Abstract
Purpose
The purpose of this work was to characterize NiMn2O4 spinel-based thermistor powder, to use it in screen printing technology to fabricate temperature sensors, to study their performance for different sintering temperatures of thermistor layer, with and without insulative cover, as well as to investigate stability of the fabricated thermistors and their applicability in water quality monitoring.
Design/methodology/approach
After the characterization of starting NiMn2O4 spinel-based thermistor powder, it was converted to thick film paste which was screen printed on alumina substrate. Thermistor layers were sintered at four different sintering temperatures: 980°C, 1050°C, 1150°C and 1290°C. An interdigitated pattern of Ag-Pd conductive layer was used to reduce the resistance. Temperature-resistance characteristics were investigated in air and water, with and without insulative cover atop the thermistor layer. Stability of the fabricated thermistors after aging at 120°C for 300 h was also examined.
Findings
Thick film NiMn2O4 spinel thermistors, prepared by screen printing and sintering in the temperature range 980°C–1290°C, exhibited good negative temperature coefficient (NTC) characteristics in the temperature range −30°C to 145°C, including high temperature coefficient of resistance, good stability and applicability in water.
Originality/value
This study explores the range of sintering temperature that can be applied for NiMn2O4 thermistor thick films without compromising on the temperature sensing performance in air and water, as well as stability of the thermistors after aging at elevated temperatures.
Keywords
Citation
Uppuluri, K. and Szwagierczak, D. (2022), "Fabrication and characterization of screen printed NiMn2O4 spinel based thermistors", Sensor Review, Vol. 42 No. 2, pp. 177-186. https://doi.org/10.1108/SR-07-2021-0218
Publisher
:Emerald Publishing Limited
Copyright © 2022, Kiranmai Uppuluri and Dorota Szwagierczak.
License
Published by Emerald Publishing Limited. This article is published under the Creative Commons Attribution (CC BY 4.0) licence. Anyone may reproduce, distribute, translate and create derivative works of this article (for both commercial and non-commercial purposes), subject to full attribution to the original publication and authors. The full terms of this licence may be seen at http://creativecommons.org/licences/by/4.0/legalcode
Introduction
The accurate measurement and monitoring of temperature are crucial for controlling most biological, chemical and environmental processes and proper operation of many domestic, industrial, electronic and medical devices. The main groups of temperature sensors comprise thermometers, thermocouples, pyrometers and thermistors, each based on different measurement principles and fitted to various temperature ranges.
Thermistors (thermally sensitive resistors) use thermo-resistance effect for measurements in low and moderate temperature ranges (Kamat and Naik, 2002; Feteira, 2009). Thermistors are classified into two main types – NTC thermistors with a negative temperature coefficient of resistance and positive temperature coefficient (PTC) thermistors with a positive temperature coefficient of resistance.
The best known materials for NTC thermistors are compounds with a spinel AB2O4 structure (A = Ni, Cu, Zn, B, Mn, Co, Fe, Cr). Low and elevated temperatures from −50°C to 150°C are their typical operating range, although some spinel compositions are also suitable for high temperature measurements. The most popular are bulk NTC thermistors manufactured as ceramic discs or chips using conventional pressing-sintering or tape casting–firing processing (Ma et al., 2013; Gao et al., 2014; Varghese et al., 2009; Ma and Gao, 2017; Csete de Györgyfalva and Reaney, 2001; Fang et al., 2009; Kumar et al., 2019; Cui et al., 2021; Muralidharan et al., 2011; Karmakar and Behera, 2019; Han et al., 2018). However, thick film screen printing technology (Aleksić and Nicolić, 2017; Park and Bang, 2003; Park and Lee, 2009; Jagtap et al., 2008; Yuan et al., 2011; Schmidt et al., 2004; Schmidt et al., 2005) is also an attractive fabrication method because it offers low cost, diversity of thermistor geometry, ease of mass production, miniaturization and integration capability. Furthermore, thick film thermistors exhibit faster response and higher sensitivity than bulk ceramic components owing to a lower thickness of the active layer and high thermal conductivity of the alumina substrate (Aleksić and Nicolić, 2017). Besides screen printing which provides films with thicknesses of tens of micrometers, other deposition methods resulting in thinner layers, with a thickness below 5 micrometers, such as electron beam evaporation and radio frequency magnetron sputtering (RFMS) (Schmidt et al., 2004; Schmidt et al., 2005) and aerosol deposition (Ryu et al., 2013; Schubert et al., 2018), were also used for thermistor fabrication. Due to the composition instability of most of spinel-based thermistors at temperatures exceeding 200°C, several thermistor materials with a perovskite structure were considered for high temperature applications (Kulawik et al., 2007; Qu et al., 2021).
