Electrical conductivity and electrical stability of Bi / Mg modiﬁed NiO ceramics for NTC thermistors

Thermistors with negative temperature coe ﬃ cient (NTC) of resistivity are important components for temperature sensors and actuators. High material constant (B value) of NTC thermistor, i.e. high-temperature sensitivity, is one of key focuses. Herein, Bi / Mg modiﬁed NiO based ceramics for NTC thermistors were prepared by conventional solid-state reaction method. Introduction of Bi 2 O 3 signiﬁcantly enhances the sintering ability of ceramics and reduces the sintering temperature from 1380 to 1250 °C. Mg-doping (i.e. preparation of Ni 1-x Mg x O ceramics, where x = 0, 0.02, 0.05, 0.07 and 0.1) has signiﬁcant e ﬀ ect on room temperature resistivity ( ρ 25 ). Phase composition, microstructure, electrical property and electrical stability were investigated. All prepared ceramics have the phase with rock-salt structure and show typical NTC characteristics with B values higher than 5300 K. The electrical stability with an optimized resistance-change rate of 1.02% after being aged at 150 °C for 500 h is achieved. The electrical properties of the ceramics were analysed by combining X-ray photoelectron spectra with complex impedance spectra.


I. Introduction
Thermistors with characteristic negative temperature coefficient (NTC) of resistivity have been widely applied in various areas such as automotives, household appliances, and aerospace. For a NTC thermistor, temperature sensitivity that is normally defined with B value can be determined by the following equation: where ρ 1 and ρ 2 are resistivities of a thermistor at temperatures T 1 and T 2 , respectively. The B values of normal-temperature type NTC thermistors are in the range of 2000-5000 K [1]. Room temperature resistivity (ρ 25 ), B value and electrical stability are the key parameters of a NTC element. Conventional NTC thermistors are usually composed of transition metal compounds with spinel structures [2,3] and have the common char-by Zhao et al. [9], the ∆R/R 0 of (Y 2 O 3 + CeO 2 )-LaCr 0.5 Mn 0.5 O 3 composite ceramics was less than 2% after 900 h ageing at 320°C. The electrical stability of NTC thermistors based on single-cation oxide or simple-oxide ceramics has also been studied [4,[10][11][12][13].
The ageing characteristics of Na-doped Zn 0.4 Ni 0.6 Obased NTC ceramics reported by Gao et al. [4] showed that the introduction of Bi 2 O 3 obviously reduced the ∆R/R 0 from 237% to 1.8%. Li et al. [10] reported that ∆R/R 0 of ZnO-based ceramics decorated with Sb-ions was as low as 1.44%-2.17% and Ga/V co-modified ZnO ceramics also showed high electrical stability with ∆R/R 0 less than 1.85% after being aged at 150°C for 1000 h [11]. NiO is a semiconductor oxide with a band gap of 3.6-4.0 eV [12]. In modern life, NiO-based materials have been extensively studied for various applications, such as thermal resistance, dielectric, magnetic, gas sensor, thermoelectric, catalytic and electrodes for lithiumion batteries or supercapacitors [14][15][16][17][18][19]. To obtain NTC thermistors with proper ρ 25 and high B values, MgO and Bi 2 O 3 modified NiO ceramics were prepared in this work. The results show that, by changing the dopant contents, B values of the prepared ceramics are always higher than 5300 K. Bi 2 O 3 modified Mg-doped NiO ceramics can achieve high electrical stability. The electrical properties and ageing characteristics are investigated.

Sample preparation
NiO based ceramics doped with different contents of Mg 2+ ions (Ni 1-x Mg x O, x = 0, 0.02, 0.05, 0.07 and 0.1, denoted as xNMO) and Bi 2 O 3 decorated Ni 0.9 Mg 0.1 O (Ni 0.9 Mg 0.1 O-yBi 2 O 3 , y = 1, 2, 3, 4 and 5 wt.% to Ni 0.9 Mg 0.1 O, denoted as 0.1NMO-yB) were prepared by a conventional solid-state reaction process. The raw materials were basic nickel carbonate (NiCO 3 · 2 Ni(OH) 2 · 4 H 2 O, ≥99.0%, Xilong Technology, China), magnesium hydroxide (Mg(OH) 2 , ≥99.0%, Aladdin, China) and bismuth oxide (Bi 2 O 3 , 99.9%, Changsha Yaguang Trading Co. Ltd, China). The raw materials were weighed according to the designed nominal formula (xNMO and 0.1NMO-yB), and mixed by ball milling in deionized water, followed by drying in an oven for 12 h. The dried mixtures were calcined at 750°C in air for 5 h. Each batch of calcined powder was fully ground and granulated with polyvinyl alcohol (PVA) as the binder, and then pressed into green pellets with a diameter of 12 mm and a thickness of about 2 mm. The xNMO green pellets were sintered at 1380°C for 1 h to obtain related ceramics. On the other hand, some of calcined 0.1NMO powder was mixed with various contents of Bi 2 O 3 , ground and granulated, and then pressed into green pellets same as that of xNMO. The 0.1NMO-yB ceramics were sintered at 1250°C in air. Both surfaces of the as-sintered ceram-ics were polished with sandpaper and then coated with silver paste followed by heating at 600°C for 5 min to make ohmic electrodes.

