POMANI-Mn3O4 based thin film NTC thermistor and its linearization for overheating protection sensor

Materials Chemistry and Physics xxx (2015) 1e13

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POMANI-Mn3O4 based thin film NTC thermistor and its linearization for overheating protection sensor Arvind Kumar a, *, Madan Lal Singla b, Amod Kumar b, Jaspreet Kaur Rajput c a

Department of ECE, UIET, Panjab University, Sector 25, Chandigarh 160014, India CSIR-Central Scientific Instruments Organisation, Sector 30, Chandigarh 160 030, India c Department of Chemistry, Dr BR Ambedkar National Institute of Technology, Jalandhar 144011, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Mn3O4- POMANI (Poly-2-methyl aniline) nanocomposites.
Low cost linearisation of NTC properties with high accuracy.
Thin film based temperature sensor for overheating protection.
Surface as well as point temperature sensing.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 September 2014 Received in revised form 3 February 2015 Accepted 21 February 2015 Available online xxx

Hydrochloric acid doped thin film of poly-o-methyl aniline (POMANI)-Mn3O4 nanocomposites have been fabricated on glass substrate. The nanocomposite films showed RT-NTC characteristics in the temperature range of 35e185  C with repeatability in the temperature range of 75e185  C. The cut off temperature of the thermistor fabricated from the nanocomposite material was found to be between 165 and 170  C. Synthesised nanocomposite material has been characterized using FT-IR, XRD, TEM for structure, morphology and TGA/DTC for thermal stability. Thermistor constant (b) observed from RT characteristics are in the range of 7363 Ke10,188 K and activation energy (DE) was calculated which was in the range 0.634 eVe0.878 eV. Further linearization of thin film based NTC thermistors was carried out using an low cost analog circuit by adding parallel (RP), series resistance (RS) and operational amplifier (OP-AMP). It has been observed that these thin film based temperature sensors have repeatable temperature sensing behavior on linearization with high sensitivity and low power dissipation (Pdiss). © 2015 Elsevier B.V. All rights reserved.

Keywords: Nanocomposite materials Semiconductor Thin films Electrical conductivity

1. Introduction In recent years inorganic-conducting polymer nanocomposites have received much attention due to capable of merging the

* Corresponding author. E-mail addresses: [email protected] (J.K. Rajput).

(A.

Kumar),

[email protected]

advantages of organic part to those of inorganic [1]. Polyaniline (PANI) is well known and most important conducting polymer having good electrical conductivity [2,3], facile synthesis [4] and environmental stability [5]. Various oxidation states [6], reversible doping/dedoping (acid/base) characteristics are associated with structural chain nitrogen [7]. The properties of PANI can be varied easily with acid/alkali doping and addition of metal oxide nanoparticles which changes the mechanical strength, thermal stability and electrical behavior.

http://dx.doi.org/10.1016/j.matchemphys.2015.02.042 0254-0584/© 2015 Elsevier B.V. All rights reserved.

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Alone PANI has some drawbacks due to poor solubility in common organic solvents, infusible polymeric backbone, poor thermal stability and strong hydrogen bonding between amine group and adjacent chains. This restricts its use and ongoing applications [8]. Various efforts have been made to overcome such issues. In the present study to overcome these drawbacks methyl substituted aniline (o-toludine) has been used to synthesise poly-omethyl aniline (POMANI). This has been found to have faster switching time between its oxidized/reduced states [7,9], better solubility, stability and processing [7] for various electronic applications [10]. The methyl group hindrance improves the properties of POMANI as compared to its counterpart PANI [11]. The NTC thermistors measure the temperature in a particular range as per design with high accuracy and sensitivity. These thermistors find applications in various electronic and electrical appliances/devices. The ceramic material i.e. oxides of transition metal oxides like Mn, Co, Ni and many others showed good NTC characteristics in different ranges. NPs of these metal oxides also exhibit metallic/semiconductor/insulator character with varying chemical and physical properties which enhances optical, magnetic and electrical behavior as compared to bulk counter parts [12e14] for sensing, transduction and other electronic applications. Some of metal oxides which have spinel crystal structure with Mþ2 and Mþ3 occupying tetrahedral/octahedral sites respectively showed conduction and NTC behavior due to possible hopping of electron between variable oxidation states. These ceramics have high resistivity characteristics at room temperature which can be reduced/ altered with the addition of conducting polymers on formation of comosite. The adduct of metal oxide NPs and conducting polymers create diverse materials with advance functions and synergistic/ complementary behavior of individual counterpart creates tuneable properties [15e18]. Among various metal oxides manganese oxide (Mn3O4) is an important metal oxide with spinel structure which find wide applications in batteries, catalysis, energy storage, supercapacitors, sensors etc. [19e23]. However it suffers from the drawback of high resistivity at ambient room temperature. In present study attempts have been made to alter the high resistance of Mn3O4 by synthesising its nanocomposites with POMANI. The focus of our investigations is synthesis of POMANI-Mn3O4 nanocomposite, fabrication of thermistor and to study their temperature sensing properties. The NTC cut off temperature range and its relation with the composition of POMANI-Mn3O4 nanocomposites have been evaluated. The thermistor parameters such as thermistor constant (b), temperature coefficient of resistance (a), power dissipation (Pdiss), activation energy (Eac), resistance at room temperature and its variation with change of temperature has been evaluated. Further the linearization of RT characteristics has been done using an analog circuit by adding parallel (RP), series resistance (RS) and operational Amplifier (OP-AMP). It has been

Fig. 2. Fabricated Thin film based temperature sensor of POMANI-Mn3O4 nanocomposite (T3-ES-H-GP).

