properties of BaBi_1−𝑥La_xO₃ ceramics as a new NTC materials system.

2. Experimental Procedures

BaBi_1−𝑥La_xO₃ ceramic samples were prepared by solid state reaction method. The nominal composition of analytical grade BaCO₃, La₂O₃, and Bi₂O₃ powders was mixed in ethanol with an Al₂O₃ mortar. The obtained mixture was calcined in air at 800^∘C for 8 hours followed by pressing the powder into disc shape pellet. The disc was sintered at 750^∘C for 4 hours followed by a cooling rate of 200^∘C/h in air.

The crystalline structure of the prepared samples was analyzed by an X-ray diffractimeter (BRUKERD8-ADVANCE) using Cu K_𝛼 radiation with 40 KV, 35 mA, at a scanning rate of 6^∘/min. The microstructure of the samples was investigated by using a scanning electron microscope (Model: JSM5610LV). The Ag pastes with thickness of about 15 𝜇m were spread on opposite surfaces of the sintered samples using a screen printer. After the pastes were dried at room temperature, the samples were heated at 550^∘C for 30 minutes. The electronic resistance of the samples in the furnace was measured with a digital multimeter (Fluke 45) in steps of 20^∘C. The accuracy of the furnace measurements was ±0.5^∘C.

3. Results and Discussion

Figure 1 displays the XRD patterns of BaBi_1−𝑥La_xO₃, respectively, showing that a single phase of BaBi_1−𝑥La_xO₃ was prepared in the range 0 ≤𝑥≤ 0.1. The ion radius of La³⁺, Bi³⁺, and Ba²⁺ are 1.15 Å, 1.03 Å, and 1.35 Å, respectively. And Bi³⁺ is more similar to La³⁺ in the radius than Ba²⁺. So Bi³⁺ was replaced by La³⁺ in the lattices prior to Ba²⁺. From the facts that Bi does not occupy the Ba site [7] and that the BaO phase is not observed in the XRD pattern, it is demonstrated that the La ion does not occupy the Ba site but the Bi site.

Figure 1 

XRD patterns for (a) BaBi0.99La0.01O3, (b) BaBi0.97La0.03O3, (c) BaBi0.95La0.05O3, and (d) BaBi0.90La0.10O3.

The SEM images obtained from the surface of as-sintered samples BaBi_1−𝑥La_xO₃ are shown in Figures 2(a)–2(d), respectively. With an increase in La content the grain size decreases rapidly. The influence of La content in the samples on grain size was attributed to La³⁺ ions substituting for Bi³⁺ ions in the lattices. And the melting point of La₂O₃ is much higher than Bi₂O₃. With an increase in La content the melting point of BaBi_1−𝑥La_xO₃ increases. Under the same sintering temperature diffusion of ions in material becomes more difficult with higher melting point. So grains in material with low melting point grow up easily.

(a)

(b)

(c)

(d)

Figure 2 

SEM images of as-sintered samples: (a) BaBiO3, (b) BaBi0.99La0.01O3, (c) BaBi0.97La0.03O3, and (d) BaBi0.90La0.10O3.

The resistivity versus 1/𝑇 curves of BaBi_1−𝑥La_xO₃ materials with different La contents sintered at 750^∘C is shown in Figure 3. The relationship between the resistivity and the reciprocal of absolute temperature of the prepared NTC thermistors is linear, indicative of NTC characteristics. Furthermore, the resistivity measured at other temperatures showed basically the same behavior as at room temperature, irrespective of the measuring temperature. BaBiO₃ shows, at room temperature, a distorted perovskite lattice with a monoclinic unit cell (I2/m) characterized by two different B sites, occupied, respectively, by Bi³⁺ and Bi⁵⁺. X-ray photoemission [8] and X-ray absorption spectroscopy [9] point to a minimal charge transfer between the two Bi sites.

Figure 3 

Resistivity-1/T curves of BaBi1−𝑥LaxO3 ceramics sintered at 750∘C for 4 hours.

The data on the electrical parameters such as room temperature resistivity, thermistor constant, and temperature coefficient of resistance are given in Table 1. The resistivity and the coefficient of temperature sensitivity first decreased with increasing 𝑥 in the range of 𝑥<0.03 and then increased with further increase in 𝑥. Room temperature resistivity and B constant of materials are related to active energy. So the 𝜌₂₅ and the B_25/125 show the same trend with an increase in La content of BaBi_1−𝑥La_xO₃ compounds. However the alpha parameter values of samples increased to a maximum value and then decreased again. The alpha parameter value of NTC materials is related to B constant. And the alpha parameter value decreases with an increase in B constant.

Table 1 

Resistivity at 25^∘C, B_25/125 constant, and temperature coefficient of resistance for the samples sintered at 750^∘C.

La substitution introduce electrons into the conduction band which is ascribed to the Bi(La)6s-O2p hybridized band [10]. And Shiro Kambe pointed out that the decrease in resistivity was due to the increase in the formal Bi valence in the solid solutions of BaBiO₃ compounds [11]. So with an increase in La content the resistivity of samples decreased. However the microstructure of samples became another main factor to affect the resistivity of them with further increasing La content. Grain size decreased rapidly with an increase in La content, and crystal boundary area enlarged. So the room temperature resistivity of samples increased when the La content in BaBi_1−𝑥La_xO₃ compounds was high (>0.03).

4. Conclusion

A single phase of BaBi_1−𝑥La_xO₃ with a monoclinic structure was prepared in the range 0 ≤𝑥≤ 0.1. The melting point of BaBi_1−𝑥La_xO₃ increases with an increase in La content. The BaBi_1−𝑥La_xO₃ compounds with different La content show typical NTC effect. As the amount of La in BaBi_1−𝑥La_xO₃ compounds increases, the resistivity at room temperature decreases to a minimum value. At high La content (>0.03), the resistivity increased again with increasing La content.

Acknowledgment

The work is supported by the Science Foundation of Guangxi Key Laboratory for Information Materials (GKN0710908-02-K).