In the regular spinel structure, oxygen anions build a close-packed face-centered sublattice, whereas the divalent and trivalent cations are situated in tetrahedral and octahedral interstices, respectively. In the thermistors with a spinel structure, tetrahedral A sites and octahedral B sites are occupied by transition metal ions which easily change the oxidation states and positions in the crystal lattice (Schmidt et al., 2005). The octahedral sites are preferentially occupied by Mn4+, Ni2+, Mn3+, Co3+ ions, whereas Co2+ and Mn2+ ions, are mainly situated at tetrahedral sites (Ma and Gao, 2017). For NiMn2O4-based spinel oxide, which is the NTC thermistor composition that dominates in industrial applications, a fraction of Ni2+ ions is transferred from tetrahedral to octahedral sites. To preserve the crystal lattice neutrality by compensating the created cation vacancies, the appropriate amount of Mn3+ ions undergoes disproportionation reaction resulting in the formation of Mn4+ and Mn2+ ions. Mn2+ ions occupy tetrahedral sites. Small polaron hopping related to charge carriers transfer between transition metal ions with different oxidation states at octahedral B sites is a widely accepted electrical conduction mechanism in spinel-based thermistors (Han et al., 2018; Schmidt et al., 2005). Depending on the hopping distance, two hopping mechanisms are distinguished – the nearest neighbor hopping (NNH) or variable range hopping (VRH). For pressed pellets and screen printed NiMn2O4-based thermistors, the VRH of small polarons formed by Mn4+ and Mn3+ couples was indicated as responsible for electrical conduction (Schmidt et al., 2005).
Spinel NiMn2O4 undergoes partial decomposition at temperatures above 900°C (Varghese et al., 2009; Ma and Gao, 2017; Csete de Györgyfalva and Reaney, 2001) resulting in NiO precipitation. To fulfill the condition of electrical neutrality of the crystal lattice after Ni2+ ions partly abandon the spinel structure, the relevant concentration of Mn4+ ions has to convert into Mn3+ ions. An excess of Mn3+ ions at octahedral B sites leads to tetragonal distortion (Ma and Gao, 2017). Consequently, the NiO formation should result in an increase in resistivity of the spinel-based thermistor due to a decrease in the number of Mn3+ – Mn4+ species and higher resistivity of the NiO phase. Alternatively, NiO can be reabsorbed into the spinel crystal lattice (Ma and Gao, 2017). Besides the composition, grain sizes and microstructure uniformity are important factors influencing the thermistor resistance. A significant increase in resistivity was reported for the grain sizes below a critical level (Cui et al., 2021).
This study presents the fabrication procedure of NiMn2O4 spinel-based thick film thermistors and the investigation of the influence of sintering temperature on the composition, microstructure, electrical properties and stability of the fabricated sensors. Testing in water and checking the possibility of a significant lowering of the sintering temperature were vital objectives of the work aimed at the evaluation of applicability of the fabricated thermistors for water quality monitoring sensors and low temperature cofired ceramics (LTCC) devices.
Materials and methods
The grain size distribution of the starting thermistor powder was analyzed based on the laser diffraction analysis (LDA) technique (Mastersizer 2000, Malvern Panalytical, UK). The density, specific surface area and average particle diameter of the powder were determined based on helium density tests (Accupyc II 1340, Micometrics, USA) and measurements using the brunaur-emmett-teller (BET) method (Quantachrome NovaWin2, USA).
The X-ray diffraction (XRD) method was used to characterize the phase composition of the NiMn2O4 spinel-based powder destined for the preparation of thick film thermistor pastes and the thermistor thick films sintered at different temperatures. Using a heating microscope (Leitz, Germany), changes in the shape and dimensions of a sample made of the pressed thermistor powder were continuously surveilled during heating from room temperature up to 1370°C. The characteristic temperatures at which the sample starts and stops to shrink enable determination of the optimal sintering range. The heating microscope observations can also provide information about softening and melting points of the sample under investigation.