Fig. 3. FTeIR spectra of (a)POMANI(T-ES) (b) POMANI-Mn3O4(T3-ES) (c) Mn3O4 (d) POMANI-Mn3O4(T3-EB).

observed that prepared POMANI-Mn3O4 nanocomposite thin film thermistors have interesting temperature sensing behavior on linearization. These may find applications for overheating protection of various prosthetic devices and may have commercial value.

Fig. 1. (a) Blue colored thin film of POMANI-Mn3O4(T3-EB) (b) Green colored thin film of POMANI-Mn3O4 after acid doping (T3-ES-H-GP). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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2. Materials All the chemicals were purchased from local supplier and were used as such without any further purification. ELIX 3 Millipore was used as deionised (DI) water source in all the synthesis.

3

ES, T2-ES and T3-ES respectively. A pure sample of POMANI was also prepared for comparison by following above procedure without adding Mn3O4 nanoparticles and was named T-ES. 2.4. Fabrication of thin films of POMANI e Mn3O4 nanocomposites for temperature sensor

2.1. General information Synthesised Mn3O4 nanoparticles, POMANI and POMANIMn3O4 nanocomposites were characterized using various techniques like TEM, XRD, DSC and FT-IR. TEM studies were carried out using Hitachi S7500 instrument. For this analysis sample was prepared by dispersing the nanomaterials in ethanol and then it was deposited by drop casted on carbon coated copper grid followed with drying. Information regarding morphology of Mn3O4, POMANI and POMANI e Mn3O4 nanocomposites was obtained from the TEM results. The XRD studies were carried out using Panlytical XPERTPRO (NDP) X-ray diffractometer using Ka radiation (l ¼ 0.154 nm) and 2q in the range from 15 to 80 . DSC analysis was carried out using EXSTAR6000 TG/DTA/DTG 6300. The materials were heated at a rate of 10  C min1 from 40 to 760  C under nitrogen atmosphere in platinum pan. The FT-IR spectra were recorded with Agilent Technologies Cary 630 Model over wave number in the range 4000e400 cm1 using KBr pellets. Sonication was carried out using Ultra-sonic bath of Loba Chemie model no. 3.5L100H1DTC, of 36 ± 3 KHz frequency with overall dimensions of 300  150  100 mm. The RT characteristics were carried out using Fluke 287 True RMS digital multimeter and PID controlled oven. 2.2. Preparation of Mn3O4 nanoparticles Mn3O4 nanoparticles were synthesised by coprecipitation method. 31.3 mmol of MnCl2.4H2O was dissolved in 30 ml of DI water with addition of 30 ml of 1.5 M aqueous solution of freshly prepared NH4OH along with stirring. Further to the reaction mixture 6 M aqueous solution of NaOH was added drop wise till pH z 12 achieved. The reaction mixture was transferred to a round bottom flask and the mixture was refluxed for 3e4 h till complete precipitation of nanoparticles occurred. Brown precipitates were separated after centrifuged at 10,000 rpm, washed several times with DI water and ethanol respectively. Finally these were dried in vacuum oven at 70e80  C for 12 hr. 2.3. Synthesis of POMANI-Mn3O4 nanocomposites The POMANI e Mn3O4 nanocomposites were synthesised by using in situ oxidative emulsion polymerization. For the synthesis of POMANI-Mn3O4 nanocomposites in each of three separate flasks 12.5 ml of 0.25 M aqueous solution of o-toludine, 0.175 M SLS and 0.75 M HCl was taken respectively and kept under ice cold conditions. All the contents of three flasks were mixed in 200 ml flask and allowed to stand in an ice bath. Then a known amount of presonicated Mn3O4 nanoparticles were added to the above solution and mixture was kept stirring under ice cold conditions to which 10 ml of 0.26 M ammonium persulphate solution was added drop wise with stirring for overnight. The formation of dark green product indicates the completion of the reaction. Methanol was added to the reaction mixture for precipitation of the product. The residue was filtered, washed with DI water and finally washed with ethanol. The final product was dried at 60 for overnight. The prepared POMANI e Mn3O4 nanocomposite have POMANI is in an emeraldine salt state. The procedure was carried out using different concentration of Mn3O4 nanoparticles i.e. 0.01 gm, 0.025 gm, 0.05 gm keeping other reagents and conditions constant and the samples were named T1-