Materials characterization
In order to determine the phase composition of the sintered ceramics, XRD patterns were recorded using an X-ray diffractometer (XRD, Rigaku D/max 2500, Japan. Cu Kα Radiation, λ = 0.154056 nm) with diffraction angles (2θ) between 20°and 80°and a scanning speed of 8°/min. The ceramic microstructures were characterized by a scanning electron microscope (SEM, JMS-7900 F, Japan). The relative density (D r ) of each sample was measured using the Archimedes method and calculated according to equation of D r = D m /D t , where D m is measured density and D t is theoretical density of NiO crystal. The energy dispersive X-ray spectrometer (EDS, Oxford Ultim Max 65) equipped with SEM was used to analyse element distribution in the 0.1NMO-yB ceramics. The valence states of Ni-ion in ceramics were analysed by X-ray photoelectron spectroscopy (XPS, Kalpha 1063, UK).
The resistivity-temperature measurement system (ZWX-C, China) was used to measure the resistances change with increase of temperature (R-T) in temperature range from room temperature to 250°C, in air. Resistivities were calculated according to the Ohm's law ρ = R · A/t, where A is electrode area, t is sample thickness and R is tested resistance. The complex impedance spectra (CIS) were obtained through an electrochemical test system (Gamry reference 600, USA) in frequency range from 1 Hz to 1 MHz. Each CIS was analysed by Gamry Echem analysis program.
Ageing test was carried out in an oven at a constant temperature of 150°C for 500 h, in air. The sample resistances at 25°C were tested using a Multimeter (FLUKE, 8808A, USA) before and after being aged, in a constanttemperature room controlled by an air conditioner. The resistance change rate (∆R/R 0 ) of each sample was calculated according to the formula ∆R/R 0 = (R 1 −R 0 )/R 0 , where R 0 and R 1 were sample resistances before and after ageing, respectively.

