Effects of Ta2O5 on the microstructure and electrical properties of ZnO linear resistance ceramics

ZnO linear resistance ceramics were synthesized from ZnO–Al2O3–MgO–TiO2–SiO2–Ta2O5 by a conventional ceramics method. Effects of Ta2O5 on the phase composition, microstructures, and electrical properties of ZnO linear resistive ceramics were investigated. The results show that doping with appropriate amount of Ta2O5 can refine the grains of the main crystalline phase ZnO and the secondary crystalline phase ZnAl2O4 in terms of microstructure, and also can reduce the grain boundary barrier and optimize the I–V characteristics in terms of electrical properties. In addition, the doping of Ta2O5 can improve the stability of the resistivity , and the impedance frequency indicates that the doping of Ta2O5 makes the sample suitable for high-frequency electric fields. The resistivity of the sample doped with 0.2 mol% Ta2O5 is 56.2 Ω·cm, and this sample has the best grain boundary barrier height, nonlinear coefficient and temperature coefficient of resistance of 0.054 eV, 1.04 and −3.48 × 10−3 °C−1, respectively.


Introduction
As a semiconductor material with wide direct band gap and large excitation binding energy, ZnO has good optical and electrical properties, Which make it widely used in short-wavelength optoelectronics devices, transparent conducting layers for displays, photocatalysts, photoconductors [1][2][3][4]. ZnO has been used as varistors [5] and linear resistance ceramics as well. ZnO linear resistance ceramics studied in this paper have special nonlinearity in current and voltage, small resistivity fluctuations, small resistance temperature coefficient and large absorbed energy [6,7]. Compared with carbon-based resistors [8], ZnO linear resistors exhibit characteristics that are less susceptible to oxidation, easy to control resistivity, good stability, and high flux density. Compared with metal-type resistors, ZnO linear resistors consist mainly of non-ohmic ZnAl 2 O 4 phase and low-ohmic phase ZnO , which have linear I-V characteristics [9], small resistance temperature coefficient, and high serviceable temperature. This makes it more suitable for application in the power-electronics sector, especially in overvoltage protection systems.
In recent years, many reports have discussed the influence of different dopants on the properties of ZnO linear resistive ceramics. According to reports, Al 2 O 3 could play an active role in controlling the grain growth and regulating the resistance [10,11]. MgO promoted the development of the temperature coefficient of resistance in the positive direction [12], and TiO 2 was more friendly to the stability of resistivity [13]. Besides, the properties of ZnO linear resistance ceramics were improved by doping some rare earth oxides (La 2 O 3 [14], Y 2 O 3 [15], Sm 2 O 3 [16]). Based on the characteristics of ZnO linear resistance materials, the Fermi energy level at the grain boundary is smaller than that at the grain. However, a high grain boundary barrier is detrimental to linear I-V characteristics [17]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Nahm [18] mentioned in the report of Ta 2 O 5 doped ZnO-V 2 O 5 -MnO 2 ceramics that when the doping amount of Ta 2 O 5 was more than 0.05 mol%, its nonlinear coefficient was significantly reduced, and the grain growth could be limited to promote the densification of ceramics. Tominc Sara [19] also mentioned that the addition of Ta 2 O 5 led to the nonlinear collapse. In addition, Ta 5+ has a large ionic radius and trivalent cation equivalence, which is similar to the ionic radius [20] of Zn 2+ , which make it easy to dissolve into the ZnO lattice and cause donor doping. All this indicated that Ta 2 O 5 may be a good additive for ZnO linear resistance ceramics. However, there are few reports on the doping of Ta 2 O 5 in related aspects. This paper presents a systematic study on the influence of Ta 2 O 5 doping on microstructure, phase composition, and electrical properties of ZnO linear resistance ceramics, and the stability of resistance was also discussed.

Preparation of samples
(75-x) mol% ZnO-12 mol% Al 2 O 3 -5 mol% SiO 2 -7 mol% MgO-1 mol% TiO 2 -x mol% Ta 2 O 5 (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.30) linear resistance ceramics were synthesized by a conventional ceramics method. In the first step, Ta 2 O 5 and ZnO were ball-milled with ZrO 2 balls in a planetary ball mill(CX-QM2L) at 180 r/min for 6 h, in which the ratio of ball to mixed powder and water is 2:1:1.2. Then put the ball-milled slurry in Electric Blast Drying Oven(XMTD-8222) to dry and calcine at 800°C for 3 h. In the second step, MgO, Al 2 O 3 , TiO 2 and SiO 2 were ball-milled in the same way, and the slurry was dried and calcined at 1150°C for 3 h to make it uniformly dispersed. In the second step, the Ta 2 O 5 -ZnO mixture obtained in the first step and the mixture obtained in the second step are ball-milled, and the slurry was dried. the dried slurry is pelletized with a polyvinyl alcohol binder, and then compressed into a circular sheet ( Φ15 mm×6 mm). In the fourth step, the binder was first excluded from the round pieces and then sintered in the high temperature furnace of silicon molybdenum rod(KSL-1700X-A2) at 1330°C for 3h in an air atmosphere to get ZnO linear resistor ceramic samples.