The POMANI e Mn3O4 nanocomposites (T1-ES, T2-ES and T3ES) and POMANI (T-ES) were prepared in step-2.4 were dedoped by stirring in 3% NH4OH solution. The ammonium hydroxide solution was added till pH ¼ 9 was attained and a blue colored suspension was obtained in each case. Each blue colored product was centrifuged, washed with DI water, ethanol and finally dried in vacuum oven at 45  C for 8e10hr. Fine blue colored powders were obtained both for POMANI e Mn3O4 nanocomposites (T1-ES, T2-ES and T3-ES) and pure POMANI (T-ES) in their emeraldine base form and the samples were renamed as T1-EB, T2-EB, T3-EB,T-EB). 0.05 g of known sample (T1-EB, T2-EB, T3-EB and T-EB) was sonicated in 1.2 ml of NMP (N-Methyl-2-pyrrolidone) till the maximum solubility was attained. It was further centrifuged at 3000 rpm to remove any insoluble particles. Finally, thin film was fabricated from each filtrate by spin coating technique on clean glass plate. A glass substrate of dimension 1  1 cm was mounted on a spin coater and films were deposited at 2500 rpm. All these fabricated films were dried at room temperature and are bluish in appearance. The thin film of T3-EB is shown in Fig. 1a. For the acid doping of fabricated thin films of POMANI e Mn3O4 nanocomposites and POMANI, each film was dipped for 12 h in 1 M HCl solution. Each film was washed with DI water and finally with ethanol which results in greenish colored films. The films were named (T1-ES-H-GP, T2-ESH-GP, T3-ES-H-GP and T-ES-H-GP). The greenish colored thin film of T3-ES-H-GP is shown in Fig. 1b. The thin film of Mn3O4 was also prepared and sample was named MN-GP. All the thin films prepared were used as such for the fabrication of sensor. For this conductive silver paste was used to draw electrical contacts from two ends of the film. Finally the film was covered with another glass plate of 1  1 cm dimension and the sample was sealed to avoid any contamination using epoxy adhesive (Fig. 2). 2.5. FT-IR studies Fig. 3 shows the FT-IR spectrum of Mn3O4 nanoparticles (Fig. 3c), POMANI (Fig. 3a), and their nanocomposites (Fig. 3b,d). In the FT-IR spectra of Mn3O4 nanoparticles (Fig. 3c), two bands are observed at 599 cm1 and 479 cm1, which are assigned to normal spinel structure having coupling between MneO stretching modes of tetrahedral and octahedral sites respectively [24,25]. Two broad peaks at 3424 cm1 and 1638 cm1 are assigned to surface adsorbed moisture on the Mn3O4 nanoparticles. It can be observed that stretching peak of eOH groups is very much identified as Mn3O4 nanoparticles is synthesized in aqueous medium which can form tunnel structure during the course of reaction, thus small amount of water molecules may be intercalated in tunnel. FTIR spectra of POMANI (T-ES) showed a broad band at 3221 cm1 which is assigned to NeH stretching of amine group, peaks at 1581 cm1 and 1494 cm1 are due to stretching vibrations of quinoid and benzenoid rings respectively (Fig. 3a). The peaks at 1301 cm1 and 1247 cm1 are assigned to CeN and C]N stretching of benzenoid rings. Band at 1148 cm1 corresponds to in-plane bending of CeH. An absorption band at 800 cm1 revealed the presence of ortho substituted benzene ring [26,27]. The spectra of nanocomposite POMANI-Mn3O4 (T-ES) indicate the main constituent POMANI however incorporation of Mn3O4 nanoparticles results into slight shift of IR absorption bands of POMANI to higher wave number (Fig. 3b). The FT-IR spectra of

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Fig. 4. XRD patterns of (a)Mn3O4 (b)POMANI(T-ES) (c)POMANI-Mn3O4(T3-ES) (d) POMANI-Mn3O4(T3-EB).

