High-performance electrical properties of La-based perovskite ceramics for the functional phase of thick film resistors


 In order to explore an economical functional phase alternative material for thick film resistors, the crystal structure, microstructure, and electrical properties of (1-x)LSCN + xLCNZ (x = 0.0–1.0) composite ceramics were studied through solid-state reaction experiments. The composite ceramics were characterized by x–ray diffraction, scanning electron microscopy, energy dispersive x–ray spectroscopy, and DC four–probe method. Results suggested that the main phases of LSCN and LCNZ were formed, along with a small part of impurity phases. The addition of LCNZ to LSCN decreased the electrical conductivity and changed the TCR from positive to negative. Zero TCR could be achieved around 0.6 < x < 0.8 and relatively low absolute TCR values could be obtained for the samples of 0.4 ≤ x ≤ 0.8. The ceramic of 0.6LSCN + 0.4LCNZ showed the optimal performances of conductivity = 1923 S/cm, TCR = 379.54 ppm/℃, and relative density = 95.05%.


Introduction
Thick lm resistors (TFRs) are one kind of thick lm component which has been applied extensively in hybrid circuits for power conversion, current limiting, voltage division, etc. [1][2][3]. TFRs can be formed by screen-printing the paste onto a substrate, followed by drying and ring in a belt furnace [4]. And the paste usually comprises functional phase, glass phase, and organic vehicle [5,6]. Ag/PdO, RuO 2 , IrO 2 , Bi 2 Ru 2 O 7 , and Pb 2 Ru 2 O 6.5 are typical functional phases of TFRs, which manifest high conductivity (for broad range of resistance value), positive TCR at temperatures of 25-125 ℃, long-term stability, and high-temperature stability [7][8][9][10]. However, all of them contains precious metal elements which is not bene cial to lower the production cost of manufacturing TFRs. Considering this problem, some base metals (like Cu, Al), SnO 2 and MoO 2 , or carbon paste, have been researched as functional phase of TFRs [11][12][13][14]. Nevertheless, either their sintering processes depend on inert atmospheres or their electrical performances (resistance and TCR) are poor [3,13]. Therefore, it still makes sense to study more economical functional phase alternative materials.
Perovskite oxides of La 1 − x Sr x CoO 3−δ and LaCo 1 − x Ni x O 3 have aroused wide concern in the eld of solid oxide fuel cell cathodes, thermoelectric conversion, and oxygen-permeable membranes due to their unusual magnetic and electrical properties [15][16][17][18][19][20]. For La 1 − x Sr x CoO 3−δ , as 50% Sr 2+ substitutes to La 3+ site, abundant hole carriers are introduced and almost the same amount of Co 3+ ions would be converted to Co 4+ in this system, which strengthens the double exchange interactions between Co 3+ and Co 4+ ions [21]. Furthermore, the ion-conversion optimizes the electron transport and provides this material with excellent conductivity [22]. For LaCo 1 − x Ni x O 3 , researchers believed that the replacement of Ni for Co can improve the electron density and carrier concentration, as well as broaden the width of the itinerant conduction band, leading to a reduction in resistivity [23][24][25][26]. In our previous investigations,  [27]. Besides, these two materials are stable in air even at high temperatures. As it is known that the TCR value of the TFRs is usually adjusted by altering the quantitative ratio between functional phase with positive TCR and glass phase with negative TCR [28]. If the TCR of the functional phase is close to zero, a new route to produce TFRs would be achieved without glass phase. Therefore, zero TCR might be realized by adding the LCNZ ceramic into the LSCN ceramic.
Thus, the present study was performed to study the crystal structure, microstructure, and electrical properties of (1-x)LSCN + xLCNZ (x = 0.0-1.0) composite ceramics, and to investigate the possibility to be an economical functional phase for TFRs.