Performance testing and characterization
The microstructure of the sample was examined by backscattered electron (BSE,JSE-6510,Japan) imaging and energy dispersive spectroscopy (EDS). Bulk density and porosity are measured by the Archimedes method. The average size of the crystal grains is measured by Nano-Measurer software. The phase composition was examined by x-ray diffraction (XRD, Rigaku, Japan) using Cu Kα radiation.
To measure the I-V characteristics, both sides of the sintered sample were covered uniformly with aluminum paste and fired at 650°C for 40 min. The nonlinear coefficient [α = lg(I 1 /I 2 )/lg(V 1 /V 2 )] is measured by the DC voltage-stabilized source, where the voltage corresponds to the current, V 1 corresponds to I 1 , and V 2  corresponds to I 2 . Respectively, the AC impedance spectrum is carried out at a frequency of 0.1 Hz to 10 6 Hz by using electrochemical stations(CHI760E). The resistance temperature coefficient α T was obtained from the 12 where x i is the resistance of any slice in a set of resistive slices,x is the average resistivity of the set, and n is the overall number of slices in the set.  Figure 2 shows the magnified XRD plots of all samples in the 2θ range of 32°-40°. There are two points of interest in the figure. Firstly, the Zn 2 SiO 4 diffraction peak gradually enhances with the increase of Ta 2 O 5 . Zn 2 SiO 4 (trigonal system) could inhibit ZnO grain growth by the particle blocking effect at the grain boundaries. Zn 2 SiO 4 also could facilitate the precipitation of excessive conductive impurities at the grain boundaries, generating more carriers, which reduced the barrier height of grain boundary. Secondly, the diffraction peak of    ZnO shifts to a higher angle with the increase of Ta 2 O 5 , which indicates that Ta 5+ gradually dissolves into the ZnO lattice. Figure 3 depicts the EDS spectrum of 0.20 mol% Ta 2 O 5 -doped ZnO linear resistance ceramics. In order to distinguish the different phases and their distribution, BSE micrographs of this sample were used. The ZnO phase is represented by the white grains marked by 01, the ZnAl 2 O 4 phase is represented by the black colored grains marked by 02, and the Zn 2 SiO 4 phase is represented by the gray grains marked by 03. The Zn 2 SiO 4 phase is mainly mixed in the ZnAl 2 O 4 phase. The element Ta is present in ZnO phase, which indicated that Ta 5+ gradually dissolves into the ZnO lattice, and which is consistent with the XRD test results. Figure 4 depicts the BSE images of ZnO linear resistance ceramics with different content of Ta 2 O 5 . It can be seen from the figure that with the increase of Ta 2 O 5 , Zn 2 SiO 4 becomes abundant and evenly distributed between ZnO and ZnAl 2 O 4 , which inhibits the growth of ZnO grains. The ZnO gradually changes from the large particles in picture a to uniform elongated 'rods', and the black holes are gradually reduced, which is beneficial to the homogenization of the microstructure. To further investigate the effect of Ta 2 O 5 doping on the grain size, the Nano-Measurer software was used to measure the average grain size of the primary crystalline phase ZnO and the secondary phase ZnAl 2 O 4 . As shown in table 1, the average grain sizes of ZnO and ZnAl 2 O 4 all were reduced with the increase of Ta 2 O 5 . However, excessive addition of Ta 2 O 5 leads to agglomeration, which results in abnormal growth of ZnO grains. It shows that there is an optimal value of 0.2 mol% for the doping of Ta 2 O 5 . Figure 5 depicts apparent porosity and bulk density of ZnO linear resistance ceramics with different content of Ta 2 O 5 . As shown in figure 5, the apparent porosity of the sample gradually decreases and the bulk density gradually increases with the increase of Ta 2 O 5 . One reason is that the grain size of ZnO and ZnAl 2 O 4 has been refined, which improves the density and uniformity of the grains.The other reason is that the theoretical density of Ta 2 O 5 (8.2 g cm −3 ) is higher than that of ZnO (5.67 g cm −3 ).