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POMANI-Mn3O4(T-EB) is also shown in Fig. 3d. The absorption bands of Mn3O4 nanoparticles also shifted to higher value, which may be due to its interaction with POMANI. 2.6. XRD analysis XRD patterns POMANI e Mn3O4 nanocomposites and POMANI are shown in Fig. 4. The XRD pattern (Fig. 4a) of Mn3O4 nanoparticles matches well with JCPDS file no 24-0734 for hausmannite structure (MnIIMnIII 2 O4) with tetragonal system (space group I41/ amd). There is some possibility of formation of Mn2O3 along with major Mn3O4, as both are isomorphous. These cannot be discriminated by XRD (Fig. 4a). The XRD of POMANI and POMANI e Mn3O4 nanocomposites show broad diffraction peaks between 18 and 28 due to both parallel and perpendicular periodicity of polymer (POMANI) chains. In the XRD of POMANI (T-ES) diffraction peak is observed at 2q ¼ 18.8 and 2q ¼ 24.4 which revealed a low crystallinity showing conductive polymer nature due to repetition of benzenoid and quinoid rings in POMANI chains (Fig. 4b). The XRD pattern of POMANI e Mn3O4 nanocomposite (T3-ES) is shown in Fig. 4c and its corresponding diffraction peaks occur at 2q ¼ 21.9 and 2q ¼ 24.3 . The XRD of T-ES and T3-ES are almost similar this is due to low contents of Mn3O4 in the nanocomposite where polymer overlaps the Mn3O4. In T3-ES the diffraction peaks of POMANI are shifted from 2q ¼ 18.8 and 2q ¼ 24.4 to 2q ¼ 21.9 and 2q ¼ 24.3 respectively. Also the intensity of these peaks is reduced thus indicate the presence of Mn3O4 nanoparticles during polymerisation affect the crystalline nature of POMANI. This may happen due to hydrogen bonding between NeH of polymer and oxygen of Mn3O4. Also the counter acid anion in quinoid part of POMANI chain interacts with d-orbital of manganese of Mn3O4 [24]. The XRD pattern of POMANI (T3-EB) as emeradine base is shown in Fig. 4d, the diffraction peak is broad and depicts amorphous nature. 2.7. TEM TEM images of Mn3O4 nanoparticles (Fig. 5a) indicate square and polyhedron shape of the nanoparticles with size of 20e24 nm Fig. 5b showed the POMANI image and it exhibits spherical and nearly monodisperse nanoparticles. But addition of Mn3O4 nanoparticles to POMANI matrix results in different morphology, which is like fine ellipsoidal shaped without agglomeration (Fig. 5c). On the addition of SLS the surfactant adsorb on the surface of Mn3O4 nanoparticles and during stirring the surfactant can debundle the nanoparticles by steric/electrostatic repulsions. The use of SLS further improves the dispersion of Mn3O4 nanoparticles in POMANI matrix and this also confirmed the uniform incorporation of Mn3O4 nanoparticles to POMANI matrix. 2.8. Thermal studies Thermal stability of Mn3O4 nanoparticles, POMANI and their nanocomposites (T1-ES, T2-ES) are presented by thermograms in Fig. 6. The thermogram of Mn3O4 nanoparticles (Fig. 6a) reveals two gradual weight losses. The initial weight loss of 2.1% occurs in temperature range 40e271  C is due to loss of adsorbed water molecules and moisture. The second weight loss is due to release of O2.

6Mn2 O3 /4Mn3 O4 þ O2 Fig. 5. TEM images of (a)Mn3O4 (b)POMANI(T-ES) (c) POMANI e Mn3O4(T3-ES).

This study shows that there is formation of Mn2O3 nanoparticles along with major Mn3O4 nanoparticles. The POMANI showed three step weight loss behaviors (Fig. 6b).

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a

b

Fig. 6. a. TGA analysis of Mn3O4 nanoparticles. 6b. TGA analysis of (a)Mn3O4 (b)POMANI(T-ES) (c) POMANI- Mn3O4(T1-ES) (d) POMANI- Mn3O4(T2-ES).

The initial weight loss occurs from ambient to 100  C due to removal of moisture or residual water, weight loss above 200  C is due to loss of surfactant SLS, dopant and thermal oxidative degradation. The degradation of POMANI starts after 370  C which continued up to 700  C. The thermograms of POMANI-Mn3O4 nanocomposites follow the same trends of weight loss as of POMANI. Degradation of POMANI in nanocomposites (T1-ES, T2ES) started at lower temperature, it may be as catalytic effect of Mn3O4 nanoparticles for degradation of POMANI and result in lowering of degradation temperature [28] (Fig. 6c, d). Also plasticization effect is observed by addition of diluents (SLS and Mn3O4). The plasticization results in lowering of temperature of degradation, crystallinity and creates more flexibility. 2.9. Electrical properties The RT characteristics of all fabricated samples (Mn-GP, T-ES-H-

GP, T1-ES-H-GP, T2-ES-H-GP, and T3-ES-H-GP) were carried out in the temperature range 35e185  C since the NTC cot off temperature was above 175  C. All the samples were repeated more than ten times at different time intervals to different days to correlate the repeatability of the materials. All the nanocomposites showed resistance values in M-Ohms at ambient temperature and RT curves matched well with negative coefficient of resistance (NTC) characteristics. However there is a small alteration in resistivity value with time, but at cut off temperature minimum values were observed. In case of Mn-GP resistance was decreasing nonlinearly with respect to increasing temperature. At ambient temperature the resistance value of this thermistor was above 30GU (Fig. 7a, b) and the resistance of T-ES-H-GP was continuously decreasing with the increase in temperature. This thermistor showed maximum repeatability in the temperature range of 75e185  C. The cut off (where the sample showed the almost constant resistance) of this

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800

40000

750

35000

700 650

30000

600

(a)

25000

0hr 6hr 12hr 24hr 1Week 1 Month MN-GP

20000 15000 10000 5000 0 60

80

100

120

550

RESISTANCE(M-Ohms)

RESISTANCE(M-Ohms)

7

140

160

500

0 hr 6 hr 12 hr 2 Day 1 Week 2 week T1-ES-H-GP

450 400 350 300 250 200 150

180

100

o

TEMPERATURE( C)

50 0 70

2700

80

90

100 110 120 130 140 150 160 170 180 o

TEMPERATURE( C)

2400

Fig. 9. RT characteristics of T1-ES-H-GP at different time intervals between temperature range 80e185  C.

