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Advanced SnO

-Based Ceramics:

Synthesis, Structure, Properties

Mihaiu Maria Susana, Scarlat Oana,

Zuca Stefania and Zaharescu Maria

“Ilie Murgulesc” Institute of Physical Chemsitry of the Romanian Academy

Romania

1. Introduction

Tin dioxide presents specific optical and electrical properties and a good chemical stability

which confers special characteristics to the SnO

based materials.

SnO

belongs to the important class of transparent conductor oxide materials that combine

low electrical resistance with high optical transparency in the visible range of the

electromagnetic spectrum. These properties are requiered for optoelectronic applications i.g

light emitting diodes, electrode materials in solar cells flat panel displays, transparent field

effect transistors[Wagner, 2003; Presley et al., 2004]. Tin dioxide is aslo an oxidation catalyst

and its activity and selectivity can be substantially improved by incorporation of various

additives [Mihaiu et al., 2002]. Another field in which tin dioxide plays a dominant role is in

solid state gas sensors. A wide variety of oxides exhibit sensitivity towards oxidizing and

reducing gases by a variation of their electrical properties, but SnO

was one of the first

considered, and still is the most frequently used, material for these applications [Caldararu

et al., 1999]. The sensor properties of SnO

depend not only on such factors as the oxide’s

surface stoichiometry, the methodology used to prepare the powder, temperature and

atmosphere of calcination but also, and mainly, on the high specific area deriving from the

low densification of this oxide. Nowadays, SnO

is certainly one of the main polycrystalline

ceramic candidates to compete with the traditional multicomponent ZnO-based varistors

(voltage-dependent resistor-VDR’s), especially because of high electrical stability and its

more simple microstructure [Bueno et al., 2007]. High density in polycrystalline ceramics is

essential for high varistor properties, since the phenomena involved for good varistor

properties occur in the region of the material’s grain boundaries. The main restriction in a

wider use of this type of material is related to poor sintering ability of the SnO

-based

compositions. This behaviour is related to the low diffusivity of the SnO

structure and

predominance of the nondensifying mechanisms (surface diffusion and evaporation-

condensation). The latter result in grain and pore growth, thus limiting the final density

[Varela et al., 1990]. The sintering problem is further complicated by the formation of

deleterious intermediate phases above 1273K and high vapor pressurre of SnO [Dolet et al.,

1992]. Dense SnO

-based materials have been obtained either by using sintering additives

(e.g.,CuO) to promote densification by liguid-phase mechanism or by applying high

pressures (e.g., in hot isostatic pressing technique).

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

102

In recent years, aluminium smelters have made great progress in reducing emissions of

various gaseous and particulate compounds that may have a negative impact on the global,

regional and local environment [Popescu et al., 2007]. The selection of a suitable anode

material has proved to be very difficult task. It should be resistant to evolving oxygen and

cryolite-alumina melt at ~ 950° C , i.e., in the melt, which is used as solvent for oxides. The

maximum allowable wear rate for a commercial use is evaluated as 1.0 cm/year at a current

density of 0.8 A/cm

. The electric conductivity and electrocatalytic activity with respect to

oxygen evolution should be high enough to prevent extra losses of electricity. The material

should be resistant to mechanical and thermal stress, and its components should not be

poisonous for the raw aluminum, to avoid further purification. The only materials that meet

these requirements are a number of oxides. Over the years, many materials have been tested

on a laboratory scale with mixed success. One such material is tin oxide (SnO

) doped with

different additives which is considered to be a promising material for manufacturing

electrodes for melts electrolysis due to their excellent corrosion resistance and heat resistant

properties. Studies on the electrical conductivity of SnO

-based ceramic materials revealed

the major influence exerted by the nature of the dopants and the thermal treatment, i.e. the

sintering conditions. The electrical resistance is claimed [ Zuca et al.,1999] to be decreased

drastically (by 4 to 5 orders of magnitude) by the addition of Sb

as, following its limited

solubility in the SnO

lattice, the concentration of charge carriers increases. On the other

hand, the sintering capacity was found to be greatly improved by the addition of CuO

[Dolet et al., 1995]. Although a list of various potential additives has been suggested by

Varela, their influence on the sintering properties (namely densification) and electrical

conductivity has not been systematically investigated so far [Varela et.al., 1992]. The

influence of a series of oxide additives (individual and various combinations), namely

, CuO, ZnO, Fe

, Cr

, TiO

, MnO

and MoO

on the sintering capacity and

electrical conductivity of the SnO

-based ceramic materials has been investigated. From

among various oxide additives tested, only CuO was found to promote the densification of

the composite. The addition of Sb

, which is claimed to increase the electrical

conductivity, is only effective in the presence of CuO addition [Zuca et al., 1991].

