O Mrooz, P Demchenko, R Gladyshevskii - Phase composition crystal structure and electrical properties of thermo-modified cuo 1nio 8coo 2mnx 9o4electroceramics - страница 1

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ВІСНИК ЛЬВІВ. УН-ТУ

Серія хім. 2007. Bun. 48. Ч. 1. С. 229-235

VISNYK LVIV UNIV. Ser. Khim. 2007. No 48. Part 1. P. 229-235

УДК 621.315.592:537.312.6 + 548.736

PHASE COMPOSITION, CRYSTAL STRUCTURE AND ELECTRICAL PROPERTIES OF THERMO-MODIFIED Cuo.1Nio.8Coo.2Mnx.9O4

ELECTROCERAMICS

O. Mrooz x, P. Demchenko 2, R. Gladyshevskii 2, I. Hadzaman x, O. Shpotyuk x, L. Akselrud 2, S. Volkov 3, V. Pekhnyo 3

1 Scientific Research Company "Carat", Stryjska Str., 202, 79031 Lviv, Ukraine

2 Ivan Franko National University of Lviv, Kyryla & Mefodiya Str., 6, 79005 Lviv, Ukraine e-mail: demchenko @franko. lviv. ua

3 V.I. Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Palladin Avenue, 32/34, 03142 Kyiv, Ukraine

The phase composition and crystal structure of Cu01Ni08Co02Mn19O4 ceramics fabricated under different high-temperature sintering regimes were studied by X-ray diffraction analysis. The seven batches of ceramics were prepared by isothermal sintering in air at Ts = 1200 °C with variable dwell times and heating (cooling) rates. The investigated ceramics consist of a mixture of the main

cubic spinel phase (space group Fd 3 m) and the additional phase - solid solution based on NiO phase

with the NaCl-type structure (space group Fm 3 m). The XRD results (the fractions of the additional phase in ceramics, the lattice and atomic parameters of both present phases), as well as the electrical properties of ceramics (the conductivity 025 and the values of constant B25/85) are considered in dependence on the technological regimes of their preparation.

Key words: ceramics, thermoelectic materials, crystal structure.

The 3d transition-metal manganites for negative temperature coefficient (NTC) thermistor applications have found wide use in diverse fields, as consumer electronics, biomedicine, industrial process control, environment monitoring [1-3]. In the previous publication we reported the sintering behaviour of Cu01Ni08Co02Mn19O4 manganite electroceramics at different sintering temperatures Ts [4]. Particularly, the single-phase cubic spinel ceramics was obtained at low Ts = 920 °C, and the decomposed ceramics, which contained the residual tetragonal spinel phase with modified composition and the separated additional phase - solid solution (SS) based on NiO, was formed at high Ts = 1170°C. The present work was performed to study in details the possibilities of thermal modification of high-temperature ceramics by changing technological regimes of their preparation. Preliminary results are given in [5].

© Mrooz O., Demchenko P., Gladyshevskii R. et al., 2007

The traditional ceramics technique was used for the preparation of the samples of CutuNia8Coch2MnL9O4 composition from the mixture of CuCO3Cu(OH)2 and MCO3x xmM(OH)2nH2O (M=Ni, Co and Mn) reagents. After calcinations at 700 °C, the products were wet-milled, dried, blended with an organic binder and pressed into discs (11.5 mm in diameter) under -100 MPa pressure. The seven batches of experimental samples were fabricated using different regimes of isothermal sintering at Ts = 1200 °C with variable dwell times (1, 2 or 3 h), heating rates up to Ts and cooling rates down from Ts (Table 1). In details, for batch No. 1 ceramics: pressed blanks were heated with a temperature gradient of +100 °/h up to 600 °C (the same heating rate up to 600 °C was applied for all the prepared batches), heated with a rate of +200 °/h up to Ts (1200 °C), isothermally sintered for 1 h at Ts, cooled with a rate of -100 °/h down to 850 °C, finally cooled with a rate of - -750 °C/h down to 100 °C and removed from the furnace (the same cooling procedure from 850 °C was applied for all batches).

The thermally modified ceramics were characterized by X-ray diffraction (XRD) technique. The powder patterns were recorded at room temperature using a powder diffractometer of type HZG-4a with FeKa radiation. The measurements were carried out in 29 steps of 0.02°-0.05° with variable scanning rate, depending on the sample quality. The profile analysis was performed using pseudo-Voigt profile function. The lattice parameters and crystal structures of the phases were refined using the Rietveld method with the program FULLPROF.2k (version 3.70) [6] from the WinPLOTR software [7]. Micro-structural analysis, i.e. determination of the average apparent size and average max strain of grains, was performed using the profile fitting procedure with Voigt approximation maintained in the WinPLOTR software [7].