(b)

1800

0hr 6hr 12hr 24hr 1Week 1 Month MN-GP

1500 1200 900

60000 55000 50000

600 300 0 70

80

90

100

110

120

130

140

150

160

170

180

o

TEMPERATURE( C) Fig. 7. RT characteristics of MN-GP at different time intervals (a) between temperature range 50e180  C (b) between temperature range 75e180  C.

RESISTANCE(M-Ohms)

RESISTANCE(M-Ohms)

2100

(a)

45000

0 hr 6 hr 12 hr 2 Days 1 Week 2 Week 3 Week 1 Month T2-ES-H-GP

40000 35000 30000 25000 20000 15000 10000 5000 0

sample was between 175 ± 10  C (Fig. 8). The RT characteristic of T1-ES-H-GP was approximately same for the temperature range 75e185  C and became almost constant at >175  C (Fig. 9).T2-ES-H

30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 o

TEMPERATURE( C)

4000 20000

3750

18000

3500 3000

14000

2750

12000

RESISTANCE(M-Ohms)

RESISTANCE(M-Ohms)

3250 16000

0hr 6hr 12hr 24hr 1Week 1 Month T-ES-H-GP

10000 8000 6000 4000

(b) 0 hr 6 hr 12 hr 2 Days 1 Week 2 Week 3 Week 1 Month T2-ES-H-GP

2500 2250 2000 1750 1500 1250 1000 750 500

2000

250

0

0 45

60

75

90

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120

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150

165

180

195

o

TEMPERATURE( C) Fig. 8. RT characteristics of T-ES-H-GP at different time intervals between temperature range 45e187  C.

60

70

80

90 100 110 120 130 140 150 160 170 180 190 200 o

TEMPERATURE( C) Fig. 10. RT characteristics of T2-ES-H-GP at different time intervals (a) between temperature range 40e185  C (b) between temperature range 60e185  C.

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A. Kumar et al. / Materials Chemistry and Physics xxx (2015) 1e13 30000 27500 25000

(a)

RESISTANCE(M-Ohms)

22500 20000

0hr 6hr 12hr 18hr 24hr 1Week 1Month T3-ES-H-GP

17500 15000 12500 10000 7500 5000 2500 0 50

60

70

80

90

100 110 120 130 140 150 160 170 180 o

TEMPERATURE( C)

Fig. 12. Arrhenius plots for T-ES-H-GP, T2-ES-H-GP, T2-ES-H-GP, T3-ES-H-GP.

700

Table 2 Thickness of Thin Films of different samples.

600

RESISTANCE (M-Ohms)

(b) 500

6 hr 12 hr 18 hr 24 hr 1 Week 1 Month T3-ES-H-GP

400

300

200

100

0 70

80

90

100

110

120

130

140

150

160

170

180

o

TEMPERATURE( C) Fig. 11. RT characteristics of T3-ES-H-GP at different time intervals (a) between temperature range 50e185  C (b) between temperature range 75e185  C.

Sr. No.

Temperature sensor

Density(r) of material in g/cm3

Loading density

Thickness by weight difference method (mm)

1 2 3 4

T-ES-H-GP T1-ES-H-GP T2-ES-H-GP T3-ES-H-GP

0.52 0.62 0.71 0.83

0.124 0.073 0.075 0.041

2.211 1.203 1.063 0.496

Polarons have the main role for charge transfer mechanism. The polarons are radical cations having spin g ¼ 1/2 and electronic charge ¼ ±e [30]. These polarons are partially delocalized on polymeric chain by rearrangement of double bonds. In case of nanocomposites the hopping of electrons between Mn(II) and Mn(III) ions in tetrahedral to octahedral sites further increases the charge mobility [31,32]. The charge mobility of POT conjugated chain and Mn3O4 increases with temperature and become maximum at cut off temperature region. After that very less change

and T3-ES-H showed the good repeatability in the temperature range of 75e185  C. However the cut off temperature for both these sample was 160  C (Figs. 10a,b and Fig. 11a,b). In all the POMANI e Mn3O4 nanocomposites, Mn3O4 and POMANI the resistance dropped exponentially with increase in temperature. The resistance drops 35e45% with every 10  C rise in temperature become almost constant at 165  C ± 10  C 185  C ± 5  C (Table 1). All the thin film samples have polymer chain with charge carrier. The exponential decrease in resistance in POMANI (T-ES-H-GP) is due to delocalization of charge carrier in polymer chain [29]. The HCl doping further introduced delocalized charges into the p electron system.

Table 1 Properties of fabricated Thin Film based Temperature Sensors. Sr. No.

Temperature sensor

Repeatability range ( C)

1 2 3 4 5

MN-H-GP T-ES-H-GP T1-ES-H-GP T2-ES-H-GP T3-ES-H-GP

92e180 75e185 75e185 75e185 75e185



C C  C  C  C 

Thermistor cut off ( C) >175 >175 >185 >160 >160



C C  C  C  C 

b (K) (75  C 185  C)

DE (eV)

10,188 7615 7549 7504 7363

0.878 0.656 0.650 0.646 0.634

Fig. 13. Temperature dependence of resistivity for T-ES-H-GP, T2-ES-H-GP, T2-ES-HGP, T3-ES-H-GP.

