Thermal synthesis of the ZnO-Bi2O3 pigments

Article abstract of DOI:10.1023/A:1010117812127

A zinc oxide pigment with an admixture of bismut oxide has been prepared as new yellow pigment for colouring of plastics and paints. The effect of the Bi₂O₃ in the starting mixture on the colour hue of the pigment has been evaluated. The calcination conditions of the pigment synthesis have been established. The synthesis of these pigments was followed by thermal analysis using a derivatograph apparatus in our laboratory. The optimum conditions for the synthesis of pigments and the properties of the products have been estimated.

Full text of DOI:10.1023/A:1010117812127

Journal of Thermal Analysis and Calorimetry, Vol. 60 (2000) 209–213
THE THERMAL SYNTHESIS OF THE ZnO–Bi O3
2
PIGMENTS
P. Šulcová and M. Trojan
Department of Inorganic Technology, Faculty of Chemical Technology, University of Pardubice
Legions Sq. 565, 532 10 Pardubice, Czech Republic
Abstract
A zinc oxide pigment with an admixture of bismut oxide has been prepared as new yellow pigment
for colouring of plastics and paints. The effect of the Bi O in the starting mixture on the colour hue
2 3
of the pigment has been evaluated. The calcination conditions of the pigment synthesis have been es-
tablished. The synthesis of these pigments was followed by thermal analysis using a derivatograph
apparatus in our laboratory. The optimum conditions for the synthesis of pigments and the properties
of the products have been estimated.
2 3
Keywords: lattice parameters, point defects, solid solutions ZnO–Bi O
Introduction
The increasing need of pigments for and the fact that the most of the pigments, espe-
cially in yellow hues, contains elements (lead, chromium, antimony, cadmium, sele-
nium), which are questionable from the hygienic point of view, opens the great neces-
sity for development and investigation of new ecological pigments. For this reason
the main attention has been directed to the preparation of new compounds of oxide
type which would be found useful as colour pigments.
The pigments based on zinc oxide in new yellow hues have been synthesized in our
laboratory. The pigments of the ZnO–Bi O system give intensive yellow hues. This type
2 3
of pigments has been prepared from commercially available zinc white as a starting raw
material of zinc. These pigments seem to be interesting because of their colour hues and
their formation which is based on the mutual suppressing of non-stoichiometry of crystal
lattices of ZnO and Bi O .
2 3
This pigment is formed by zinc oxide grains on surface of which is a layer of bis-
muth oxide formed by thermal decomposition of bismuth nitrate at temperature of
about 500°C. An idea of interaction of zinc oxide with bismuth oxide is based on the
fact that zinc oxide (n-type electrical semiconductor) has a deficiency of oxygen
which is characterized by vacancies of oxygen in crystal lattice or Zn-interstitials.
Bismuth oxide is a p-type electrical semiconductor which has overstoichiometry of
oxygen. An interaction of both oxides can be based on a transportation of over-
stoichiometric oxygen atoms from Bi
2
O
3
towards the contact regions between both
1
418–2874/2000/ $ 5.00
2000 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
©
Kluwer Academic Publishers, Dordrecht