Results and discussion
To test the effect of Ta 2 O 5 doping on its grain resistance and grain boundary resistance, the AC impedance complex plane of the sample was obtained by separating the real and imaginary parts of the complex impedance (Z′-Z″). And the equivalent circuit fitting with Z-view software, as shown in figure 6. The impedance spectra of all samples are arcs, which can be described by the following formula [17]: Where ω refers to the circular frequency, Cgb refers to the grain boundary capacitance, Rgb and Rg are the resistance of the grain boundary and the grain. The impedance arc fitted by the Z-view software is used for analysis. The grain and grain boundary resistivities (ρg, ρgb) shown in table 1 can be simply converted from Rg and Rgb. The grain boundary resistivity and grain resistivity both decrease and then increase with the doping of Ta 2 O 5 , showing a 'U' shape trend. In terms of electrical properties of ZnO linear resistor ceramics, the main defects considered are donor-type defects [21]. Ta, which is a VB group element, is suitable for n-type doping of ZnO [22]. The ionic radius of Ta 5+ is slightly smaller than that of Zn 2+ , and the valence state difference between Ta 5+ and Zn 2+ is 3. Therefore, Ta 5+ can easily replace Zn 2+ , and one dopant atom can provide 3 free electrons. The equation [18] for the possible defects arising in this period is as follows: In this equation, Ta 5+ replaces Zn 2+ in the zinc oxide lattice as a donor, introducing conductive electrons, which leads to an increase in conductivity. At this time, the resistivity of the sample showed a downward trend. But when the donor concentration exceeds a certain amount, the electronic compensation becomes vacancy compensation, and the carrier concentration decreases with the increase of the Ta 2 O 5 value, which leads to a decrease in conductivity. At this time, the resistivity of the sample shows a rising trend. It is worth mentioning that when the doping amount of Ta 2 O 5 is more than 0.05 mol%, the dominant position changes from grain boundary resistivity to grain resistivity. The significant decrease in the resistivity of the grain boundary makes the difference between the resistance of the grain and the grain boundary smaller, which is beneficial to suppress the interface polarization and promote the sample to be suitable for a high-frequency electric field [23]. Figure 7 depicts the impedance-frequency(0-10 wHz, 10 wHz-100wHz) characteristics of ZnO-based linear resistance ceramics doped with 0-0.05 mol% and 0.10-0.30 mol% Ta 2 O 5 . Surprisingly, the resistivity of the samples with 0.00-0.05 mol% Ta 2 O 5 are significantly more strongly affected by the electric field frequency than that of the samples with 0.10-0.30 mol%Ta 2 O 5 . Especially under the frequency of 10 WHz-100 WHz, the sample doped with Ta 2 O 5 is more stable. This is mainly because the addition of Ta 2 O 5 significantly reduces the grain boundary resistivity of the sample. The resistivity of the grain boundary is more susceptible to the influence of the electric field, so the decrease of the resistivity of the grain boundary is beneficial to the stability of the sample resistivity. This also shows that Ta 2 O 5 doped ZnO-based linear resistance ceramics are excellent candidates for high-frequency electric field applications. Figure 8 depicts the resistivity-temperature(R-T) properties of ZnO-based linear resistance ceramics with different content of Ta 2 O 5 , and the resistance temperature coefficients (α T ) could be calculated as the table 1 listed. As shown in the table, all the values are negative. Because ZnO-based linear resistance ceramics follow the principle of 'thermal excitation', which makes them exhibit negative temperature coefficient (NTC) characteristics [24]. α T develops in a positive direction as the mole percentage of Ta 2 O 5 doping increases from 0.00 mol% to 0.20 mol%. Doping Ta 2 O 5 promotes the increase of Zn 2 SiO 4 , and the thermal expansion coefficient of Zn 2 SiO 4 (7.0×10 −6 K −1 ) [25] is higher than that of ZnO (5.0×10 −6 K −1 ). Therefore, the Zn 2 SiO 4 grains may thermally induce disconnection of ZnO grains, thereby increasing α T , which is consistent with the results of TiO 2 -doped ZnO ceramics [13]. But when the mole percentage of Ta 2 O 5 exceeds 0.20mol%, α T develops in a negative direction. Because the thermal expansion coefficient of Ta 2 O 5 [26] is lower than that of ZnO, excessive Ta 2 O 5 will cause α T to develop in a negative direction. Figure 9 depicts the nonlinear coefficient and grain boundary barrier height of Ta 2 O 5 doped ZnO-based linear resistance ceramics. The height of the grain boundary barrier is calcurilated by Arrehenius fitting and thermal expansion theory. Arrehenius plots of numerically fitted ZnO compound conductive ceramics dc resistivity is shown in figure 10. It can be seen that the changing trends for the nonlinear coefficient and grain boundary barrier height are similar, which decrease first and then increase with the increase of Ta 2 O 5 . Previous studies have shown that the nonlinear coefficient (α) is positively associated with grain boundary barrier height (j 0 ). The relevant expression for the grain boundary barriers height is as follows [7]: where N d and N s represent the donor concentration and the acceptor concentration. As mentioned above, Ta 5+ can replace Zn 2+ for donor doping. When the content of Ta 2 O 5 is not more than 0.2 mol%, the donor concentration is in the dominant position. Therefore, the donor concentration also gradually increases with the increase of Ta 2 O 5 , which reduces j 0 and α. However, excessive doping of Ta 2 O 5 will cause the excess Ta 5+ to segregate to the grain boundary, which will increase j 0 . Of course, the refinement of crystal grains, the decrease of porosity, and the improvement of resistance stability also affect the internal electric field, and all of those optimizes the I-V characteristics. Figure 11 depicts the standard deviation of resistivity and nonlinear coefficient of Ta 2 O 5 doped ZnO-based linear resistance ceramics. As shown in the figure, with the increase of Ta 2 O 5 content, the standard deviation of resistivity and nonlinear coefficient shows a trend of decreasing first and then increasing. This shows that the proper doping of Ta 2 O 5 provides great help for the stability of resistivity and nonlinear coefficient. The main reason may be the significant reduction of the grain boundary resistivity and the optimization of the microstructure.

Declaration of interest statement
No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described is original research that has not been published previously.