Structural and Electrical Properties of Zn1-xCrxO NTCR Nanoceramics Synthesized by High Energy Ball Milling

Cr doped ZnO (x = 0-0.04) nanoceramics was successfully synthesized by high energy ball milling (HEBM) technique. The structural and electrical properties of the synthesized sample has been studied in detail. The doping of Cr into ZnO has been verified by X-ray diffraction (XRD) and also from the variation in structural parameters. Rietveld refinement XRD pattern of calcined sample showed the hexagonal wurtzite structure and it did not induce impurity phases. From the XRD, it has been confirm that maximum result confirms that up to 4 atomic% of Cr can be doped into ZnO. The strain of the sample reduced with increase in particle size. After sintering, there is a growth of particle size of Cr doped ZnO sample. The impedance spectroscopy data shows a single semicircle in the high frequency region corresponding to the bulk properties of the nanoceramic sample. The decrease in real part of the impedance with temperature suggests the NTCR behavior of the sample in the temperature range of 300-500 ºC. The temperature dependent relaxation phenomena are also observed for the synthesized ceramic sample at high temperature. Distributed relaxation time suggests that the relaxation in the synthesized samples is of non-Debye type. The equivalent electrical circuit of the semicircular pattern in the impedance spectrum of ZnO and Zn 1-x Cr x O nanoceramics sample is a parallel combination of bulk resistance (R b ) and bulk capacitance (C b ). temperature range 300 - 500 ºC.The impedance spectroscopy data shows a single semicircle in the high frequency region corresponding to the bulk properties of the ceramic sample. The impedance pattern suggests a decrease in bulk resistance with increase in temperature. Distributed relaxation time suggests that the relaxation in the synthesized sample is of non-Debye type and the relaxation time decreases with increase in temperature. Our observation was found to be suitable for high temperature sensor applications.


Introduction
Temperature sensors are used in domestic and industrial sector, laboratory and medical processes to regulate the temperature. The electrical behaviour of NTC ceramic depends on factors such as purity, composition, cation distribution [1]. The distribution of cation and oxygen parameter (u), gives information to build up physical properties for application in industry [2]. Nano metal oxide has paying attention of scientific community because of their unique properties like electronic properties, optical properties and large surface to volume ratio. The properties of a materials depend on particle size, crystalline structure, their surface conditions and shape [3]. Different behaviour of metal oxides such as ZnO, In2O3, CdO, CuO and SnO2 have been extensively investigated in doped and undoped form [4][5][6]. ZnO is an important semiconductor material among different transition metal oxides [7], it has a with wide band gap of ~3.37 eV, large exciton binding energy of ~60 meV having hexagonal wurtzite structure and belong to the space group P63mc (C6v) [8]. Because of its unique properties like non-toxic nature, abundance in nature, low cost, suitability for doping, high thermal and chemical stability and is an important semiconductor materials [9]. ZnO has versatile applications like UV photodetectors [10] gas sensors [11], solar cells [12], spintronics [13] luminescent materials [14], piezoelectric devices [15], light emitting devices [16], Thermistor [17] and cancer treatment [18].
Optical, electrical and other material properties of ZnO can be enhanced and control for its practical applications by doping with selective elements [19]. From the literature, it is found that doping with metal ions makes suitable for specific needs [7]. Several synthesis method has been used to synthesis doped and undoped ZnO nanoparticles such as sol-gel [20], solid state [21], hydrothermal [22] ball milling [23], microwave method [24], sonochemical method [25] etc.
Among these methods, ball milling is superior one because it reduces the phase transition temperature, sintering temperature, increases the particle reactivity. Particle size can be easily decreased to nano scale level and also low temperature solubility limit can be extended [26]. The sintering temperature is important in controlling the intrinsic defects in ZnO and also the properties of the samples [27].
By doping zinc oxide with different transition metals (TM), [28][29][30][31] or rare earth (RE) metals properties like electrical, Optical, and magnetic can be changed [32][33][34][35].This can be achieved by modifying the electronic structure and band gap of ZnO [36,37] [17], Sr [20], Ca [39], Mn [40], La [41] etc. into ZnO shows NTCR behaviour which is very much suitable for thermistor application. Among different phases of chromium oxide, under normal conditions Cr2O3 is the most stable phase. Therefore conduction properties of ZnO can be modified by Cr doping [7]. As the ionic radius of Cr is very close to that of Zn 2+ , Cr 3+ can easily enter into ZnO crystal lattice [42].
The structural and electrical behaviour of oxides can be correlated by complex impedance spectroscopy (CIS) in a broad range of frequencies and also to study their temperature and frequency dependent phenomena [43]. The ac conduction mechanism in amorphous semiconductors can be described by Correlated Barrier Hopping (CBH), Quantum Mechanical Tunneling (QMT) [44], and Hoping over a Barrier [45][46][47].
Although there are few reports available for ac conductivity study of Cr doped ZnO, a detailed investigation is not done. In this communication, we have presented the study on the structural, morphological and NTCR behaviour of Cr doped ZnO nanoceramics in details. The impedance studies of Cr-ZnO pellets is also carried out for a clear understanding of the suitability of Cr doped ZnO nanoceramics for thermistor application.