The particle sizes of the starting thermistor powder and the microstructure, compatibility with alumina substrate and elemental composition of the sintered thermistor layers were studied using scanning electron microscopy (SEM) and energy dispersive spectroscopy (Nova Nano SEM 200 with EDAX Genesis EDS system, FEI, USA).
NiMn2O4 spinel-based thermistor powder was prepared by the thermistor manufacturer (TEWA TERMICO, Poland) by ball milling the starting nickel and manganese oxides in stoichiometric proportions, followed by solid state synthesis at 1200°C–1300°C and final ball milling of the synthesis product. In this work, thick film thermistor paste was fabricated by mixing the thermistor powder (provided by the manufacturer) with ethyl cellulose of analytical grade purity and anhydrous terpineol (Fluka Analytical, Switzerland) in agate mortar for 20 min until the best consistency was achieved for screen printing. Furthermore, grinding the powder in the mortar enabled elimination of soft agglomerates present in the starting thermistor powder. The prepared thermistor paste was screen printed on alumina substrates (96% Al2O3) and dried at 120°C for 15 min. These operations were performed twice to decrease the resistance of the thermistors by an increase of the resistive layer thickness. Thermistor thick films were then fired in a chamber furnace at different temperatures − 980°C, 1050°C, 1150°C and 1290°C for 1–4 h to find optimal sintering conditions. It was checked, how much the temperature of 1290°C used by the manufacturer for sintering of bulk thermistors can be lowered for thick films, without degradation of their resistance-temperature characteristics, compactness and good adhesion to alumina substrate. For every sintering temperature, six thermistors were fabricated.
A conducting layer made of silver-palladium thick film paste (9695, ESL) was screen printed in an interdigitated pattern on top of the thermistor layer and fired at 850°C for 10 min in a belt furnace after drying for 15 min at 120°C. To protect the thermistors from degradation in future applications in water, a final layer of insulative screen printing ink (MaraPur PU, Marabu, UK) was applied on the top of the sensor working area and cured in a dryer at 120°C for 30 min. However, to investigate the impact of the used insulative ink on the performance of the thermistors, three of the six thermistors were left uncovered in each type of thermistor. Figures 1(a), (b), (c) show the three consecutive stages of screen printing required for fabrication of the thermistor. A digital optical microscope (Hirox, Japan) was used to measure the dimensions of the interdigitated pattern and thermistor layer [Figure 1(d)]. The thickness of the layers sintered at different temperatures was found using SEM.
For a simple determination of the thick film thermistor resistivity of the rectangular shaped resistors sintered at different temperatures, calculations were made using the formula:
The resistances of the rectangular shaped resistors were too high (about 108 Ω at room temperature) to ensure their easy measurements in the whole temperature range of thermistor operation (−30°C to 150°C). Thus, an interdigitated pattern was designed to adjust the resistance to the preferred kΩ range at room temperature.
The application of interdigitated electrodes converted the resistor shape close to one square to the shape corresponding to a small part of the square. As a result, a very effective decrease in the thermistor resistance (from 108 to 105 Ω at room temperature) was attained as compared with the rectangular shaped thermistor. The number of fingers of the interdigitated Ag-Pd electrode was 16. The average distance between the fingers, measured using a digital microscope was about 550 µm [Figure 1(d)].
The temperature dependence of the thermistor resistance can be described using the following expression:
The material constant B is also known as thermistor constant or beta constant (β) (Veres et al., 2007), and it describes the temperature sensitivity of thermistors. It was calculated based on the following formula:
A critical property in the characterization of the thermistor is temperature coefficient of resistance TCR (α) which describes the rate of change of resistance in relation to the change in temperature. It was calculated using the following formula:
The following relationship was used to calculate the activation energy Ea:
Resistance of the fabricated thermistors was measured as a function of temperature by placing them in a chamber furnace and monitoring the temperature change using a programmable temperature profile. A high accuracy (± 0.5%) Philips meter which has a measurement range of 1 Ω–2 GΩ was used to measure the resistance. Thermistors covered with insulative ink were measured in the temperature range of −20°C to 85°C, whereas thermistors without the insulative cover were measured in the range of −30°C to 145°C.