During the electrochemical investigations some irregularities were found (it seemed that a

small amount of oxygen was consumed on the anode). To understand the causes of these

phenomena in order to diminish or remove them it is considered necessary to investigate

the chemical processes between the three oxides (SnO

, Sb

, and CuO) that take place

during the thermal treatment of their mixture.

In this work we propose a review of chemical processes that occur in the SnO

- Sb

- CuO

system over the whole concentration range of the components. The obtaininig of the

sintered materials by conventional ceramic method and spark plasma sintering and the

electriacal behaviour of the oxide materials belonging to this system is also approached.

2. Phase formation in the SnO

-Sb

-CuO ternary system

A summary of physical characteristics of individual SnO

, Sb

, and CuO compound is

given in the Table 1.

Based on the experimental data determined under non-isothermal conditions up to 1773K

for pure components [Zaharescu et al., 1991], the following has been established:

1. no effects were observed for SnO

in the 293-1773 K temperature range;

Advanced SnO

-Based Ceramics: Synthesis, Structure, Properties

103

2. the endothermal reduction of CuO to Cu

O which starts at 1348 K, is followed by

eutectic melting of the two copper oxides;

3. the oxidation of Sb

to Sb

, which takes place within a wide temperature interval

from 713 to 933 K is accompanied by a well marked exothermal effect and a mass

increase; the endothermal effect observed at 1433 K indicates the vaporization of Sb

Physical and

structural

characteristics

SnO

CuO

Mineral cassiterite valentinite cervantite tenorite

Culour white colorless white black

Molecular

weight

150,69 291,50 307,50 78,54

Symmetry tetragonal romboedric orthorhombic monoclinic

Space group D

(P4/mnm) V

) Pna21 C

(c2/c)

Lattice

parameters

(Å)

=4,7374

=3,1864

4,92

=12, 46

= 5,42

a = 5.43

b = 4.79

c = 11.73

4,92

=12, 46

= 5,42

β =99˚29’

Ionic

radius (Å)

= 0,71

=0,76

=0,62

=0,72

Density

(g/cm

)

6,95 5,67 6.64 6,3- 6,49

Melting

point (K)

-O;1800;>2200* 1433 sublimes 1599

Boiling

point (K)

2073

sublimes

2273

Refraction index

┴

2,11// c

2,18

2,35

2,00 2,63

Table 1. Physical and structural characteristics of SnO

, Sb

, CuO [Wyckoff, 1963, 1964]

2.1 Solid state reactions in the component subsystems

2.1.1 SnO

-CuO binary system

The experimental data on the SnO

-CuO binary system have been published in several

papers [Zaharescu et al., 1991; Scarlat et al., 1999]. The thermal analysis of (1-x)SnO

2-x

CuO

compositions, in non-isothermal conditions, in air, up to 1773 K, underlined that the

thermal effects registrated could be assigned exclusively to the presence of CuO in the

reaction mixture. The reduction process of CuO to Cu

O over 1273 K as well as the

formation over 1373 K of the liquid phase have been evidenced. No binary compounds

were formed.

The results obtained by isothermal treatments at 1173, 1273 and 1473 K of the (1-x)SnO

CuO mixtures were also analyzed. Thus, for the samples with 1-10% content of CuO only

SnO

was identified by X-ray diffraction, no matter of treatment temperature. Although the

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

104

ionic radii of the two ions are close (0.71 Å for the Sn

4 +

and 0.72 Å for the

2 +

) due to their

different structures and especially their different valence, the formation of a a solid solution

between the two oxides is unlikely to occur. The disappearance of characteristic diffraction

lines of copper oxide and the sharp contraction of these samples was assigned to its

penetration into the liquid phase (eutectic melt), which facilitates rapid rearrangement of the

ions [Zaharescu et al., 1991; Dolet et al., 1992]. It was observed that in isothermal condition

the reaction of CuO reduction, namely: CuO → Cu

O takes place at substantially lower

temperature compared with the non-isothermal measurements. The phase composition of

the binary mixtures with a content of CuO >30%, thermally treated at 1273 K consists in a

mixture of SnO

, CuO and Cu

O. The samples with a CuO content exceeding 10% fused

when thermally treated at 1473 K. The shrinkage of the tested samples increased with

increasing temperature while the porosity decreased; at 1273 and 1473 K samples with a

zero porosity were obtained, apparently owing to the presence of the liquid phase.

As a conclusions, at temperature over 1273 K the SnO

-CuO initial binary system transforms

into the SnO

- CuO- Cu

O pseudo-ternary system. In the presence of CuO the sintering

ability of SnO

is promoted. Below 1173 K, the rate of densification can be explained in

terms of viscous flow model [Zaharescu et al.,1993], while at higher temperature a copper-

rich liquid is formed and rearrangement is occuring even for short sintering time.