2
10
2 3
SULCOVÁ, TROJAN: ZnO–Bi O PIGMENTS
phases, where these atoms occupy oxygen vacancies on the surface of the zinc oxide
grains. Diffusion of oxygen into the crystal lattice of ZnO, where can occupy oxygen
vacancies as another type of the interaction of oxides ZnO and Bi O , is also ac-
2
3
cepted.
Experimental
The starting mixtures containing the required content of admixtures were calcinated in
porcelain crucibles in an electric resistance furnace LM 112.10 (VEB Electrobad
–1
Frankenhausen, Germany) for two h. The increase of the temperature was 10°C min .
The pigments of the ZnO–Bi O type were calcinated at temperature of 600°C.
2
3
The temperature region of the formation of these types of pigments was fol-
lowed by thermal analysis using a Derivatograph-C apparatus (MOM Budapest, Hun-
gary; system of Paulik, Paulik and Erdey) which allows the evaluation of data and si-
multaneous registration of the thermoanalytical curves TG, DTG and DTA. The start-
ing raw bismuth material and some starting mixtures prepared were studied by ther-
–
1
mal analysis in ceramic crucible in air. The increase of temperature was 5°C min .
As a heat-resistance standard was used α -Al O .
2
3
The powder pigments were studied by X-ray diffraction analysis. The X-ray
diffractograms of the samples were obtained using a vertical X-ray diffractometer
HZG-4B (Freiberger Präzisionsmechanik, Germany).
Results and discussion
The main aim was to propose, verify and analyse conditions of the synthesis of this
type of pigments based on the wurtzite structure of zinc oxide with an admixture of
bismuth oxide. Our attention was focused on finding such pigments which would
give the most intensive colour.
The starting mixtures containing the increasing content of bismuth oxide (3, 5, 8,
1
0, 13, 15, 18, 20 and 25 mol% Bi O ) were homogenized by wet process. The prepared
2 3
mixtures were calcinated at the temperature of 600°C for 2 h. This calcination tempera-
ture was determined on the base of results of thermal analysis of bismuth nitrate which
was used as starting material of bismuth for the preparation of the (1–x)ZnO ⋅B i O pig-
2
3
ments.
The thermal analysis provided the first information about the temperature region
of the formation of this type of pigments. From the thermoanalytical curves TG, DTG
and DTA it follows that bismuth nitrate decomposes in several steps. The supposed
decomposition of bismuth nitrate can be represented schematically by the following
reactions.
1
4
00–150°C:
50°C:
40–500°C:
60°C:
Bi(NO
Bi(NO ) →BiO (NO ) +1/2N O
5
3 3 2 3 3 2
) ⋅5 H O→Bi(NO ) +5H O
2
3
3
0.5
3 2
2
BiO (NO ) →BiONO +1/2N O
5
0
.5
3 2
3
2
5
BiONO →1/2Bi O +1/2N O
3 2 3 2 5
J. Therm. Anal. Cal., 60, 2000

ŠULCOVÁ, TROJAN: ZnO–Bi
2
O
3
PIGMENTS
211
On the base of results of thermal analysis (Fig. 1) it follows that bismuth nitrate
partially decomposes into reactive intermediates. The decomposition of bismuth ni-
trate to bismuth oxide is ended at the temperatures of about 560°C (forming bismuth
oxide). This process represents the formation of the monoclinic modification of
Bi O oxide (the temperature maximum T of this endothermic effect on the DTA
2
3
max
curve is 563°C). This result is in an agreement with the published data [1] about the
stability of individual modifications of Bi O . The DTA curve shows the endother-
2
3
mic effect with the temperature maximum Tmax at the temperature of 632°C. At this
temperature the cubic modification of Bi O starts to appear. This modification is
2
3
characterized by the lattice constant a=1.0223 nm.
Fig. 1 Thermoanalytical curves of bismuth nitrate Bi(NO
3
)
3
⋅5H
2
O, Apparatus
–1
Derivatograph-C, heating rate 5°C min , standard α-Al O , atmosphere air, ce-
2 3
ramic crucible
From Fig. 1 it follows that bismuth nitrate partially decomposes into bismuth
oxide. Its decomposition to bismuth oxide is ended at the temperatures of about
5
60°C. Figure 2 presents the DTA curve which indicates the endothermic process at
the temperature maximum Tmax of 533°C. This endothermic effect represents the for-
mation of the oxide Bi O which reacts with zinc oxide forming of the (1–x)ZnO⋅
2
3
xBi O pigments. The endothermic effect with the temperature maximum T of
2
3
max
6
65°C represents the tendency of these pigments to sintering.
The temperature of 600°C for the pigment synthesis is also in agreement with
the results of thermal analysis of the starting mixture ended at the temperature of
about 520°C. The mixture had light yellow hue of Bi . The pigment calcinated at
O
3
2
the temperature of 600°C had the intensive yellow colour. This means that the calci-
J. Therm. Anal. Cal., 60, 2000

