G. HARDAL, B. Y. PRICE: INFLUENCE OF NANO-SIZED COBALT OXIDE ADDITIONS ON THE STRUCTURAL ...
923–928
INFLUENCE OF NANO-SIZED COBALT OXIDE ADDITIONS ON
THE STRUCTURAL AND ELECTRICAL PROPERTIES OF
NICKEL-MANGANITE-BASED NTC THERMISTORS
VPLIV DODATKA NANODELCEV KOBALTOVEGA OKSIDA NA
ZGRADBO IN ELEKTRI^NE LASTNOSTI NTC TERMISTORJEV
NA OSNOVI NIKLJEVEGA MANGANITA
Gökhan Hardal, Berat Yüksel Price
Istanbul University, Engineering Faculty, Metallurgical and Materials Engineering Department, Avcýlar, Istanbul, Turkey
berat@istanbul.edu.tr
Prejem rokopisa – received: 2015-07-15; sprejem za objavo – accepted for publication: 2015-12-15
doi:10.17222/mit.2015.228
The structural and electrical properties of NiMn2O4 and Ni0.5CoxMn2.5-xO4 (where x = 0.5, 0.8 and 1.1) NTC thermistors have
been investigated. The samples, prepared by conventional ceramic processing techniques, were calcinated at 900 °C for 2 h and
then sintered at 1100 °C and 1200 °C for 5 h. The cubic spinel phase was observed by XRD analysis in the NiMn2O4 and
Ni0.5Co0.8Mn1.7O4 samples sintered at 1100 °C for 5 h. The sintering at 1200 °C resulted in much denser microstructures with a
larger grain size. The room-temperature electrical resistivity (25) and material constant (B) value of the NiMn2O4 sample
sintered at 1100 °C were 7710  cm and 3930 K, respectively. The electrical resistivity of the samples decreased significantly
with the addition of Co3O4. The B25/85 values of the Ni0.5CoxMn2.5-xO4 (where x = 0.5, 0.8 and 1.1) samples sintered at 1100 °C
were found to be 3820 K, 3525 K and 3270 K, respectively.
Keywords: cobalt oxide, electrical properties, microstructure, NTC thermistor
Preiskovana je bila zgradba in elektri~ne lastnosti NiMn2O4 in Ni0.5CoxMn2.5-xO4 (kjer je x = 0,5, 0,8 in 1,1) NTC termistorjev.
Vzorci, pripravljeni po obi~ajni tehniki priprave keramike, so bil kalcinirani 2 h na 900 °C in potem 5 h sintrani na 1100 °C in
1200 °C. Kubi~na {pinelna faza je bila opa`ena pri XRD-analizi, v vzorcih NiMn2O4 in Ni0.5Co0.8Mn1.7O4, sintranih 5 h na 1100 °C.
Sintranje na 1200 °C je povzro~ilo mnogo bolj gosto mikrostrukturo z ve~jimi zrni. Vrednosti za elektri~no upornost pri sobni
temperaturi (25) in materialne konstante (B) vzorca NiMn2O4, sintranega na 1100 °C, sta bili 7710  cm in 3930 K. Elektri~na
upornost vzorcev se je ob~utno zmanj{ala po dodatku Co3O4. Vrednosti B25/85 pri vzorcih Ni0.5CoxMn2.5-xO4 (kjer je bil x = 0,5,
0,8 in 1,1) sintranih na 1100 °C so bile: 3820 K, 3525 K in 3270 K.