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9

Fig. 17. General linearisation arrangement for thin film NTC Thermistor.

in hopping which further increased the charge mobility, hence resistance of nanocomposite T1-ES-H is lower than T-ES-H. To gain further insight into the sensing behavior Arrhenius plots are obtained. The temperature resistance plot in the form of lnR and 1000/T, known as Arrhenius plot which are almost straight line. By measuring the slope of Arrhenius plot of a linear zone, activation energy of the samples has been calculated. The dependence of resistance and temperature can be approximated by the following equation for NTC thermistors [33]. Fig. 14. Plot of lns vs 1000/T for T-ES-H-GP, T2-ES-H-GP, T2-ES-H-GP, T3-ES-H-GP.

R1 ¼ R2 expðbð1=T1  1=T2 ÞÞ The thermistor constant (b) reflects the temperature sensitivity of NTC thermistors. The Thermistor constant (b) has been obtained from the slope of the Arrhenius plot and by using following equation:

b ¼ ðlnR1 =R2 Þ=ð1=T1  1=T2 Þ

Fig. 15. Heating on/off studies for the sensor T3-ES-H-GP along with sensitivity and heating/cooling rate.

in hopping occur which result in almost constant resistance at cut off temperature region (Table 1). The nanoparticles of Mn3O4 present in the nanocomposites result in increase of electrons involved

Fig. 16. Heating on studies for the sensor T2-ES-H-GP along with sensitivity and heating rate.

The R1 and R2 are measured electrical resistance at temperature T1 and T2 respectively [24,31]. The thermistor constant values for these various thin films are obtained in the range (7363 Ke10,188 K) given in Table 1 (Fig. 12). Further the activation energy (DE) has been calculated by using the relation b ¼ DE/K, where K is the Boltzmann constant and b is thermistor constant to study the ion conduction of the various thermistor sensors. The activation energy of these samples was calculated to be in the range 0.634 eVe0.878 eV. The lowest activation energy is obtained for T3ES-H-GP and highest for Mn-GP. The DE values indicate that hopping mechanism is prominently operating for charge transport. Resistance of a thin film can be approximated by using formula R ¼ rd/Lt, Where r is the resistivity of the thin film, t is its thickness, d is the gap size between the silver finger contacts and L is their length. For all samples the gap size between the silver contacts was 0.2 cm and its length was 1 cm. The contact width is higher than thickness of the thin film therefore it can be ignored [34]. The thickness of the thin film was obtained using weight difference method in high precision quartz crystal balance [35,36]. The loading density of the films was calculated as 0.12e0.14 mg/cm2. The thickness of the film was measured/calculated and found to be in the range of 0.497 mme2.2 mm. The calculated thickness of each film is given in Table 2. In each sample the silver finger contacts were drawn and connected by tin coated copper wire. The resistivity of all the samples at various temperatures has been evaluated from their corresponding RT characteristics. The resistivity (r) of T-ES-H-GP is highest and T3-ES-H-GP is lowest (Fig. 13) and becomes almost constant at the cut off temperature. The resistivity (r) decreases with addition of Mn3O4 in all the samples. Also the value of thermistor constant (B), activation energy (DE) along with resistivity (r) are found low as the content of Mn3O4 increases in the POMANI matrix. This may be happen probably due to increase in hopping between cations at the octahedral sites which in turn increases the charge mobility [37e40]. Both the micro and macro structure of the nanocomposites decide these properties which include chain length, conjugation, crystalline nature and the amount of adduct. The DE values also confirmed this behavior. The electrical conductivity of the materials can be

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A. Kumar et al. / Materials Chemistry and Physics xxx (2015) 1e13

Fig. 18. Analog circuit for linearisation.

Fig. 19. Linearisation set up.

analysed by the Arrhenius equation s ¼ s exp(Ea/kT), where s is constant, Ea is the activation energy, k is Boltzmann constant and T is temperature in kelvin. Temperature dependent conductivity for all the samples is given in Fig. 14. The plots of log s versus reciprocal of absolute temperature (1000/T) obeys Arrhenius behavior and indicates semiconducting transport with 3D hopping mechanism [41e43]. These characteristics indicate NTC characteristics described by NernsteEinstein relation [44].

a

s ¼ 1=r ¼ so =T$NCð1  CÞexpðEH=ktÞ with so ¼ Noct e2 d2 no =k where Noct is concentration of octahedral sites per cubic centimeter, d represents the jump distance for charge carrier, no is lattice vibrational frequency, k is boltzmann constant, e is the electronic charge, N is concentration per formula unit of sites which are

b

Fig. 20. Linearisation plot for sample (a) T2-ES-H-GP (b) T3-ES-H-GP.