Experimental details
HEBM technique is used to synthesize Cr doped ZnO (Zn1-xCrxO with x = 0 -0.04) nanocrystalline samples. Calculated quantity of ZnO and Cr2O3 has been milled for 10h in a high energy ball mill (PM400 Retsch) in dry milling conditions having ball to powder weight ratio 10:1 with a speed of 300 rpm. Milling parameters like rotation speed, time of milling etc. important role for synthesis of ZnO nanoparticles [48]. The milling process was stopped for 30 min after every 1h of milling. Because of the kinetic energy of grinding medium heat generated during milling and the milling process is exothermic [49]. The milled sample was calcinated at 900 C for 2h with a heating rate of 2 ºC/min. Then the calcined powder mixed thoroughly with a small amount of PVA for making pellet. The pelletization was done using a hydraulic press at a pressure of 500 Mpa and sintered at 1000 ºC for 2h. Silver paste was applied on the surface of the pellets and dried at 700 ºC for 15 minutes, to make conductor. The structure of the ball milled calcined powder was determined by XRD D8 Advance, Bruker and field emission scanning electron microscopy (FESEM, Carl Zeiss NTS Ltd, UK). By using the "FullProf" program rietveld refinement of the XRD patterns was carried out [50] to quantify different structural parameter. The electrical behavior of the samples were studied using Impedance analyzer (Hioki LCR Hi-tester-3532-50) in a frequency range of 100 Hz -1 MHz and in a temperature range of 300 C to 500 C. The schematic representation of material synthesis experimental process is given in Figure 1. The XRD pattern of calcined sample (900 ºC, 2 h) ( Figure 2(b)) confirms that the samples up to 4 atomic % (x = 0.04) of Cr retains their structure intact. It indicates when the sample is calcined at the diffused particle when calcined at 900 ºC the substituted Cr 3+ ion in the Zn 2+ ion positions in ZnO lattice up to x= 0.04 which shows the single phase Zn1-xCrxO material and no precipitation of Cr2O3 was observed. Thus there is no impurity within range of XRD. Therefore, we believe that upto 4% of Cr (x = 0.04) doping is lower than the solubility limit of Cr ion in ZnO lattice by high energy ball milling. More than 4% of Cr doping in ZnO was reported by sol gel method and by a number of researchers [51][52][53]. K. Sebayang et al. reported more than 3.5% of Cr doping by solid state reaction [54]. It is also noticed that upon calcination the peak intensity increased and FWHM decreased which indicates the crystallinity of the Zn1-xCrxO sample increased after calcination [55].

Structural parameters analysis
The structural parameters are obtaind from the Rietveld refinement of the XRD patterns of 10h ball milled Zn1-xCrxO nanoceramics (( Figure 3(a-f)). It has been observed from Figure 3(a) there is no significant modification in the value of lattice parameter (a and c) due to Cr doping. But a closer study indicates slight increase in the value of both a and c after Cr doping which can be due to the lattice distortion cause due to doping. But it decreases with increases in Cr concentration. This result is consistent with the reported observation [52,53,56,57] . The c/a ratio of ZnO is not much changed by Cr doping while the volume of unit cell reduces slowly with increase in Cr concentration (Figure 3(b)).
The value of c/a = 1.60 shows that the hexagonal structure of pure ZnO is not distressed due to Cr doping. The crystallite size and strain was determined by using Scherer equation and W-H method.
The variation of crystallite size and strain with increase in Cr concentration were illustrated in Figure   3(c). It was observed that the crystallite size reduces with increases in Cr concentration in ZnO. This variation of crystallite size is as a result of the substitution of Cr ion in ZnO lattice [52].
The reduction in size may be due to lattice distortion of the ZnO crystal structure due to incorporation of smaller size Cr atom into ZnO lattice [58]. Similar result is also reported by a number of researchers [57,[59][60][61] The lattice strain was increased after Cr doping but by increasing the Cr concentration it was found to be decreased. The result is quite similar as observed by Santi Septiani et al. [56]. Where S = specific surface area, Dp = size of the particle (nm), ρ = density of ZnO (5.606gm/cm 3 ).
The specific surface area of Cr doped ZnO shows larger value upto 2% of Cr doping than pure ZnO which is slightly decreases for 3 and 4% of doping. This large value of S shows that sample is more reactive [62].
The u parameter was calculated by using the formula In our case, the lattice parameter (a and c) and Zn-O bond length was found to be decreased with increasing the Cr concentration, which confirms the reduction of crystallite size also the unit cell volume and applicable modification in micro strain .
The average value of δ was calculated by using the relation