The electrical resistance R was measured for ceramic discs with pre-fired Ag-electrodes in the temperature range from 25 to 170 °C with digital multimeter, using temperature chamber HPS 222 and precise digital thermometer. The electrical conductivity <x25 was calculated from R25 value, measured at T=25±0.01 °C, using the results of geometry determination for 3 disc samples. The constant B25/85 was calculated according to

the equation: #25/85=1778-ln(R25/R85).

As expected, a pattern characteristic of a two-phase sample is obtained for all the investigated batches of ceramics. Reflections of the additional NiO phase besides the reflections of the main spinel phase are observed. The fractions of the additional phase in ceramics, the lattice and atomic parameters of both present phases were determined from the Rietveld refinements. The summarized results of XRD analysis for the investigated ceramics are given in Table 1. The results of refinement of the composition for spinel phase, microstructural parameters and electrical properties for the ceramics are given in Table 2. The full data of the crystal structure refinement are given on example of batch No 4 ceramics (Tables 3-4, Fig. 1). During the refinement of the crystal structures it was empirically established that Co and Cu cations tend to occupy the tetrahedral sites, Ni cations the octahedral sites, while Mn cations occupy both tetrahedral and octahedral sites in the cubic spinel structure. So, the investigated ceramics consist of a mixture of the main cubic spinel phase (space group Fd 3 m) and the additional phase - SS based on NiO phase with the NaCl-type structure (space group Fm 3 m). The unexpected result is the existence of cubic modification of spinel phase in ceramics. Our preceding data indicated the tetragonal spinel phase (space group I41/amd) at Ts = 1170 °C [4]. In fact, slight variations of synthesis conditions essentially modify ceramics structure. The refinements of spinel crystal structure according to tetragonal model were less satisfactory in view of higher agreement factors, thermal parameters, standard deviations, etc.

Table 1

Phase composition and results of crystal structure refinement for thermally modified Cu01Ni08Co02Mn19O4 ceramics in dependence on technological regimes of their preparation

Batch No

Regimes of sintering

Present phases

Lattice parameter a, A

Agreement factors (%)

RBragg,            Rwp, X

1

+200 °C/h tTsa; Ts, 1 h;

Spinel

8.4156(7)

3.93

4.04

4.66

1.26

 

-100 °C/h 4850 °C

SS NiOb

4.1959(4)

3.33

1.91

 

 

 

 

15.0(6)c

 

 

 

 

 

2

+200 °C/h YTs; Ts, 2 h;

Spinel

8.4170(7)

8.57

7.99

5.60

1.43

 

-100 °C/h 4-850 °C

SS NiO

4.1960(4)

5.61

3.63

 

 

 

+200 °C/h YTs; Ts, 3 h;

14.6(7)

 

 

 

 

 

3

 

Spinel

8.4195(4)

4.25

4.10

3.33

1.47

 

-100 °C/h 4850 °C

SS NiO

4.1969(3)

3.30

2.24

 

 

 

 

15.8(5)

 

 

 

 

 

4

+750 °C/h YTs; Ts, 1 h;

Spinel

8.4176(6)

4.26

4.20

3.45

1.43

 

-100 °C/h 4850 °C

SS NiO

4.1963(3)

2.61

1.43

 

 

 

+750 °C/h YTs; Ts, 2 h;

15.1(5)

 

 

 

 

 

5

 

Spinel

8.4208(7)

11.6

12.5

3.36

2.31

 

-100 °C/h 4850 °C

SS NiO

4.1966(4)

4.91

3.19

 

 

 

 

15.8(6)

 

 

 

 

 

6

+750 °C/h YTs; Ts, 3 h;

Spinel

8.4203(5)

3.43

3.83

2.42

1.48

 

-100 °C/h 4850 °C

SS NiO

4.1971(3)

4.93

2.87

 

 

 

 

15.9(5)

 

 

 

 

 

7

+200 °C/h YTs; Ts, 1 h;

Spinel

8.4096(9)

4.74

3.42

4.83

1.41

 

-35 °C/h 4850 °C

SS NiO

4.1915(7)

8.44

5.91

 

 

 

 

12.1(1.0)

 

 

 

 

 

a Isothermal sintering temperature Ts = 1200 °C, for more explanation see preparation part;

b solid solution based on NiO-compound;

c fraction (in weight %) of the separated additional phase.