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A. Kumar et al. / Materials Chemistry and Physics xxx (2015) 1e13

a

11

b

Fig. 21. Comparison of Linearisation using different parameters (a) T2-ES-H-GP (b) T3-ES-H-GP.

a

b

Fig. 22. Power dissipation w.r.t voltage and temperature (a) T2-ES-H-GP (b) T3-ES-H-GP.

available to the charge carriers, C is a fraction of available sites occupied by charge carriers, EH is a hopping energy. The rate of change of resistance with time and heating/cooling rate has also been reviewed. It was observed that when heating was on resistance dropped exponentially and on heating off mode the resistance followed almost same exponential path with respect to

a

the cooling rate (Figs. 15 and 16). The senor has high sensitivity even for surface as well as point sensing. The sensor was tested with a hot needle and fast response was obtained. The electrical resistivity of synthesised POMANI and POMANIMn3O4 nanocomposites was compared with other reported methods. The electrical resistivity of prepared POMANI

b

Fig. 23. Comparison of Power dissipation w.r.t temperature using different linearization parameters (a) T2-ES-H-GP (b) T3-ES-H-GP.

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A. Kumar et al. / Materials Chemistry and Physics xxx (2015) 1e13

(85  104 U cm) is lesser than that of PANIs synthesised by other methods (2.1  107 U cm) [30], where as higher than few (3.3  104 U cm to 5  104 U cm) [45], 4.5e0.3 U cm [46]. However the POMANI-Mn3O4 nanocomposites showed resistivity in the range of 10.6  104 U cm- 66.5  104 U cm whereas Singla et al. reported PANI-Mn3O4 nanocomposites showed resistivity in the range of 2.63  104 U cm to 6.75  104 U cm [24]. Thus from these results it is suggested that degree of doping and composition of adduct can adjust the resistivity of POMANI. 2.10. Linearization of NTC as temperature sensor NTC thermistors find extensive usage in measurement and control of temperature and other physical variables. The nonlinear RT characteristics of NTC thermistor is a challenge for wide range temperature sensors. Non linear characteristics restrict the utility of these thermistors although they have high sensitivity for temperature sensing. In this study a low cost linearising scheme for above fabricated thin film based NTC thermistors have been reported using an analog circuit. The general linearization scheme for NTC thermistor is given in Fig. 17, where Temperature (T) is the input signal. Output signal V(T) and Vout are the function of temperature. It is desired that the relationship between output V(T) and input T(temperature) should be linear for a certain range of temperature, which is desired to be measured in terms of voltage. The linearising circuit has parallel resistance (Rp), series resistance (Rs) and operational amplifier (OP-AMP) (Figs. 18 and 19a,b). Operational amplifier with low bias current of the range mA, high input impedance (>10 MU) and input supply of 10e15 V can be used to amplify the V(T) signal for increasing the sensitivity and operating range of the thermistor. The gain of the operational amplifier can be adjusted according to the designed values of R1 and R2. The R2/R1 ratio was selected 0.2 for the proposed linearization circuit. To design a linearization scheme parallel, series resistance and inflection temperature (TI) is derived for better characteristics of temperature sensor. The inflection temperature TI, is a temperature at which the flattest possible transfer function V (T) and Vin is obtained, preferably in the middle of temperature range of operation. V (T)/Vin gives the resistor ratio m, at temperature TI. RS and RP can be determined by setting the second derivative of V(T)/Vin [47].

 b  2$T 2$b$ð1  mÞ

 RS ¼ RT ðTI Þ$ RP ¼

RS $RT ðTI Þ$ðb  2$TI Þ RS $ðb þ 2$TI Þ  RT ðTI Þ$ðb  2$TI Þ

mCritical ¼

ðb  2$TI Þ 2$b

The Vin is 5 V in each case. The value of TI is 135  C and m ¼ 0.49 for the sample T2-ES-H-GP (Fig. 20a). Similarly for the sample T3ES-H-GP TI is 135  C and m ¼ 0.48 (Fig. 20b). The proposed circuit exhibit acceptable linearity (Fig. 20a, b). However slight variations occurred due to high values of RS and RP used in the linearsing circuit can be ratified by using operational amplifier (OP-AMP) in the circuit (Fig. 21a, b). By using this linearising scheme the useful temperature measurement range of the temperature sensor can be increased under practical conditions. The proposed circuit has advantages like simple configuration, good linearity and lower cost. A number of these above fabricated temperature sensors can be interfaced to a microcontroller for monitoring and controlling the temperature of various devices simultaneously. 2.11. Power dissipation in fabricated thermistor Heat dissipation occurs when a current flows through the resistors or electronic device. This concept of heating of the device is known as self-heating. The self-heating effect depends on the load

