Characterization and electrical conductivity of La_1−xSr_xCrO₃ NTC ceramics

Abstract

The La_1−xSr_xCrO₃ (x = 0–0.1) negative temperature coefficient (NTC) ceramics have been prepared by the traditional solid state reaction method. X-ray diffraction (XRD) analysis has revealed that the as-sintered ceramics crystallize in a single perovskite structure. Scanning Electron Microscope (SEM) images show that the doped Sr²⁺ contributes to in the decrease in porosity. X-ray photoelectron spectroscopy (XPS) analysis indicates the existence of Cr³⁺ and Cr⁶⁺ ions on lattice sites, which result in hopping conduction. The presence of the Cr⁶⁺ is one of the key factors that affect the electrical conductivity of La_1−xSr_xCrO₃. Resistance–temperature characteristics were studied in the range of −80 to 10 °C for the ceramic samples, the electrical characterizations show that the electrical resistivity and material constant B decrease with the increase of the strontium content.

1 Introduction

LaCrO₃ material is stable perovskite rare earth oxide having attracted extensive attention of researchers because of their variety of properties, such as good conductivity, excellent catalytic performance, outstanding ferroelectric and etc. [1–5]. LaCrO₃ material by doping other metallic elements in A or B-sites get rapid development owing to their excellent properties, such as high melting points, high mechanical, chemical stability and good electrical conductivity [6–8]. Furthermore, LaCrO₃ doped with appropriate impurity ions can be applied to the NTC thermistor materials [9, 10]. NTC thermistors are thermally sensitive resistors whose resistance decreases with increasing temperature [11, 12]. However, LaCrO₃ material has too high electrical resistivity at room temperature because of the weak sintering properties in the air. In order to improve the electrical properties of LaCrO₃, the A-site was doped by alkaline-earth metals [13, 14]. Considering the ions radius of Sr²⁺ and La³⁺ is similar, and the introduction of Sr²⁺ into the lattice could be compensated by the oxidation from Cr³⁺ to Cr⁶⁺ [15, 16], the substitution of Sr²⁺ for La³⁺ in LaCrO₃ will promote the hopping conductivity due to the presence of Cr³⁺ and Cr⁶⁺ ions and thereby decrease resistivity [15, 17]. Therefore, it is possible that NTC thermistor material could be obtained through the substitution of Sr²⁺ for La³⁺ in LaCrO₃. In this paper, the new NTC ceramic material La_1−xSr_xCrO₃ (x = 0–0.1) were studied. The structure and electrical properties of La_1−xSr_xCrO₃ (x = 0–0.1) ceramics were investigated.

2 Experimental procedures

Powders of La_1−xSr_xCrO₃ (x = 0–0.1) were prepared by solid-state reaction. Stoichiometric amounts of La₂O₃ (purity >99.99%, Sinopharm Chemical Reagent co., Ltd, China), Cr₂O₃ (purity >99%, Sinopharm Chemical Reagent co., Ltd, China), and SrCO₃ (purity >99%, Sinopharm Chemical Reagent co., Ltd, China) were well mixed, ground, and calcined at 1100 °C for 5 h. The calcined powders were then milled in mortar for 8 h. Subsequently, the powders were pressed at 10 MPa to form a disk of 10 mm in diameters and 3 mm in thickness, and then cold isostatic pressing at 150 MPa was used to enhance densities. The pressed disks were sintered in the temperature of 1600 °C for 5 h.

X-ray diffraction (XRD; BRUKERD8-ADVANCE, Cu K _α radiation) analysis was used to identify the crystalline phases of the sintered ceramic samples. Scanning Electron Microscope (SEM; Zeiss SUPRA 55 VP, Germany) was used to observe the microstructure of the sintered ceramic samples. X-ray photoelectron spectroscopy (XPS; Thermo, ESCALAB 250XI) was used to analyze the chemical states of the sintered ceramics.

In order to obtain the electrical conductivity, the disks were coated with a platinum paste, and then annealed at 1200 °C for 2 h. The resistivity was measured in the temperature of −80 to 10 °C by Agilent34970A multimeter in an oil bath.