For the thermistors sintered at 1290°C, destined for measurements in water (with a protective layer), a long-term stability test was performed after aging at 80°C for 1000 h (Vötsch chambers, Germany). In addition, the changes of R-T characteristics after aging at 120°C for 300 h were studied for all samples sintered at 980°C, 1050°C, 1150°C and 1290°C.
Results and discussion
Thermistor powder characteristics
Table 1 displays the results of the characterization of the starting thermistor powder (as delivered by the manufacturer) by the LDA and BET methods.
Figure 2 illustrates the particle size distribution in the examined powder. Three modes were analyzed which correspond to D10 = 0.54 μm, D50 = 1.40 μm and D90 = 20.81 μm. A distinct difference between average particle diameter dBET determined using BET method and the particle sizes derived from the LDA analysis implies a significant agglomeration of the starting thermistor powder. A SEM image of the starting thermistor powder shown in Figure 3 confirms that the powder consists of small particles (0.5–1 µm in diameter) which form bigger agglomerates.
Figure 4 displays selected images from a heating microscope for a sample of the thermistor powder during heating in the range 25–1370°C. Based on these studies, the temperature about 980°C was found as the onset of shrinking. The relevant decrease in the sample size was not distinctly visible in Figure 4 due to a high heating rate in the microscope. However, during firing of thermistor thick films at 980°C for much longer period of 4 h, quite good densification was attained [Figure 6(a)]. In the temperature range 1050–1290°C the sample exhibits more intensive shrinking. A slight softening effect was observed at 1290°C implying the formation of a small amount of liquid phase, although no melting or even a significant progress in shrinking occurred up to 1350°C.
Microstructure and composition of thermistor thick films
Figure 5 presents the comparison of the diffraction patterns of thermistor thick films screen printed on Al2O3 substrate and sintered at various temperatures of 980°C, 1050°C, 1150°C and 1290°C. The main crystalline phase in all samples is NiMn2O4 spinel, which crystallizes in cubic structure, space group Fd-3m. Furthermore, the peaks related to Al2O3 substrates were found in the samples fired at 980°C, 1050°C and 1150°C. Such peaks were not observed for a sample fired at 1290°C, probably due to very compact microstructure of the layer sintered at this temperature. All thermistor layers contained small amounts of ZrO2 originating presumably from contamination by grinding media.
The sintering temperature of thermistor films significantly exceeded 900°C, indicated as the starting temperature of NiMn2O4 spinel partial decomposition (Csete de Györgyfalva and Reaney, 2001). The presence of the NiO phase was not revealed for the investigated samples, although such possibility cannot be excluded due to overlapping of the NiO peaks of the highest intensity with some peaks for the spinel.
SEM images in Figure 6 illustrate the microstructure of the sintered thermistor thick films. It follows from the comparison of Figure 6(a), (b), (c) that the morphology of the samples sintered at 980°C, 1050°C and 1150°C does not differ distinctly. The grains are relatively uniform and small, with sizes not exceeding 3 µm and slightly growing with increasing sintering temperature. All layers are well sintered. The grains are mainly of submicrometer size (300 nm-1 µm) for the layers sintered at 980°C and at 1050°C [Figure 6(e)] and 0.5–3 µm in diameter for the layer sintered at 1150°C [Figure 6(f), (g)]. For a thermistor sintered at the highest temperature of 1290°C, distinct grain growth was observed [Figure 6(d)], the grains show irregular shapes and are much bigger ranging from 2 to 10 µm. The thicknesses of the double screen printed thermistor layers slightly decrease with the sintering temperature from 56 µm at 980°C and 1050°C to 50 µm at 1150°C. For the layer sintered at 1290°C, the shrinkage is much more pronounced and the thermistor layer thickness is 34 μm.