Morphological study by FESEM
The surface morphology of all samples was studied by FESEM. Figure 4(i) illustrates the FESEM pattern of 10h milled undoped and Cr doped ZnO nanoceramics. All compositions exhibit spherical shape with diverse particle sizes. From these pictures it is seen that the particle size reduces

Impedance (Z*) Analysis
The electrical behaviour of all the synthesized material was carried out by Complex Impedance spectroscopy (CIS) in the frequency range 100 Hz -1 MHz. The output response of such measurement in a complex plot shows in the pattern of a sequence of semicircles, which helps to split the contribution of grain boundary and bulk. The complex impedance of a system can be described as [65]: * = ′ + j ′′ (6) where ′ = cos and ′′ = sin .
Similarly Figure 5(x = 0 -0.04) illustrates the variation of real part of impedance (Z ) with frequency at temperature range of 300-500 C for Zn1-xCrxO nanoceramics sintered at 1000 C with different concentration of x. From the figure it was found that the magnitude of Z (i.e resistance) reduces with rise in temperature and frequency for all samples which indicate the chances of increase in ac conductivity with temperature and frequency [66]. Figure 5(f) shows the variation of Z with the Cr concentration at a particular temperature and frequency (300 ºC and 20 kHz). It was observed that with Cr doping the impedance (Zʹ) becomes higher which increases linearly with raise in Cr concentration.
The increase of Z with increasing Cr concentration in ZnO at 300 ºC for all Cr doped samples shows a decrease in the value of ac conductivity with increase in concentration. The decrease in degree of Z on raise in temperature shows their NTCR behavior in the studied temperature and frequency range.
Again at low frequency and temperature region the value of Z is high which may be because of the polarization in the samples. Relaxation process in the materials was observed in the low frequency range [67]. In the temperature range of 300-500 C Zn1-xCrxO nanoceramics shows NTCR behaviour which suggests that, the material can be a suitable candidate for NTCR thermistor applications. The variation of imaginary part of impedance (Z") with frequency at different temperature for Zn1-xCrxO samples with different concentration of Cr was shown in Figure 6. The effect of doping is observed from the variation in the magnitude of Z", peak expansion and asymmetry. At higher temperature each curve exhibit peak. A single peak (Z"max) which is temperature dependent is seen for all the samples.
The peak shifts towards higher frequencies as the temperature is increased and a broadening observed with the decrease in peak height. This broadening suggest the temperature dependent relaxation phenomena in the material [68]. The relaxation phenomena in the material may be due to the presence of immobile species at low temperature and defect at high temperature [55]. The variation of the peak position and height with concentration of Cr doping was shown in Figure 6(f). The Cr shifts peak frequency to the higher value and it increases linearly with increase in Cr concentration up to x = 0.04. distribution [50].
The semicircular pattern in the impedance spectrum is representative of the electrical processes taking place in the nanoceramic samples. The equivalent electrical circuit of Zn1-xCrxO nanoceramics was carried out with ZVIEW software by fitting the experimental impedance data. It was found that the circuit model of ZnO and Zn1-xCrxO nanoceramics sample is a parallel combination of bulk resistance (Rb) and bulk capacitance (Cb) and the circuit diagram was as shown in Figure 8.The value of bulk resistance (Rb) and bulk capacitance (Cb) obtained at temperature 300-500C was shown in Table   1 Figure 1 Schematic representation of methodology    Variation of frequency with Z of the pellet sample prepared by the 10h milled powder calcinated at 900 ºC for 2h and sintered at 1000 ºC for 2h of Zn1-xCrxO (x = 0 -0.04) nanoceramics and (f) with different Cr concentration at a particular temperature (300 ºC) and frequency 20kHz. Variation of Z" with frequency of Zn1-xCrxO (x = 0 -0.04) of the pellet sample prepared from the 10h milled powder calcinated at 900 ºC for 2h with sintering at 1000 ºC for 2h and (f) Variation of fmax and Z"max with different Cr concentration at a particular temperature (300 ºC) and frequency 20kHz.