Analysis of the obtained values of the lattice parameter of the additional phase a - 4.195 A leads to the conclusion, that the additional phase is a Ni1-xMnxO (NiO-MnO) solid solution as the a value is intermediate between the values of the lattice parameters for the NiO and MnO binary phases (a = 4.177 A and a = 4.488 A, respectively) [8-9]. The lowest value of lattice parameter of additional phase is observed for slowly cooled batch No 7 ceramics. This suggests the lowest content of Mn in Ni1-xMnxO SS, however, according to the composition refinement, the lowest content was observed for the batch No 5. Frac­tion of the additional phase in ceramics, as well as lattice parameters of both present phases exhibit rising tendency (taking into account the precision of experiments) with the increase of dwell times at Ts (Table 1, ceramic batches No 1—No 2—No 3, No 4—No 5 —No 6). Effect of dwell times can be explained by the enhancement of spinel decomposition (de-oxidation) reactions at Ts, leading to separation of additional phase [2, 4]. The correspon­dent cr25 values of decomposed ceramics, which depend on intrinsic conductivity of each phase and their ratio, show evidence of decrease (Fig. 2). This is mainly due to an increase of the fraction of additional phase with lower intrinsic conductivity, compared with the spi­nel phase [1]. The lowest content of the additional phase and the highest cr25 value is obtained for slowly cooled after Ts batch No 7 ceramics, most probably is a result of rever­sal (re-oxidation) reactions in ceramics [10]. The investigated ceramics show typical for thermistor materials semiconductor behavior with high values of constant B25/85 > 3300 K.

Table 2

Results of refinement of the composition for spinel phase, microstructural parameters and electrical properties (conductivity a25 and constant B25/85) for thermally modified Cu01Ni08Co02Mn19O4 ceramics

Batch

No

Refined composition of the spinel phase"

Average apparent size of grains, A

Average max strain in grains, %

025,

Ohm-1 m-1

K

1

Cu0.1Ni0.72(2)Co0.2MnL98(2)O4

260 (±153)

3.8(2)

0.28

3330

2

Cu0.1Ni0.69(3)Co0.2Mn2.01(3)O4

291 (±164)

3.4(1)

0.24

3350

3

Cu0.1Ni0.81(2)Co0.2Mn1.89(2)O4

310 (±169)

3.1(1)

0.22

3400

4

Cu0.1Ni0.63(2)Co0.2Mn2.07(2)O4

291 (±162)

3.32(8)

0.21

3390

5

Cu0.1Ni0.46(7)Co0.2Mn2.24(7)O4

259 (±121)

3.8(2)

0.24

3400

6

Cu0.1Ni0.76(2)Co0.2Mn1.94(2)O4

243 (±128)

3.5(±1.1)

0.21

3415

7

Cu0.1Ni0.63(3)Co0.2Mn2.07(3)O4

293 (±150)

3.3(1)

0.34

3320

a The proposed composition is empirical in view of small difference of scattering amplitudes for the Ni and Mn.

Table 3

Atomic coordinates, atomic displacement parameters and site occupancy factors (s.o.f.) for spinel phase of batch No 4 Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics

(cubic spinel, space group Fd3 m) a

Atom

Site

x

y

z

Biso'> A

s.o.f.

Ni

8a

1/8

1/8

1/8

0.5(2)

0.022(2)

Mn

8a

1/8

1/8

1/8

0.5(2)

0.678(2)

Co

8a

1/8

1/8

1/8

0.5(2)

0.2

Cu

8a

1/8

1/8

1/8

0.5(2)

0.1

Ni

16d

1/2

1/2

1/2

0.3(2)

0.305(2)

Mn

16d

1/2

1/2

1/2

0.3(2)

0.695(2)

O

32e

0.2623(4)

0.2623(4)

0.2623(4)

0.5(2)

1

a The proposed distribution of the transition metals is hypothetical in view of small difference of their scattering amplitudes.

Table 4

Interatomic distances (S A) and coordination numbers (C.N.) of atoms for spinel phase of batch No 4 Cu01Ni08Co02Mn19O4 ceramic sample (cubic spinel, space group Fd 3 m)

Atoms

S, A

C.N.

Atoms

S, A C.N.

(Ni, Mn, Co,Cu) - 4O

2.002(3)

4

O - 1(Ni, Mn, Co, Cu)

2.002(3) 4

(Ni, Mn) - 6O

2.006(3)

6

- 3(Ni, Mn)

2.006(3)

Fig. 1. Observed and calculated XRD profiles for batch No 4 Cu01Ni08Co02Mn19O4 ceramics (Upper bars: cubic spinel, space group Fd 3 m; lower bars: SS based

on NiO-compound, space group Fm 3 m)

і--1--1--1--1--1--г

h_I_і_I_і_I_і_I_і_I_і_I_і_I_d

2.2 2.4 2.6 2.8 3.0 3.2 3.4

10[1]/T, K"[2]

Fig. 2. The electrical conductivity (7 vs. 10[3]-T[4] for Cu01Ni08Co02Mn19O4 ceramics of batches No 1-7 prepared at different sintering conditions

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