Resistance-temperature characteristics of thick film thermistors
Resistance-temperature characteristics in air
The electrical parameters – TCR, resistivity at 25°C, material constant B25/85°C and activation energy Ea, are listed in Table 2 for the fabricated thermistors with and without insulative cover. The TCR, when expressed in %/°C, describes the per cent decrease in resistance when the temperature increases by 1°C. The resistance-temperature characteristics of all the fabricated thermistors sintered at different temperatures were similar to each other. The electrical resistivity ρ of all the thermistors decreased exponentially with increasing temperature. Therefore, there is a linear relationship observed in the Arrhenius plot between natural logarithmic values of resistivity and the reciprocal of absolute temperature (1/T), indicating good and typical NTC characteristics of the fabricated thermistors (Figure 7). The material constant B25/85°C for all the fabricated thermistors is within the industrial standards for NTC thermistors (2800 to 4820 K) (Park and Lee, 2009). Across the temperature range of −30°C to 145°C, the TCR values range from −4.9 to −2.3%/°C, respectively.
The resistance-temperature characteristics across the applied range of sintering temperatures do not vary significantly (Table 2, Figure 8). In spite of a significant difference between the lowest and highest sintering temperatures of 310°C, the difference in the resistivities at 25°C of thermistors sintered at those temperatures is about 7%, and the difference in TCR values is lower than 0.1%/°C. Therefore, the environmental impact during fabrication can be greatly reduced without compromising on the characteristics of the thermistors. Apart from conservation in energy due to reduced temperature of the firing process, the ability of NiMn2O4 spinel thick film to show good NTC characteristics over a wide range of sintering temperatures was also attained. The latter feature allows better flexibility in fabrication processes, such as co-firing with other functional layers, which may be crucial for LTCC multifunctional applications.
It is supposed that the reason for small impact of the sintering temperature on resistances of thick film thermistors is high temperature used by the manufacturer to carry out the solid state synthesis of the thermistor powder. The synthesis temperature was equal to or higher than that applied in this study to sinter thermistor thick films. As a result, the composition and stoichiometry of the spinel layer was fixed already at the stage of the starting powder and remained the same independently of the sintering temperature of a thick film thermistor. Consequently, the resistivities of films sintered at different temperatures were very similar.
The fabricated temperature sensors exhibit excellent repeatability with very low standard deviation among the tested samples regardless of the sintering temperature and the presence or absence of the insulative cover. The insulative cover increases the resistivity of the thermistor but the increment is quite low (∼0.2 kΩm). On the other hand, the insulative layer decreases the material constant B25/85, but this reduction is also quite low (4 to 10 K).
When the resistivity and material constant B25/85°C of all thermistors with insulative cover is compared to the resistivity and material constant of those sintered at the same temperature but without insulative cover, the difference does not exceed 6% and 2.5%, respectively (Table 2). Therefore, the used insulative ink is a good material for covering the thermistor layer because it does not adversely affect the NTC characteristics of the temperature sensors. The ink provides a protective layer which helps to greatly delay the effects of wear and tear from repetitive measurements in real life water samples as well as from dust and small particles when used in air.
Resistance-temperature characteristics in air after aging
The changes in resistance-temperature characteristics after aging are presented in Table 3 and in Figure 9. The comparison of Table 2 and Table 3 shows that the resistivities of the thermistors decreased 2.7–6.5% after aging at 120°C for 300 h, whereas the B25/85°C values were 0.7–2.7% lower. The lowest changes in resistivity were found for the samples sintered at 1050°C, whereas the B constant change was the lowest for the samples sintered at 1290°C. The thermistors sintered at 1290°C subjected to aging at 80°C for a longer period of 1000 h exhibited less dynamic change of resistance over time (2.5%) in comparison to the thermistors without insulative cover aged at 120°C for 300 h (Figure 9, Table 4). Aging phenomenon in spinel-based thermistors is a widely observed effect attributed to changes of the oxidation states and distribution of Ni and Mn cations between different lattice sites at elevated temperatures (Yuan, et al., 2011; Schubert, et al., 2018). Further studies are needed to determine if after the initial aging, long term stabilization of the thermistors can be attained.
Resistance-temperature characteristics in water
The resistance of the thermistor layers sintered at 1290°C with the insulative cover was measured as a function of temperature in water in the range of 0°C to 85°C. The thermistors were found to retain their good NTC characteristics in water (Figure 10). The resistivity of the thermistor was slightly lower in water than the resistivity of the thermistor in air. At 0°C, this difference is 17%, while above 10°C the difference in resistivity of thermistor measured in air and water is negligible and does not exceed 9%. Therefore, in positive temperatures the resistivity of thermistor is similar in air and water.