Klju~ne besede: kobaltov oksid, elektri~ne lastnosti, mikrostruktura, NTC termistor
1 INTRODUCTION
Sensors for monitoring and controlling temperature
are very important, not only in our daily life but also in
many industrial and laboratory applications such as aero-
space and automotive industries, circuit compensation,
cryogenic systems etc.1,2 NTC thermistors are useful for
precision temperature measurements as their resistance
decreases with increasing temperature.3 The most exten-
sively used negative temperature coefficient (NTC) ther-
mistor materials are nickel-manganite-based semicon-
ducting materials which exhibit the spinel-type crystal
structure with the general formula AB2O4.4 In the spinel
structure, there are two sites available for the cations,
i.e., the tetrahedral site, A-site, and the octahedral site,
B-site. The distribution of the ions over the sites is as
follows: Mn3+ will predominantly occupy the B-site,
while Mn2+ will be placed on the A-site and the majority
Ni2+ will go to the B-site.5 The electrical resistivity, , of
NTC thermistors varies exponentially with temperature,
T, by the well-known Arrhenius equation  = o exp
(B/T), where o is the resistivity of the material at infinite
temperature and B is a constant, which is a measure of
the sensitivity of the materials over a given temperature.6
The material constant B, can be calculated using
Equation (1):
B
T T
T
T
1
2
1 2
1 2
1 1
=
−
−
ln ln 
(1)
where 1 and 2 are the electrical resistivity at tempe-
ratures T1 and T2, respectively. The activation energy Ea
can also found by the equation B = Ea/kB, where kB is
the Boltzmann constant.7
The electrical properties of nickel-manganite-based
NTC thermistors closely depend on the ratio of the com-
positions (type and amount of additives), initial particle
size of raw materials and processing conditions (selected
synthesis method, calcination and sintering temperature,
sintering time etc.). Attainment of high-density, con-
trolled-grain-size microstructures and appropriate dimen-
sional designs are important factors in good sensor
design.8 Previous studies have been focused on the effect
Materiali in tehnologije / Materials and technology 50 (2016) 6, 923–928 923
UDK 661.873:62-911-026.772:669.24:549.521.61 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(6)923(2016)
of composition ratios and different production routes on
the electrical properties of various metal-oxide-doped
NTC thermistors. In this study, nano-sized cobalt-oxide-
added, nickel-manganite-based NTC thermistors were
fabricated by the solid-state reaction method, the effect
of dopant concentration and sintering temperature on the
structural and electrical properties of NTC materials
were investigated.
2 EXPERIMENTAL PART
The particle size of Co3O4 powder was less than 50
nm, purchased from Sigma-Aldrich. NiO (99 % purity,
Alfa Aesar), Co3O4 (99.5 % purity, Sigma-Aldrich) and
Mn2O3 (99 % purity, Sigma-Aldrich) powders were
weighed according to the compositions of NiMn2O4 and
Ni0.5CoxMn2.5-xO4 (where x= 0.5, 0.8 and 1.1). The molar
ratios of these compositions are given in Table 1. The
raw powder mixture was ball-milled using ZrO2 balls as
a grinding media with ethyl alcohol in a jar for 5 h. The
obtained slurries were dried and powders were calcinated
at 900 °C for 2 h. The powders were pressed to form
disc-shaped specimens and then sintered at 1100 and
1200 °C for 5 h employing a 360 °C/h heating rate in the
air and then cooled naturally in the furnace. The bulk
density (, g cm–3) of the sintered samples was calculated
from their weights and dimensions. The phases in the
sintered samples were determined by X-ray diffraction
(XRD, Rigaku D/Max-2200/PC) analysis using Cu-K
radiation at 60 kV/2 kW.
Table 1: Molar ratio of Ni, Mn and Co in all compositions
Tabela 1: Molarno razmerje Ni, Mn in Co v vseh spojinah
Composition
code
Ni
(moles)
Mn
(moles)
Co
(moles)
A1 1 2 -
A2 0.5 2 0.5
A6 0.5 1.7 0.8
A10 0.5 1.4 1.1
In order to calculate the lattice parameter of the sam-
ples Equation (2) was applied:
a d h k l= + +2 2 2 (2)
where h, k and l are the miller indices, a (nm) is the
lattice parameter of cubic structure, d is the interplanar
spacing of the peaks corresponding to (311).
The volume of the unit cell (V, nm3) for the cubic sys-
tem is obtained from Equation (3):
V = a3 (3)
The average values of the crystallite size (D, nm) of
the samples were calculated by means of X-ray line
broadening method, using the Debye Scherrer formula:
D =
0 9.
cos


 
(4)
where 0.9 is a constant related to crystallite shape, 
 is
the X-ray radiation wavelength in nanometres (nm),  is
the full width at half-maximum (FWHM) of the peaks
corresponding to (311) and  is Bragg’s angle.9 The
value of  from the 2 axis of the diffraction profile
must be in radians.10 The microstructure of the samples
was observed using scanning electron microscopy
(SEM, JEOL, JSM 5600) on fracture surfaces. The sin-
tered samples were coated with silver paste to form
electrodes. The electrical resistance was measured in a
temperature programmable furnace between 25 °C and
85 °C in steps of 0.1 °C. The material constant, B, the
activation energy, Ea, and the sensitivity coefficient, ,
values were calculated for the NTC thermistors.