Phase composition and microstructure
XRD patterns of the as-sintered ceramics are shown in Fig. 1a. All samples can be indexed to rock-salt type structure with space group of  NiO matrix. If Bi 3+ cations were incorporated into the NiO matrix, the lattice parameters would have been increased since the ionic radius of Bi 3+ (0.103 nm) is much larger than those of Mg 2+ (0.072 nm) and Ni 2+ (0.069 nm). However, the XRD pattern of 0.1NMO-2B ceramics does not have any peak of impurity Bicontained phase due to the limited amount of Bi 2 O 3 and resolution of the X-ray diffractometer.
In order to reveal the valence status of Ni-ion in the sintered ceramics, XPS spectra of the 0NMO, 0.1NMO and 0.1NMO-2B samples were analysed. The related XPS spectra of Ni 2p 3/2 orbit are shown in Fig. 1b. All sample have similar XPS spectra and each XPS spectrum is composed of one main part (right one) and a satellite part (left one). The XPS spectra were fitted and analysed by Gaussian Lorentz curves. The fitted results show that the fitting peaks near 853, 855 and 860 eV should reflect Ni 2+ ions, and the fitting peaks near 856, 857 and 863 eV come from Ni 3+ ions [20]. According to the areas of fitting peaks, the ratio of Ni 2+ and Ni 3+ ions (denoted as [Ni 2+ ]/[Ni 3+ ]) in the 0NMO, 0.1NMO and 0.1NMO-2B ceramics are 2.52:1, 3.02:1 and 3.04:1, respectively. This indicates that Ni 2+ and Ni 3+ ions coexist in all sintered ceramic samples. Generally, the transition metals may have multiple valence states in oxides for the various sintering conditions [7,23]. Part of Ni 2+ ions changed to Ni 3+ in NiO during sintering due to oxygen entering the NiO lattice [24]. The related defect chemical reaction formula can be described by the following equation: Under the similar sintering conditions, the amount or proportion of oxygen entering the lattice should be similar. The relative densities of the sintered ceramics are 88% (0NMO), 88% (0.02NMO), 86% (0.05NMO), 84% (0.07NMO) and 84% (0.1NMO). With the addition of Bi 2 O 3 , the relative densities of the sintered 0.1NMO-yB ceramics were 90%, 92%, 92%, 91% and 90% for y = 1, 2, 3, 4 and 5, respectively. These reflect that the addition of Bi 2 O 3 improved the sintering-ability. Figure 2 shows SEM images of the sintered 0.1NMO-2B ceramics. Figure 2a is secondary electron image and Fig. 2b is the backscatter electron image taken from the same area. Small pores appear in the ceramic grains, as shown in Fig. 2a. During the sintering process, the pores between particles migrate together with grain boundaries. When the migration rate of grain boundaries is inconsistent with the diffusion rate of pores, as seen in Fig. 2b, there are some light-grey traces located at grain boundaries, implying the existence of impurity. In order to determine the possible matter of the impurity, EDS elemental analysis was carried out and the results are shown in Figs. 2c-f. Figures 2c, 2d and 2f indicate that Ni, Mg and O elements were distributed almost homogeneously in the ceramics. By comparing Figs. 2b and 2e, one can conclude that the light-grey traces should belong to Bi-rich area. The distribution of Bi-rich phase along the grain boundary should be due to the low melting point of Bi 2 O 3 (825°C), which is much lower than the used sintering temperature (1250°C). The molten phase was formed in the sintering process, but solidified after cooling. The formation of liquid phase at high temperatures is useful for enhancing the ceramics sinterability.

Electrical property and NTC effect
The electrical properties of prepared ceramics were investigated by testing the resistance at various temperatures to analyse the related conductivity and temperature-resistance effect.
where ρ T represents resistivity at temperature T , A is a factor related to material properties, k is the Boltzmann constant and E a is activation energy of conduction. For a normal-temperature NTC thermistor, the temperature sensitivity is usually represented using B value (Eq. 1). Here, T 1 and T 2 were selected as 25°C (298 K) and 85°C (358 K), respectively, so the B value is usually rewritten as B 25/85 . Figure 3b   However, as shown in Figs. 3a and 3c, each ln ρ-1000/T plot does not show single linear relationship over the test temperature range, and can be divided into two linear regions which respectively show high and low temperature ranges. For example, for the 0.05NMO ceramics, E a in the high-temperature region is 0.242 eV, while E a is 0.443 eV in the low-temperature region. These indicate that there should be at least two kinds of conduction models for the prepared ceramics, and each conduction mechanisms may play a different role in different temperature region. Therefore, Eq. 3 can be reexpressed as: where ε (0 ≤ ε ≤ 1) is a factor related to the content of conduction mode and depends on temperature, E a1 and E a2 are activation energies of conduction for two conduction models, respectively. In order to reveal the influence of grain effect and grain-boundary effect in electrical properties of the prepared ceramics, complex impedance spectroscopies (CIS) tested at various temperatures were analysed. Figure 4 shows the CISs of the 0NMO, 0.1NMO and 0.1NMO-2B ceramics tested at different temperatures (Figs. 4a, 4b and 4c, respectively). Only one arc could be observed in each CIS. Here, a (R g -CPE g )(R gb -CPE gb ) equivalent circuit shown as the inset in Fig. 4a was set up to fit each CIS, where R g and R gb represent impedance from grain effect and grain boundary effect, respectively. CPE g and CPE gb are related constant phase elements. The fitting curves are in good agreement with the measured data (scattered points), implying that each impedance spectrum consists of grain effect and grain boundary effect. Table 1 lists some of the fitting results. Figure 4d shows the plots of R g and R gb dependence on temperature in the NiO based ceramics, obtained by fitting the CIS result with equivalent circuit. Both R g and R gb show NTC characteristics. The R gb is greater than R g at each measured temperature for each sample. These indicate that the NTC effect of each NiO based ceramics is caused by both grain effect and grain boundary effect.