Conclusions
The fine NiMn2O4 spinel powder was used for preparation of a paste and screen printing of NTC thick film thermistors on alumina substrates. An interdigitated pattern of conductive layer enabled effective decreasing of the thermistor resistance to kΩ range. The fabricated thermistors independently of the applied sintering temperature showed good characteristics in the temperature range −30 to 145°C – high temperature coefficient of resistance (4.1%/°C at 25°C), high B25/85°C constant (4300 K), good stability after aging and applicability in water. The resistivity and temperature coefficient of resistance slightly differed for the thermistors sintered at 980, 1050, 1150 and 1290°C. The possibility to significantly lower the sintering temperature of thick film thermistors and their good performance in water are advantageous for the cost reduction, energy saving during firing process and applicability in LTCC systems for water quality monitoring.
Figures
Density, specific surface area and particle size distribution of the starting thermistor powder
| Helium density (g/cm3) | 5.072 ± 0.006 |
| Specific surface area (BET) (m2/g) | 4.53 |
| Average particle diameter dBET (μm) | 0.26 |
| Specific surface area (DLA) (m2/g) | 1.70 |
| D10 (μm) | 0.54 |
| D50 (μm) | 1.40 |
| D90 (μm) | 20.81 |
Electrical properties of fabricated thermistors with and without insulative cover sintered at different temperatures and measured in air
| Sintering temperature, °C | Insulative cover |
TCR at 25°C, %/°C |
Resistivity ρ at 25°C, kΩm |
Material constant B25/85°C, K |
Activation energy q, eV |
|---|---|---|---|---|---|
| 980 | Yes | −4.10 ± 0.004 | 4.44 ± 0.023 | 4270 ± 6.9 | 0.37 ± 0.0006 |
| No | −4.09 ± 0.004 | 4.19 ± 0.351 | 4274 ± 2.9 | 0.37 ± 0.0002 | |
| 1050 | Yes | −4.12 ± 0.020 | 4.27 ± 0.058 | 4293 ± 1.6 | 0.37 ± 0.0001 |
| No | −4.10 ± 0.007 | 4.08 ± 0.195 | 4298 ± 10.1 | 0.37 ± 0.0009 | |
| 1150 | Yes | −4.14 ± 0.001 | 4.29 ± 0.055 | 4346 ± 3.0 | 0.37 ± 0.0002 |
| No | −4.13 ± 0.002 | 4.11 ± 0.147 | 4357 ± 2.2 | 0.38 ± 0.0003 | |
| 1290 | Yes | −4.06 ± 0.110 | 4.15 ± 0.170 | 4298 ± 24.0 | 0.37 ± 0.0020 |
| No | −4.04 ± 0.005 | 3.99 ± 0.251 | 4306 ± 14.5 | 0.37 ± 0.0090 |
Electrical properties of fabricated thermistors without insulative cover, sintered at different temperatures and measured in air after aging at 120°C for 300 h
| Sintering temperature, °C | TCR at 25°C, %/°C | Resistivity ρ at 25°C, kΩm | Material constant B25/85°C, K | Activation energy q, eV |
|---|---|---|---|---|
| 980 | −4.01 ± 0.005 | 3.94 ± 0.022 | 4220 ± 168.7 | 0.36 ± 0.014 |
| 1050 | −3.81 ± 0.050 | 3.97 ± 0.224 | 4244 ± 26.40 | 0.36 ± 0.002 |
| 1150 | −3.95 ± 0.010 | 3.87 ± 0.002 | 4238 ± 7.054 | 0.37 ± 0.006 |
| 1290 | −3.89 ± 0.020 | 3.73 ± 0.070 | 4275 ± 75.60 | 0.37 ± 0.007 |
Electrical properties of thermistors with insulative cover, sintered at 1290°C and measured in air after aging at 80°C for 1000 h
| Sintering temperature, °C | TCR at 25°C, %/°C | Resistivity ρ at 25°C, kΩm | Material constant B25/85°C, K | Activation energy q, eV |
|---|---|---|---|---|
| 1290 | −3.90 ± 0.07 | 3.89 ± 0.08 | 4238 ± 108.17 | 0.36 ± 0.009 |
References
Aleksić, O.S. and Nicolić, P.M. (2017), “Recent advances in NTC thick film thermistor properties and applications”, Facta Universitatis – Series: Electronics and Energetics, Vol. 30, pp. 267-284, doi: https://doi.org/10.2298/FUEE1703267A.