3 Results and discussion

Figure 1 shows the XRD patterns of the as-sintered La_1−xSr_xCrO₃ ceramics. Analysis of these diffractograms has revealed that the ceramics are indexed to be orthorhombic perovskite phase isomorphic to LaCrO₃ (32-1240 card) described by the space group is P bmn. There is no other impurity phase, which shows that Sr²⁺ has all entered the perovskite lattice to form the solid solution. It is noted that the peaks corresponding to the perovskite phase in the ceramic samples slightly shift toward higher angles with the increase of Sr²⁺, which means that the unit cell volume decreases. This result from many factors such as the ionic radius of the acceptor dopant (Sr²⁺), the mixed valence state of the chromium ion, and the oxygen deficiency. The substitution of size larger Sr²⁺ (0.144 nm) for La³⁺ (0.136 nm) should cause an incremental increase in the until cell volume. However, the crystal structure cell volume decreased with the increase of Sr²⁺. Comparing the sizes of the relevant ions, it is clear that the amount of the relative increment in the unit cell volume due to the substitution of larger Sr is less than that of the relative decrement caused by the formation of chrome ions, which leads to the lattice contraction. La_1−xSr_xCrO₃ ceramics introduction of Sr²⁺ contributes to the Cr³⁺ was oxidized to Cr⁶⁺, which results in the decrease in the cell volume as a result of Cr⁶⁺ (0.044 nm) radius less than Cr³⁺ (0.055 nm). The results will be discussed with XPS analysis.

Fig. 1

XRD patterns of as-sintered La_1−xSr_xCrO₃ ceramics

Figure 2 shows the SEM micrographs of the as-sintered La_1−xSr_xCrO₃ ceramics. As can be seen, there was high porosity on the surface of ceramics, and the size of agglomerate Sr²⁺ doped LaCrO₃ samples slightly decreases compared to pure LaCrO₃. This may be due to the chromium volatilization at the high temperature in air atmosphere [15, 18]. Therefore, the chromium vaporization will be results in a poor sinterability of these ceramics and thereby producing little pore.

Fig. 2

SEM micrographs of the La_1−xSr_xCrO₃ ceramics: a x = 0, b x = 0.01, c x = 0.02, d x = 0.05, e x = 0.1

Figure 3 shows the XPS spectra of La 3d obtained from the ceramic samples. It can be seen that the spectra peaks are associated with the presence of lanthanum in the form of La³⁺ [19]. Figure 4 shows XPS spectra of Sr 3d obtained from the ceramic samples. It can be seen that the spectra only contained one single peak associated with the presence of strontium in the form of Sr²⁺ [20].

Fig. 3

La 3d XPS spectra collected for La_1−xSr_xCrO₃ ceramics: a x = 0, b x = 0.01, c x = 0.02, d x = 0.05, e x = 0.1

Fig. 4

Sr 3d XPS spectra collected for La_1−xSr_xCrO₃ ceramics: a x = 0.01, b x = 0.02, c x = 0.05, d x = 0.1

Figure 5 shows the XPS spectra and the curve-fitting example of La_1−xSr_xCrO₃ ceramics in the regions of Cr 2p core-level peaks. As can be seen, the peaks in the Cr 2p3/2 and Cr 2p1/2 spectra can be split into two peaks. The peak at 576.0 eV is assigned to Cr³⁺ and 580.0 eV to Cr⁶⁺ [21]. Similar result has been reported on other doped lanthanum chromite systems [13, 21]. From Fig. 5, it can be seen that Cr⁶⁺ concentration increases with increasing Sr concentration, which lead to decreasing the cell volume. This is agreement with the previous XRD results.