applied, thermal dissipation factor and the geometry of the thermistors itself [48]. The self-heating effect of NTC thermistors can be obtained by Pdiss ¼ VI, where V and I represents the instantaneous voltage and current values across the NTC respectively. Further, Pdiss ¼ dH/dt ¼ dth(T  TA) þ CthdT/dt, where dH/dt represents the rate of change of stored thermal energy of the thermistor. Cth and dth represent the heat capacity and dissipation factor of NTC thermistors respectively. TA represents the ambient temperature. dT/dt represents rate of change of temperature with time. The value of Pdiss is very meager in this study due to very small current flowing through the thermistor (Fig. 22a,b). The Pdiss by using RS and RP in the linearization circuit was less as compared to using RS alone (Fig. 23a,b). 3. Conclusion In summary thin films of HCl doped POMANI-Mn3O4 nanocomposite material has been fabricated successfully on a glass substrate for thermistor use. The RT data was studied and further analysed by fitting into a linearsing circuit. These fabricated thermistors after linearisation can work as temperature sensors, which may be used for incorporation in various devices for overheating protection, controlling temperature, temperature sensing gloves for prosthetic limbs etc. Thus the fabricated temperature sensors may be a promising device applied use. Acknowledgments Authors are thankful to Panjab University, Chandigarh, CSIO, Chandigarh and NIT, Jalandhar for providing research facilities. References [1] H.R. Zhao, D.P. Li, X.M. Ren, Y. Songand, W.-Q. Jin, J. Am. Chem. Soc. 132 (2009) 18. [2] T. Cao, L. Wei, S. Yang, M. Zhang, C. Huang, W. Cao, Langmuir 18 (2002) 750. [3] J. Han, Y. Liu, R. Guo, J. Appl. Polym. Sci. 112 (2009) 1244. [4] K.R. Reddy, B.C. Sin, K.S. Ryu, J.C. Kim, H. Chung, Y. Lee, Synth. Met. 159 (2009) 595. [5] A. Choudhury, Mater. Chem. Phys. 130 (2011) 231. [6] X.Y. Zhang, W.J. Goux, S.K. Manohar, J. Am. Chem. Soc. 126 (2004) 4502. [7] K.R. Reddy, K.P. Lee, Y. Lee, A.I. Gopalan, Mater. Lett. 62 (2008) 1815. [8] S.H. Ding, X.F. Lu, J.N. Zheng, W.J. Zhang, Mater. Sci. Eng. B 135 (2006) 10. [9] V. Bavastrello, S. Carrara, M.K. Ram, C. Nicolini, Langmuir 20 (2004) 969. [10] K. Mallick, M.J. Witcomb, A. Dinsmore, M.S. Scurrell, Langmuir 21 (2005) 7964. [11] M. Wan, J. Yang, Synth. Met. 73 (1995) 201. [12] L. Sun, Q. Li, W. Wang, J. Pang, J. Zhai, Appl. Surf. Sci. 257 (2011) 10218. [13] B.H. Shambharkar, S.S. Umare, Mater. Sci. Eng. B 175 (2010) 120. [14] X.H. Huang, J.P. Tu, X.H. Xia, X.L. Wang, J.Y. Xiang, Electrochem. Commun. 10 (2008) 1288. [15] J.G. Deng, X.B. Ding, Y.X. Peng, Polymer 43 (2002) 2179. [16] J. Zhang, A.L. Barker, D. Mandler, P.R. Unwin, J. Am. Chem. Soc. 125 (2003) 9312. [17] G.V. Kurlyandskaya, J. Cunanan, S.M. Bhagat, J.C. Aphesteguy, S.E. Jacobo, J. Phys. Chem. Solids 28 (2007) 1527. [18] S.K. Pillalamarri, F.D. Blum, A.T. Tokuhiro, M.F. Bertino, Chem. Mater. 17 (2005) 5941. [19] J. Gao, M.A. Lowe, H.C.D. Abrunpa, Chem. Mater. 23 (2011) 3223. [20] L. Zhang, Q. Zhou, Z. Liu, et al., Chem. Mater. 21 (2009) 5066. [21] Y. Tan, L. Meng, Q. Peng, et al., Chem. Commun. 47 (2011) 1172. [22] Y.F. Han, F. Chen, Z. Zhong, et al., J. Phys. Chem. B 110 (2006) 24450. [23] J.H. Kim, K.H. Lee, L.J. Overzet, et al., Nano Lett. 11 (2011) 2611. [24] K. Majid, S. Awasthi, M.L. Singla, Sens. Actuators A Phys. 135 (2007) 113. [25] P.S. Kohli, Pooja Devi, Pramod Reddy, K.K. Raina, M.L. Singla, J. Mater. Sci. Mater. Electron 23 (2012) 1891. ~es, J.M.M. Cordeiro, Mat. Res. [26] M.A. de M.Cordeiro, D. Gonçalves, L.O. de S.Bulho 8 (2005) 5. [27] S. Chaudhari, A.B. Gaikwad, P.P. Patil, J. Coat. Technol. Res. 7 (2010) 119. €zeri, Phys. B Condens. Matter 406 (2011) [28] Z. Durmus, A. Baykal, H. Kavas, H. So 1114. [29] F. Yakuphanoglu, B.F. Senkal, J. Phys. Chem. C 11 (2007) 1840. [30] F. Yakuphanoglu, B.F. Senkal, J. Electron. Mater. 37 (2008) 930. [31] M.L. Singla, S. Awasthi, A. Srivastava, D.V.S. Jain, Sens. Actuators A Phys. 136 (2007) 604. [32] S. Sun, H. Zeng, J. Am. Chem. Soc. 124 (2002) 8204.

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Please cite this article in press as: A. Kumar, et al., POMANI-Mn3O4 based thin film NTC thermistor and its linearization for overheating protection sensor, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.042