3 RESULTS
The XRD patterns of the NiMn2O4 and
Ni0.5Co0.8Mn1.7O4 samples sintered at 1100 °C for 5 h are
given in Figure 1. The calculated lattice parameter,
unit-cell volume, , peak position corresponding to (311)
and crystallite size of the samples are given in Table 2.
The XRD analysis of these samples demonstrated only
the cubic spinel phase (PDF No: 71-0852). A compa-
rison of the XRD patterns of the sintered samples and the
data is given in Table 2, the diffraction peaks of the
Ni0.5Co0.8Mn1.7O4 sample shifted to higher 2 angles, and
G. HARDAL, B. Y. PRICE: INFLUENCE OF NANO-SIZED COBALT OXIDE ADDITIONS ON THE STRUCTURAL ...
924 Materiali in tehnologije / Materials and technology 50 (2016) 6, 923–928
Figure 1: XRD patterns of NiMn2O4 (A1) and Ni0.5Co0.8Mn1.7O4
(A6) samples in the 2 range 20–65°
Slika 1: Rentgenogram vzorcev NiMn2O4 (A1) in Ni0.5Co0.8Mn1.7O4
(A6) v podro~ju 2 med 20° in 65°
as a result the lattice parameters and the unit-cell volume
decreased. The value of  increased to 0.7692° and the
value of average crystallite size decreased to 10.86 nm.
Table 2: The lattice parameter, unit-cell volume, , peak position and
crystallite size of samples sintered at 1100 °C
Tabela 2: Parameter mre`e, prostornina enotne celice, , polo`aj
vrhov in velikost kristalnih zrn vzorcev sintranih na 1100 °C
Composition a(L)
V
(L3)

(311)
(o)
2
(311)
(o)
D
(nm)
NiMn2O4 (A1) 0.8365 0.585 0.2538 35.6 32.87
Ni0.5Co0.8Mn1.7O4 (A6) 0.8273 0.566 0.7692 36 10.86
The bulk densities of the sintered NiMn2O4 and
Ni0.5CoxMn2.5-xO4 samples are shown in Table 3. The
bulk density of the A1 sample sintered at 1100 °C was
found to be 4.23 g cm–3 and it increased to 4.78 g cm-3
when the sample was sintered at 1200 °C. The bulk
density of the samples decreased first and then increased
with the addition of Co3O4.
Table 3: The bulk density of samples sintered at 1100 °C and 1200 °C
for 5 h
Tabela 3: Gostota osnove po 5 urnem sintranju na 1100 °C in 1200 °C
Composition code
 (g cm–3)
1100 °C 1200 °C
A1 4.23 4.78
A2 4.05 4.43
A6 4.27 4.63
A10 4.30 4.72
The SEM micrographs of the A1, A2, A6, A10
samples sintered at 1100 and 1200 °C for 5 h are given in
Figure 2. It can be seen in this figure that all the samples
sintered at 1100 °C had a fine-grained microstructure
with most of the pores at the grain boundaries. The grain
size of A1 was larger relative to the A2, A6 and A10
samples sintered at 1100 °C. When the sintering
temperature was increased to 1200 °C, all the samples
had a much denser microstructure and larger grains with
a number of small grains on their surface. In addition,
G. HARDAL, B. Y. PRICE: INFLUENCE OF NANO-SIZED COBALT OXIDE ADDITIONS ON THE STRUCTURAL ...
Materiali in tehnologije / Materials and technology 50 (2016) 6, 923–928 925
Figure 2: SEM micrographs of sintered samples: A1 a) 1100 °C, b) 1200 °C, A2 c) 1100 °C, d) 1200 °C, A6 e) 1100 °C, f) 1200 °C, A10 g) 1100 °C,
h) 1200 °C
Slika 2: SEM-posnetki sintranih vzorcev: A1 a) 1100 °C, b) 1200 °C, A2 c) 1100 °C, d) 1200 °C, A6 e) 1100 °C, f) 1200 °C, A10 g) 1100 °C, h)
1200 °C
the A10 sample had much bigger grains in comparison
with the A2 and A6 samples sintered at 1200 °C.
The plot of resistivity versus Co content (moles) and
the plots of log  versus 1000/T are given in Figures 3a
and 3b for all the sintered samples. The plots of log 
versus 1000/T exhibited a linear dependence in the range
25–85 °C, indicating semiconducting NTC thermistor
characteristics. The activation energy, the sensitivity
coefficient and the material constant can also be calcu-
lated from this plot. The room-temperature electrical
resistances, R25, of the A1, A2, A6 and A10 samples sin-
tered at 1100 °C were 1487, 360, 167 and 107 , res-
pectively. For the same sintering temperature, the room
temperature electrical resistivity of the A1, A2, A6 and
A10 samples were calculated as 7710, 1870, 890 and
590  cm, respectively.