Fig. 5

XPS spectra results showing the Cr 2p regions of La_1−xSr_xCrO₃ ceramics: a x = 0, b x = 0.01, c x = 0.02, d x = 0.05, e x = 0.1. Filled circle and filled diamond indicate the peaks of the Cr 2p spectra attributed to Cr³⁺ and Cr⁶⁺, respectively

Figure 6 shows the relationship between the natural logarithm of the resistivity (In ρ) and the reciprocal of the absolute temperature (1000/T) for the as-sintered La_1−xSr_xCrO₃ NTC thermistors. It can be seen that the samples exhibited typical NTC characteristic whose resistivity decreases with increasing temperature over the measured temperature range. The resistance of NTC thermistors can be expressed by the equation [22, 23]:

Fig. 6

Relationship between In ρ and 1000/T (k⁻¹) for the sintered sample corresponding for various compositions La_1−xSr_xCrO₃

$$\uprho = \uprho _{0} \exp \left( {\frac{{{\text{E}}_{{\text{a}}} }}{{{\text{KT}}}}} \right)$$

(1)

where $\uprho$ is the resistance at infinite temperature, E_a is the activation energy, K is the Boltzmann constant and T is the absolute temperature. E_a can be calculated from the slope of the In ρ versus 1/T plots. It is also noted that the relationship between ln ρ and 1/T of all the samples turned out to be linear from Fig. 6. This linear dependence indicates an electrical conduction phenomena explained by polarons jumps [24]. The chromium volatilization at the high temperature in air atmosphere lead to the formation of Cr⁶⁺ ions. Due to the presence of these higher valent cations, holes are generated as per the following defect reaction [25]:

$${\text{Cr}}^{{3 + }} + 3h^{ \cdot } \to {\text{Cr}}^{{6 + }}$$

(2)

The holes, generated from the above equation, along with the lattice interaction form small polarons. In the present study, it not create any additional defect in LaCrO₃ ceramics, and therefore the conductivity is due to the polarons generated from Eq. (2). When 0.01 ≤ x ≤ 0.1, for the substitution of La³⁺ by Sr²⁺, the valence state of Cr³⁺ ions partially oxidized to Cr⁶⁺ to maintain the electrical neutrality. The ratio of Cr⁶⁺ to Cr³⁺ increases with x, this lead to the decrease in resistivity of La_1−xSr_xCrO₃ system [17, 26] as the concentration of small polarons increases. This is agreement with the previous XPS results. As shown in Table 1, the values of ρ₋₇₀, ρ₋₁₀, B_−70/−10 constant and activation energy Eα of the thermistors were in the range of 16.51–84100, 4.77–2170 Ωcm, 1106–3268 K, and 0.095–0.283 eV, respectively. The B value can be calculated by the following equation [12]:

Table 1 ρ₋₇₀, ρ_−10, B_−70/−10 constant and activation energy for the La_1−xSr_xCrO₃ ceramics

$$B = \left[ {\frac{{T_{1} T_{2} }}{{T_{2} - T_{1} }}} \right]ln\left( {\frac{{R_{1} }}{{R_{2} }}} \right)$$

(3)

where R₁ and R₂ are the resistance value that measured by the temperature at T₁ and T_2, respectively. From Table 1 and Fig. 6 the variation trend of activation energy for different compositions of La_1−xSr_xCrO₃ indicates that activation energy continuously decreases with Sr²⁺ ions increases. These results indicate that the electrical properties can be adjusted by the Sr²⁺ ions content.

In order to reveal the relation between ρ₋₇₀, B_−70/−10 and Sr ions content clearly, Fig. 7 is presented. As shown in Fig. 7, the two lines have similar shapes, these results indicate that ρ₋₇₀ and B_−70/−10 of La_1−xSr_xCrO₃ ceramics have the same tendency to increase of Sr²⁺ ions content.

Fig. 7

Evolution of ρ₋₇₀ and B_−70/−10 constants as a function of Sr composition (x) for the sintered pellets

4 Conclusions

The structure and electrical properties of La_1−xSr_xCrO₃ ceramics has been investigated. The sintered ceramic samples are a single orthorhombic perovskite phase. The electrical conductivity in these ceramics is due to the polarons jumps between different oxidation states of chromium ions. The resistivity decreases with the increase of Sr²⁺ content, which is attributed to the increase of Cr⁶⁺. The values of ρ₋₇₀, ρ₋₁₀, B_−70/−10 constant and activation energy Ea of the thermistors are in the range of 16.51–84100, 4.77–2170 Ωcm, 1106–3268 K, and 0.095–0.283 eV, respectively. The electrical properties can be adjusted by adjusting the Sr²⁺ concentration. These compounds could be used as potential candidates for NTC thermistors.