2
12
2 3
SULCOVÁ, TROJAN: ZnO–Bi O PIGMENTS
Fig. 2 Thermoanalytical curves of mixture Zn –0 .70Bi0.30O1.15. Apparatus
1
2 3
Derivatograph-C, heating rate 5°C min , standard α-Al O , atmosphere air, ce-
ramic crucible
nation temperature of 600°C is sufficiently for the preparation of the (1–x)ZnO⋅
xBi O pigments.
2
3
The attention was focused on the possibilities of the production of the ZnO–
Bi O pigments as new inorganic pigments with the intensive yellow hue.
2
3
Table 1 Lattice parameters of samples of the ZnO+x mol% Bi
2
O
3
pigments
3
x/mol% Bi
2
O
3
a/nm
c/nm
c/a
V/nm
0
3
5
0.32499
0.32498
0.32499
0.32498
0.32493
0.32493
0.32494
0.52071
0.52062
0.52058
0.52055
0.52061
0.52052
0.52058
1.6022
1.6019
1.6018
1.6017
1.6022
1.6019
1.6021
0.047631
0.047624
0.047617
0.047611
0.047601
0.047594
0.047603
1
1
1
2
0
5
8
5
The effect of the increasing content of bismuth oxide on the colour hue of the
ZnO ⋅B i O pigments was followed. On the basis of the measured colour coordinates
2
3
both powder pigments and pigments applied in an acrylate copolymer was selected
*
pigment which had the highest value L and at the same time the lowest abundance of
green tone in yellow colour of the pigment [2]. The most intensive yellow colour of
the pigment was attributed to the pigment containing 18 mol% Bi O . This pigment is
2
3
J. Therm. Anal. Cal., 60, 2000

ŠULCOVÁ, TROJAN: ZnO–Bi
2
O
3
PIGMENTS
213
*
described by the formula Zn0.70Bi0.30O1.15. Powder pigment has coordinates L =93.93,
*
*
*
a = –1.42, b =57.11. In acrylate copolymer this pigment has coordinates L =8.33,
*
*
a = –2.59, b =79.19. At the higher content of Bi O oxide in starting mixture the col-
2
3
our hue is shifted to light yellow hue.
The samples with the increasing content of Bi O were studied by X-ray diffraction
2
3
analysis [2]. Next to the observed diffraction lines corresponding with characteristic lines
of wurtzite structure of ZnO our experiments have shown peaks which have been as-
signed to tetragonal β-modification of Bi O . All samples prepared were heterogenous.
2 3
The volume of the elementary cell of zinc oxide decreases. The minimum value of the el-
ementary cell was determined to the pigment containing 18 mol% Bi O which is charac-
2
3
terized by the most intensive yellow hue from all pigments prepared.
Conclusions
The pigments of the ZnO–Bi O system are characterized by intensive yellow colour.
2
3
They are environmentally friendly and therefore very progressive too. The yellow
pigments of the ZnO–Bi O system could complete the basic assortment of colour in-
2
3
organic pigments and substitute some problematic pigments from the environmental
point of view, especially, chrome yellows (lead chrome) which belong to the most
problematic pigments at the present time.
The preparation of this pigment type was investigated within the framework of a
grant project dealing with new ecological coloured inorganic pigments [3].
References
1
2
3
H. A. Harwig, Z. Anorg. Allg. Chem., 444 (1978) 151.
P. Šulcová and M. Trojan, Dyes and Pigments, 36 (1998) 287.
Grant No. 104/98/P227. Grant Agency of Czech Republic.
J. Therm. Anal. Cal., 60, 2000