The relationship between the B25/85 constant of the
samples and the increase in Co3O4 content is given in
Figure 4. The activation energy and sensitivity coeffi-
cient value of the samples is given in Table 4. With in-
creasing Co content, the B25/85 constant and activation
energy of the samples sintered at 1100 °C decreased
from 3930 K to 3270 K and from 0.338 to 0.282 eV, res-
pectively. A similar tendency was also seen in the A1,
A2 and A6 samples sintered at 1200 °C. For the A10
sample, the B25/85 constant and activation energy values
were found to be 3620 K and 0.312 eV, respectively. The
sensitivity coefficient value of all samples sintered at
1100 °C decreased from -4.426 to -3.683 %/K. When the
sintering temperature increased to 1200 °C, the sensi-
tivity coefficient value of all samples decreased from
-4.311 to -4.078 %/K.
Table 4: The activation energy and sensitivity coefficient of A1, A2,
A6 and A10 samples sintered at 1100 °C and 1200 °C for 5 h
Tabela 4: Aktivacijska energija in koeficient ob~utljivosti A1, A2, A6
in A10 vzorcev, sintranih 5 ur na 1100 °C in 1200 °C
Composition
code
Ea (eV) 25 (%/K)
1100 °C 1200 °C 1100 °C 1200 °C
A1 0.338 0.330 –4.426 –4.311
A2 0.329 0.313 –4.301 –4.097
A6 0.303 0.306 –3.970 –4.007
A10 0.282 0.312 –3.683 –4.078
4 DISCUSSION
The cubic spinel phase was found by XRD analysis
in NiMn2O4 and Ni0.5Co0.8Mn1.7O4 samples sintered at
1100 °C for 5 h. No secondary phase was found in these
samples. As it is well known from the binary phase dia-
gram of Mn-Ni-O, the spinel phase can only form when
the ratio of Ni/(Ni+Mn) is less than 0.35 at a calcination
temperature of 900 °C.11 The diffraction peaks of
Ni0.5Co0.8Mn1.7O4 samples shift to the higher 2 angles,
indicating a decrease in the lattice parameter with the
addition of Co3O4 due to the differences between the
ionic radii of the Mn and Co ions. Wu et al.12 reported
that the peak shift toward higher 2 angles with the in-
creasing of Co content indicates lattice constriction when
G. HARDAL, B. Y. PRICE: INFLUENCE OF NANO-SIZED COBALT OXIDE ADDITIONS ON THE STRUCTURAL ...
926 Materiali in tehnologije / Materials and technology 50 (2016) 6, 923–928
Figure 4: Effect of cobalt content on B25/85 value of A1, A2, A6 and
A10 samples sintered at 1100 and 1200 °C
Slika 4: Vpliv vsebnosti kobalta na vrednost B25/85 vzorcev A1, A2,
A6 in A10, sintranih na 1100 °C in na 1200 °C
Figure 3: a) The change of resistivity as a function of cobalt content,
b) the relationship between log  and 1000/T (K–1) for A1, A2, A6 and
A10 samples
Slika 3: a) Sprememba upornosti v odvisnosti od vsebnosti kobalta, b)
odvisnost med log  in 1000/T (K–1) pri vzorcih A1, A2, A6 in A10
Co substitutes Mn. It was also reported that the decrease
in the lattice parameter with the addition of Co should be
attributed to the fact that the ionic radius of Co2+
(0.072 nm) is smaller than that of Mn2+ (0.080 nm) for
occupying the tetrahedral sites and/or Co3+ (0.068 nm) is
smaller than Mn3+ (0.072 nm) for occupying the octa-
hedral sites.12,13 As can be seen in Figure 1, we observed
a significant broadening and a decrease of the diffraction
peak intensities in the XRD pattern of the
Ni0.5Co0.8Mn1.7O4 sample. This could be attributed to a
decrease in the average crystallite size as given in Table
2 due to the nano-size of the Co3O4 starting powder.
Savic et al. reported that the increase in the diffraction
peak width and the decrease in the peak intensities in the
XRD patterns are associated with a decreasing of the
crystallite size and an increasing of the strain.14
Since the desired NTC thermistor properties strongly
depend on the densification and grain size, we also inve-
stigated the microstructure properties of these samples.