Analysis of conduction models
NiO is a transition metal oxide and is also a semiconductor oxide with bandgap of about 3.5 eV. As it is shown in Fig. 1, the prepared NiO-based ceramics have single cubic phase (see in Fig. 1a), but there are Ni 2+ and Ni 3+ ions coexisting in the ceramics (see in Fig.  1b). These indicate that all prepared NiO-based ceramics should be non-stoichiometric compounds. The occurrence of Ni 3+ ions should be accompanied with production of hole charges, as described by defect chemical equation in Eq. 2. The hole charge is weakly bound at site of Ni 3+ ion, and located at the acceptor energy level in NiO-based semiconductor. The hole charge can be easily transferred to valence level when it is activated by surrounding environment such as electrical field or temperature, and acts as a charge carrier, resulting in the  electrical conductivity of ceramics. This kind of electrical conductivity is so-called band conduction. As shown in Fig. 3a, the substitution of Mg 2+ ions into NiO lattice leads to the increase of ceramic resistivity. This should be due to the substitution-induced change of energy level structure and lattice distortion of NiO. The bandgap E g of NiO semiconductor may be changed due to the substitution of Mg 2+ ions, and E g(total) of xNMO solid solution can be calculated according to Vegard's law [22]: where, E g(MgO) and E g(NiO) are bandgaps of MgO and NiO, respectively. MgO has a much higher bandgap (about 7.8 eV) than that of NiO crystal (about 3.5 eV). The bandgap of xNMO ceramics increased with the in-crease of Mg 2+ contents, and will increase the difference between the acceptor energy level (i.e. Ni • Ni as shown in Eq. 4) and top level of valence band. This increases the difficulty of hole charge migration and ceramics resistivity for band conduction model.
Meanwhile, for the coexistence of Ni 2+ and Ni 3+ ions in NiO-based ceramics, the migration of charge carriers might also result from polaron hopping as taken place in spinel compounds and other transition metal oxides [2][3][4]6,21]. This kind of electrical conductivity is so-called hopping conduction. So the polaron hopping could be proposed as Ni 2+ + Ni 3+ ↔ Ni 3+ + Ni 2+ in NiO-based ceramics. On the other hand, for the different ionic radii between Mg and Ni ions, the substitution of MgO should enhance the crystal distortion, and restricts the polaron hopping between Ni 2+ and Ni 3+ ions. However, the substitution of MgO in NiO lattice also decreases the total content of Ni ion and also hopping pairs per unit cell. All these lead to the increase of resistivity of NiO-based ceramics.
As it is shown in Fig. 4, both grain effect and grain boundary effect contribute to the electrical properties (resistivity and NTC effect). Therefore, besides the band conduction and hopping conduction taking place inside the grains (or bulk part), the charge carriers transferring through grain boundaries should also be another conduction model. For the existence of impurities or secondary phase such as Bi-rich one, the charge carriers passing through grain boundaries must overcome potential barrier, resulting in large E a as shown in Table 1. It can be seen from Fig. 3 that introduction of Bi 2 O 3 reduced ρ 25 and B 25/85 of ceramics. Bi 2 O 3 is a low melting point material and can enhance the sintering property and improve overall conductivity of materials. In addition, although the radii of Bi and Ni ions vary greatly, there may be a very small amount of Bi dissolved in the lattice, resulting in the enhanced lattice distortion, thus increasing E a and B values.
As temperature increases, all three conduction modes of band conduction, hopping conduction and the one of charge carriers overcoming grain boundary barrier trigger easier migration of charge carriers, resulting in higher conductivity and lower resistivity, i.e. the ceramics present typical NTC characteristics.
Among these three mechanisms, the band conduction should be the main one responsible for reducing the room temperature resistivity (ρ 25 ) of the ceramics since the difference between acceptor energy level and valence level is always small, thus its contribution to the material constant (B value) of NTC is small. The hopping conduction should also be one of the main factors for reducing ρ 25 of the ceramics because of the high concentration of polaron pairs between Ni 2+ and Ni 3+ cations, which gives large contribution to the material constant (B value) of NTC, especially, in low temperature region as shown in Fig. 3. This is because the charge transition for polaron hopping mode needs to overcome large lattice potential barriers. For the conduction mode of charge carriers overcoming grain boundaries barriers, the higher grain boundaries barriers, the higher ρ 25 of ceramics, and the higher B values. As shown in Fig.  4d, the conduction mode of charge carriers overcoming grain boundaries barriers is always the main contribution to B values. In the higher temperature region, the conduction modes of band conduction and hopping conduction obviously should give lower contribution to the NTC material's B values.