Csete de Györgyfalva, G.D.C. and Reaney, I.M. (2001), “Decomposition of NiMn2O4 spinel: an NTC thermistor material”, Journal of the European Ceramic Society, Vol. 21 Nos 10/11, pp. 2145-2148, doi: 10.1016/S0955-2219(01)00190-X.
Cui, M.M., Zhang, X., Liu, K.G., Li, H.B., Gao, M.M. and Liang, S. (2021), “Fabrication of nano-grained negative temperature coefficient thermistors with high electrical stability”, Rare Metals, Vol. 40 No. 4, pp. 1014-1019, doi: 10.1007/s12598-019-01294-3.
Fang, D.I., Zheng, C.H., Chen, C.S. and Winnubst, A.J.A. (2009), “Aging of nickel manganite NTC ceramics”, Journal of Electroceramics, Vol. 22 No. 4, pp. 421-427, doi: 10.1007/s10832-008-9467-5.
Feteira, A. (2009), “Negative temperature coefficient resistance (NTCR) ceramic thermistors: an industrial perspective”, Journal of the American Ceramic Society, Vol. 92 No. 5, pp. 967-983, doi: 10.1111/j.1551-2916.2009.02990.x.
Gao, H., Ma, C. and Sun, B. (2014), “Preparation and characterization of NiMn2O4 negative temperature coefficient ceramics by solid-state coordination reaction”, Journal of Materials Science: Materials in Electronics, Vol. 25 No. 9, pp. 3990-3995, doi: 10.1007/s10854-014-2118-5.
Han, H.S., Park, K.R., Hong, Y.R., Shim, K. and Mhin, S. (2018), “Effect of Fe incorporation on cation distributions and hopping conductions in Ni-Mn-Co-O spinel oxides”, Journal of Alloys and Compounds, Vol. 732, pp. 486-490, doi: 10.1016/j.jallcom.2017.10.216.
Jagtap, S., Rane, S., Gosavi, S. and Amalnerkar, D. (2008), “Preparation, characterization and electrical properties of spinel-type environment friendly thick film NTC thermistors”, Journal of the European Ceramic Society, Vol. 28 No. 13, pp. 2501-2507, doi: 10.1016/j.jeurceramsoc.2008.03.027.
Kamat, R.K. and Naik, G.M. (2002), “Thermistors – in search of new applications, manufacturers cultivate advanced NTC techniques”, Sensor Review, Vol. 22 No. 4, pp. 334-340, doi: 10.1108/02602280210444654.
Karmakar, S. and Behera, D. (2019), “small polaron hopping conduction in NiMnO3/NiMn2O4 nano-cotton and its emerging energy application with MWCNT”, Ceramics International, Vol. 45 No. 10, pp. 13052-13066, doi: 10.1016/j.ceramint.2019.03.237.
Kulawik, J., Szwagierczak, D., Gröger, B. and Skwarek, A. (2007), “Fabrication and characterization of bulk and thick film perovskite NTC thermistors”, Microelectronics International, Vol. 24 No. 2, pp. 14-18, doi: 10.1108/13565360710745548.
Kumar, S.K.N., Aniley, A.A., Kumar, A.A., Fernandez, R.E. and Bhansali, S. (2019), “Nanoceramic NiMn2O4 Powder-Based resistance thermometer for soil temperature measurement application in agriculture”, ECS Transactions, Vol. 88 No. 1, p. 455, doi: 10.1149/08801.0455ecst.
Ma, C. and Gao, H. (2017), “Preparation and characterization of single-phase NiMn2O4 NTC ceramics by two-step sintering method”, Journal of Materials Science: Materials in Electronics, Vol. 28 No. 9, pp. 6699-6703, doi: 10.1007/s10854-017-6361-4.