The bulk density and grain size of the A1, A2, A6 and
A10 samples sintered at 1100 °C were less than the
samples sintered at 1200 °C. Smaller grains result in a
large number of grain boundaries, which act as scattering
centres for the flow of electrons and therefore higher
electrical resistivity values were obtained when the sam-
ples were sintered at 1100 °C.15 As expected, the in-
creasing of the sintering temperature gave rise to an
increase in the bulk density and the grain size of these
samples, thus the room-temperature resistivity of the
samples decreased. In addition, the cation distribution in
the octahedral and tetrahedral sites changes with an
increasing sintering temperature in the spinel ceramics.
The ratio of Mn3+/Mn4+ in the octahedral sites increases
with the increasing sintering temperature and also results
in a decrease in the resistivity.16 The electrical resistivity
of the samples decreased significantly with the increas-
ing of the Co3O4 content. Muralidharan et al. observed
that the resistivity and B-value decreased with the
increasing Co content. Their observation is expected as
the Co2+ and Co3+ ions can also occupy the octahedral
sites and contribute to the electrical conductivity along
with Mn3+/Mn4+ ion pairs in the octahedral sites. This
gives rise to a decrease of the resistivity, B-value and
temperature coefficient of resistance.2 This phenomenon
is prominent for all samples sintered at 1100 °C, while
the Co content was increasing in the samples. Similar
trends were also observed for the A1, A2 and A6 com-
positions when the samples were sintered at 1200 °C.
When the sintering temperature was increased from 1100
to 1200 °C for the A10 sample, similar resistivity values
were found, but the B-values and activation energy of
samples were nearly constant. Moreover, the lattice para-
meters were found to be 0.8365 nm and 0.8273 nm for
the NiMn2O4 and Ni0.5Co0.8Mn1.7O4 samples, respectively.
This may be due to the fact that the hopping distance of
the charge carriers becomes easier with the decreasing
lattice parameter, thus the resistivity value decreases.17
The sensitivity coefficient and the activation-energy
values of all the samples were found in the range –4.426
to –3.683 %/K and 0.282 eV to 0.338 eV, respectively. It
is well known that the desired sensitivity coefficient, 25,
and the activation energy of the NTC thermistors are in
the range –2.2 %/K to –5.5 %/K and 0.1–1.5 eV, respec-
tively.18,19
5 CONCLUSION
The influence of nano-sized cobalt oxide additions on
the structural and electrical properties of nickel-man-
ganite-based NTC thermistors was investigated. Our
results in this work indicate that a wide range of elec-
trical properties of nickel-manganite-based NTC ther-
mistors can be obtained by the addition of nano-sized
cobalt oxide. The particularly interesting finding in this
study demonstrated that the Ni0.5Co1.1Mn1.4O sample
sintered at 1200 °C for 5 h has a low electrical resistivity
and a high B-constant.
Acknowledgements
This study is supported by TÜBÝTAK (The Scientific
and Technical Research Council of Turkey), Project
number 3001-114M860. We would like to thank
TÜBÝTAK for its financial support.
6 REFERENCES
1 R. N. Jadhav, S. N. Mathad, V. Puri, Studies on the properties of
Ni0.6Cu0.4Mn2O4 NTC ceramic due to Fe doping, Ceramics
International, 38 (2012), 5181–5188, doi:10.1016/j.ceramint.2012.
03.024
2 M. N. Muralidharan, P. R. Rohini, E. K. Sunny, K. R. Dayas, A.
Seema, Effect of Cu and Fe addition on electrical properties of
Ni–Mn–Co–O NTC thermistor compositions, Ceramics
International, 38 (2012), 6481–6486, doi:10.1016/j.ceramint.2012.