Experimental
La 2 O 3 , SrCO 3 , Co 2 O 3 , NiO, and ZnO (all of these powders were Analytical Reagent and were purchased from Chron Co. Ltd., Chengdu, China) were used to synthesize the composite ceramics via the conventional solid-state reaction. The powders were weighted in accordance with the stoichiometric ratio of LSCN and LCNZ ceramics and were ball-milled for 24 h separately. Subsequently, both of the milled mixtures were dried and calcined at 900 ℃ for 12 h. The resulting powders were mixed and ballmilled for 24 h again based on the x value of (1-x)LSCN + xLCNZ (x =0.0-1.0, molar ratio). The dried powders were ground thoroughly and shaped into disk by adding 5 wt% binder. Eventually, these disks were sintered at 1200 ℃ for 6 h in air.
The crystal structure of these composite ceramics was determined through x-ray diffraction (XRD: DX-2700, Haoyuan Co.) with Cu-K α radiation. The structure re nement was performed using Fullprof software based on the XRD data. Scanning electron microscopy (SEM: JEOL, JSM-6490LV) and energy dispersive x-ray spectroscopy (EDS) were adopted to observed the microstructure and element distribution. The bulk density was obtained through Archimedes method, and relative density was calculated by Eq. (1), where W 1 and W 2 denote the weight fraction of two materials; ρ 1 and ρ 2 are the corresponding theoretical density [29]. The electrical resistivities were examined using the DC four-probe (RST-9) method within the range of 25-125 °C. The TCR value could be obtained by Eq. (2), where ρ 25 and ρ 125 mean the resistivity at 25 °C (T 0 ) and 125 °C (T), respectively. indexed to the combination of these two standard patterns. As the x value increases, the diffraction peak intensity of LaCo 0.4 Ni 0.6 O 3 strengthens. This variation can be noted at the peaks of (110) and (104).

Results And Discussion
However, the overall peak intensity of the composite ceramics weakens monotonously with the increase of x except for the specimen of x = 1.0. Moreover, the impurity phase of NiO emerges as x ≥ 0.2, while the phase of La 1.2 Sr 0.8 NiO 4 becomes apparent in the range of x ≥ 0.4. The XRD re nements were performed based on the Rietveld method. The re nement pattern and detail tting parameters are showed in Fig. 1c and Table I. The pro le of re nement matches well with the experiment data, and the tting parameters are acceptable. The calculated fraction and designed fraction values of each phase are displayed in  The microstructure of (1-x)LSCN + xLCNZ (x = 0.2-0.8) composite ceramics was examined by SEM and is displayed in Fig. 4(a-d). Closely compact microstructures can be seen in these samples except the specimen of x = 0.8. For the composite ceramic of x = 0.2, large grains account for the majority, along with some unobvious small grains and trapped pores. With further addition of LCNZ, the average grain size decreases and small grains grow in the samples of x = 0.4 and x = 0.6 ( Figs. 4b and 4c), which might due to the insu cient energy required for grain boundaries migration and/or the migration was blocked by impurity phases during sintering [34]. As the LCNZ content reaches 0.8, visible pores and strip-like grains emerge. The variation of microstructures con rms the outcome of relative density. The EDS mapping images of 0.6LSCN + 0.4LCNZ ceramic are exhibited Fig. 4(e-k), which manifest that the phases of LSCN and LCNZ are almost homogeneously distributed in the composite ceramic. Nevertheless, the element distribution of La, Sr, and Ni is not as uniform as that of Co, Zn, and O. In detail, there are some Lalacking, Sr-rich, and Ni-rich regions in this range; and the region lacking La basically corresponds to the region rich in Sr. These uneven regions could be the existence of impurity phases of La 1.2 Sr 0.8 NiO 4 and NiO. Figure 5 shows the elemental composition of composite ceramics of x = 0.4 and x = 0.6 detected by EDS point scan. Results demonstrate that the basic elements can be detected in these materials. For these two specimens, the element ratio at spot-1 is essentially consistent with the designed value, while the element of spot-2 is rich in Sr. These outcomes are in accordance with the phenomena of EDS mapping. Moreover, the results also testify the observations displayed in XRD.