Ma, C., Liu, Y., Lu, Y., Gao, H., Qian, H. and Ding, J. (2013), “Preparation and characterization of Ni0.6Mn2.4O4 NTC ceramics by solid-state coordination reaction”, Journal of Materials Science: Materials in Electronics, Vol. 24 No. 12, pp. 5183-5188, doi: 10.1007/s10854-013-1542-2.
Muralidharan, M.N., Sunny, E.K., Dayas, K.R., Seema, A. and Resmi, K.R. (2011), “Optimization of process parameters for the production of Ni–Mn–Co–Fe based NTC chip thermistors through tape casting route”, Journal of Alloys and Compounds, Vol. 509 No. 38, pp. 9363-9371, doi: 10.1016/j.jallcom.2011.07.037.
Park, K. and Bang, D.Y. (2003), “Electrical properties of Ni-Mn-Co-(Fe) oxide thick-film NTC thermistors prepared by screen printing”, Journal of Materials Science: Materials in Electronics, Vol. 14, pp. 81-87, doi: 10.1023/A:1021900618988.
Park, K. and Lee, J.K. (2009), “The effect of ZnO content and sintering temperature on the electrical properties of Cu-containing Mn1.95-xNi0.45Co0.15Cu0.45ZnxO4(0≤ x≤ 0.3) NTC thermistors”, Journal of Alloys and Compounds, Vol. 475 Nos 1/2, pp. 513-517, doi: 10.1016/j.jallcom.2008.07.076.
Qu, J.J., Li, S., Liu, F., Liu, X., Chen, Z.X., Yuan, C.L., Liu, X.Y., Zhao, Y.Y. and Hou, D.J. (2021), “Effect of phase structures and substrate temperatures on NTC characteristics of Cu-modified Ba–Bi–O-based perovskite-type thermistor thin films”, Materials Science in Semiconductor Processing, Vol. 121, p. 105375, doi: 10.1016/j.mssp.2020.105375.
Ryu, J., Han, G., Lee, Y.P., Lim, Y.S., Park, D.S. and Jeong, D.Y. (2013), “Co and Fe doping effect on negative temperature coefficient characteristics of Nano-Grained NiMn2O4 thick films fabricated by Aerosol-Deposition”, Journal of Nanoscience and Nanotechnology, Vol. 13 No. 5, pp. 3422-3426, doi: 10.1166/jnn.2013.7232.
Schmidt, R., Basu, A. and Brinkman, A.W. (2004), “Production of NTCR thermistor devices based on NiMn2O4+δ”, Journal of the European Ceramic Society, Vol. 24 No. 6, pp. 1233-1236, doi: 10.1016/S0955-2219(03)00415-1.
Schmidt, R., Basu, A. and Brinkman, A.W. (2005), “Small polaron hopping in spinel manganates”, Physical Review B, Vol. 72 No. 11, p. 115101, doi: 10.1103/PhysRevB.72.115101.
Schubert, M., Münch, C., Schuurman, S., Poulain, V., Kita, J. and Moos, R. (2018), “Thermal treatment of aerosol deposited NiMn2O4 NTC thermistors for improved aging stability”, Sensors, Vol. 18 No. 11, p. 3982, doi: 10.3390/s18113982.
Varghese, J.M., Seema, A. and Dayas, K.R. (2009), “Ni–Mn–Fe–Cr–O negative temperature coefficient thermistor compositions: correlation between processing conditions and electrical characteristics”, Journal of Electroceramics, Vol. 22 No. 4, pp. 426-441, doi: 10.1007/s10832-008-9489-z.
Veres, A., Noudem, J.G., Perez, O., Fourrez, S. and Bailleul, G. (2007), “Manganese based spinel–like ceramics with NTC–type thermistor behaviour”, Solid State Ionics, Vol. 178 Nos 5/6, pp. 423-428, doi: 10.1016/j.ssi.2007.01.028.
Yuan, C., Liu, X., Liang, M., Zhou, C. and Wang, H. (2011), “Electrical properties of Sr–Bi– Mn–Fe–O thick-film Mn–Fe–O thick-film NTC thermistors prepared by screen printing”, Sensors and Actuators A: Physical, Vol. 167 No. 2, pp. 291-296, doi: 10.1016/j.sna.2011.02.047.
Acknowledgements
This project AQUASENSE has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement number 813680.
European Commission.
H2020-MSCA-ITN-2018–813680.