05.025
3 J. Fraden, Handbook of Modern Sensors Physics, Designs, and
Applications, Springer Science+Business Media, New York 2010
4 E. D. Macklen, Thermistors, Electrochemical Publications Limited,
Scotland 1979
5 K. Park, Fabrication and Electrical Properties of Mn-Ni-Co-Cu-Si
Oxides Negative Temperature Coefficient Thermistors, Journal of the
American Ceramic Society, 88 (2005), 862–866, doi:10.1111/
j.1551-2916.2005.00170.x
6 J. Wang, J. Zhang, Effects of Mg substitution on microstructure and
electrical properties of NiMn2-xMgxO4 NTC ceramics, Journal of
Materials Research, 27 (2012), 928–931, doi:10.1557/jmr.2012.29
7 K. Park, S. J. Kim, J. G. Kim, S. Nahm, Structural and electrical pro-
perties of MgO-doped Mn1.4Ni1.2Co0.4–xMgxO4 (0 = x = 0.25) NTC
thermistors, Journal of the European Ceramic Society, 27 (2007),
2009–2016, doi:10.1016/j.jeurceramsoc.2006.07.002
8 M. Hosseini, B. Yasaei, Effect of Grain Size and Microstructures on
Resistivity of Mn-Co-Ni Thermistor, Ceramics International, 24
(1998), 543–545
9 S. Talam, S. R. Karumuri, N. Gunnam, Synthesis, Characterization,
and Spectroscopic Properties of ZnO Nanoparticles, ISRN Nanotech-
nology, 2012 (2012) 372505, 1–6, doi:10.5402/2012/372505
10 A. Monshi, M. R. Foroughi., M. R. Monshi, Modified Scherrer
Equation to Estimate More Accurately Nano-Crystallite Size Using
XRD, World Journal of Nano Science and Engineering, 2 (2012),
154–160, doi:10.4236/wjnse.2012.23020
G. HARDAL, B. Y. PRICE: INFLUENCE OF NANO-SIZED COBALT OXIDE ADDITIONS ON THE STRUCTURAL ...
Materiali in tehnologije / Materials and technology 50 (2016) 6, 923–928 927
11 M. N. Muralidharan, E. K. Sunny, K. R. Dayas, A. Seema, K. R.
Resmi, 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, 509 (2011), 9363–9371,
doi:10.1016/j.jallcom.2011.07.037
12 J. Wu, Z. Huang, Y. Hou, Y. Gao, J. Chu, Structural, electrical, and
magnetic properties of Mn2.52–xCoxNi0.48O4 films, Journal of Applied
Physics, 107 (2010) 053716, 1–6, doi:10.1063/1.3309780
13 R. D. Shannon, Revised Effective Ionic Radii and Systematic Studies
of Interatomic Distances in Halides and Chalcogenides, Acta
Crystallography, A32 (1976), 751–767
14 S. M. Savic, L. Mancic, K. Vojisavljevic, G. Stojanovic, Z. Branko-
vic, O. S. Aleksic, G. Brankovic, Microstructural and electrical
changes in nickel manganite powder induced by mechanical activa-
tion, Materials Research Bulletin, 46 (2011), 1065–1071,
doi:10.1016/j.materresbull.2011.03.008
15 S. A. Kanade, V. Puri, Electrical properties of thick-film NTC
thermistor composed of Ni0.8Co0.2Mn2O4 ceramic: Effect of inorganic
oxide binder, Materials Research Bulletin, 43 (2008), 819–824,
doi:10.1016/j.materresbull.2007.05.008
16 W. Kong, L. Chen, B. Gao, B. Zhang, P. Zhao, G. Ji, A. Chang, C.
Jiang, Fabrication and properties of Mn1.56Co0.96Ni0.48O4 free-stand-
ing ultrathin chips, Ceramics International, 40 (2014), 8405-8409,
doi:10.1016/j.ceramint.2014.01.049
17 C. Ma, Y. Liu, Y. Lu, H. Gao, H. Qian, J. Ding, Preparation and cha-
racterization of Ni0.6Mn2.4O4 NTC ceramics by solid-state coordina-
tion reaction, Journal of Materials Science: Materials in Electronics,
24 (2013), 5183–5188, doi:10.1007/s10854-013-1542-2
18 A. Feteira, Negative Temperature Coefficient Resistance (NTCR)
Ceramic Thermistors: An Industrial Perspective, Journal of the Ame-
rican Ceramic Society, 92 (2009), 967–983, doi:10.1111/j.1551-
2916.2009.02990.x
19 K. Park, I. H. Han, Effect of Cr2O3 addition on the microstructure
and electrical properties of Mn-Ni-Co oxides NTC thermistors, Jour-
nal of Electroceramics, 17 (2006), 1069–1073, doi:10.1007/s10832-
006-8317-6
G. HARDAL, B. Y. PRICE: INFLUENCE OF NANO-SIZED COBALT OXIDE ADDITIONS ON THE STRUCTURAL ...
928 Materiali in tehnologije / Materials and technology 50 (2016) 6, 923–928