Ageing induced resistance change
Resistance change rates (∆R/R 0 ) of the 0NMO, 0.1NMO and 0.1NMO-2B ceramics after being aged at 150°C for 500 h are shown in Fig. 5. Each of ∆R/R 0 increased with ageing time in the initial ageing stage and then almost did not change after further ageing. The ∆R/R 0 of the 0NMO and 0.1NMO ceramics were 5.33% and 2.74% after ageing for 500 h, while for the 0.1NMO-2B ceramics it was 1.02%. These indicate that modification with Bi 2 O 3 can significantly improve the aging stability of the xNMO ceramics.
In order to explore the ageing characteristics of the NiO-based ceramics, the valence states of Ni ion in the 0NMO, 0.1NMO and 0.1NMO-2B ceramics after ageing treatment were analysed by XPS analysis (Fig.  6). Similar to those of the original ceramics (Fig. 1b), Ni 2+ and Ni 3+ co-exist in the aged ceramics. According to the fitting results, the content ratios of Ni 2+   As discussed in section 3.3 with conduction models, band conduction and hopping conduction were the key conduction mechanisms in NiO-based ceramics. Ageing induced the increase of content ratios ([Ni 2+ ]/[Ni 3+ ]), implying the increase of content of Ni 2+ ions and the decrease of content of Ni 3+ ions. As shown in Eq. 2, the existence of Ni 3+ ions should be accompanied with the production of hole charges. The decrease of content of Ni 3+ ions after ageing indicates that the concentration of charge carriers decreased during the process aging and resulted in the increase of ceramic resistivity. At the same time, the decrease of content of Ni 3+ ions also reduced the number of hopping pairs of Ni 2+ /Ni 3+ . For hopping conduction model, the temperature dependent conductivity (σ) is commonly described by the Nernst-Einstein equation [25]: where k is the Boltzmann constant, e is electron charge, d is hopping distance, ν 0 is hopping frequency, T 0 is defined as the characteristic temperature and N oct is the concentration of octahedral sites. The factor N C (1 − c) denotes the probability that Ni 2+ and Ni 3+ ions occupy adjacent octahedral sites, where N is the concentration of sites per formula unit which are available to the charge carriers, and c can be defined as . So the decrease of content of Ni 3+ ions must induce the increase of resistivity.
As reported by Li et al. [10], in the process of ceramics ageing, the adsorbed water (H 2 O ads ) on the ceramic surface from the air may capture the intrinsic electron holes (h • ) from the surface and interface of xNMO ceramics, thus forming •OH free radicals, h + protons and O 2 molecules: Therefore, the reduction of the concentration of electron holes in ceramics increased the resistivity of the ceramics. For the 0.1NMO-2B ceramics, [Ni 2+ ]/[Ni 3+ ] values changed slightly during the ageing process. As it is shown in Fig. 2, Bi-rich phase (e.g. Bi 2 O 3 or Bi 2 O 3based solid solutions) was located at grain boundaries in the 0.1NMO-2B ceramics and oxygen vacancies could form in the Bi-rich phase during the sample preparation and provide electrons. During the ageing process, oxygen molecules preferentially capture electrons from the Bi-rich phase and hardly affect the concentration of charge carriers inside the 0.1NMO grains and resistivity of grain effect. Meanwhile, the adsorbed oxygen molecules would also occupy the sites of oxygen vacancies, thus eliminating the oxygen defects at grain boundaries, thus ensuring the electrical stability of ceramics.
Therefore, the addition of Bi 2 O 3 can further improve the ageing properties of the xNMO ceramics.

IV. Conclusions
Bi/Mg modified NiO based ceramics for NTC thermistors were prepared by conventional solid-state reaction method and sintering. The Mg-doped NiO (Ni 1-x Mg x O, xNMO) ceramics possess typical NTC characteristics. The xNMO ceramics have adjustable room temperature resistivity (ρ 25 ) for different concentrations of Mg ions and show high thermal sensitivity with B 25/85 values higher than 5300 K. Both grain and grain boundary effects contribute to the electrical conductivity and NTC performance of the prepared ceramics. Introduction of Bi 2 O 3 enhanced the ceramic sinterability, reduced ρ and even improved effectively the electrical stability of xNMO thermistors. When modified with Bi 2 O 3 , the resistance change rates ∆R/R 0 was 1.02% after ageing in the air at 150°C for 500 h. Thus, due to the adjustable resistivity, high thermal sensitivity and electrical stability, the Bi 2 O 3 modified xNMO ceramics have good application potential as NTC thermistors.