2.1.2 SnO

-Sb

binary system

Widely used as catalysts in catalytic oxidation reactions, the oxide powders belonging to the

SnO

-Sb

system are studied by many research groups in this field [Harrison et al., 1999;.

Park et al., 1999 ] especially in the subsolidus domain.

In accordance with the rules of formation of substitional solid solutions (the sizes of two

ions differ by less than 15%, have similar valency factor, chemical affinity and structure

type) tin dioxide can form a limited solid solution with antimony oxides. Solid solubility

limit depends on the method of preparation and treatment temperature. Volta obtained

monophasic powders with rutile type structure by coprecipitation of tin (IV) and antimony

(V) chlorides and heat treatment at 773 for 16 hours [Volta et al., 1985] of the resulted

hidroxides. Similar results have been obtained by Vlasova for the powders with the

antimony content of 40% and thermal treatment at 873 K temperature[Vlasova et al.,1990].

At 1273 K the amount of antimony atoms in solid solution decreases to 30% and at 1473 K to

4%. Orel et al. have found that regardless of the initial concentration of antimony, the

maximum amount of Sb

entered into the SnO

lattice is of 2.38 mol at 1473 K [Orel et al.,

1995]. The interactions that occur between the two oxides in the whole concentration range

in non isothermal (up to 1773 K) and isothermal conditions at 873, 1073, 1273 and 1473 K

temperatures were investigated in paper [Zaharescu et al., 1991]. The temperature interval

of the exothermal effect corresponding to the oxidation of Sb

to Sb

is substantially

narrowed in the binary mixtures (695-716 K) compared to the pure Sb

(713-933 K) which

suggests the catalysing effect of SnO

in the oxidation process. It may be also assumed that

in the reaction with SnO

, antimony participates as a heterovalent mixture of the

Sb(III)Sb(V)O

type. The endothermal effect observed at high Sb

concentrations at

temperatures above 1423 K which is accompanied by a mass loss, is assigned to the

vaporization of Sb

The exothermal effect indicates the oxidation of Sb

to Sb

at 873

K however, the latter oxide has not been identified by X-ray diffraction at concentrations up

to 10% (all concentrations are given in mass %). This can be apparently assigned to the

Advanced SnO

-Based Ceramics: Synthesis, Structure, Properties

105

formation of the solid solution owing to the substitution of antimony for tin in the SnO

crystal structure. Although the samples with a higher Sb

concentration undergo

deformation and peeling at higher temperatures which make them unsuited for the

subsequent measurements. The concentration of Sb

(up to 30% ) in the solid solution

apparently increases with increasing temperature (up 1273 K) and only SnO

is identified in

the undeformed samples. The XRD results indicate the displacement of the pure SnO

diffraction peaks due to the formation of this rutile type of solid solution. All peaks were

slightly shifted to the right after the solid solution was formed (larger angle or smaller both

a and c lattice paramaters) with respect to pure SnO

Taking into account that Sb

in the presence of SnO

over 773 K in air, completly turns

into Sb

and the oxidation state of tin ions do not change, the question is what form of

antimony ions enter the tin dioxide lattice. Vlasova proposed the following “formula”

[Vlasova et al., 1990]:

(a) Sn

1-2x

(b) Sn

1-2x

; (c) Sn

1-x

;

(d) Sn

1-5x

(e) Sn

1-x

2-x

; (f) Sn

1-x

2-x/2

ax/2

;

(g) Sn

1-3x

;

where e

= free electrons

and V

cation and anion vacancies, respectively

When contents of antimony atoms do not excede 3% in the SnO

latice, the following

“compounds” were identified by EPR: Sn

1-x

2-x

; Sn

1-x

;

1-2x

In the case of higher concentration of antimony embeded in the SnO

lattice the most likely

the resulted solid solution has the the following composition:

1-2x

. By

EXAFS it has been established that Sb dopants occupy Sn sites in the rutile structure

[Chadwick et al.,1993; Rockenberger et al., 2000]. XANES measurements showed that the

largest fraction of Sb has a valency of 5

, although 3

was also observed in nanocrystalline

SnO

. The Sb

species may be due to a strong tendency of Sb to segregate to the surface

[Dusastre et al., 1998; Slater et al., 1999].

2.1.3 Sb

-CuO binary system

-CuO binary system is the most complex subsystems of the SnO

-Sb

-CuO ternary

system. In this system, the formation of CuSb

binary compound with pentavalent

antimony has been reported. The compound has a tri-rutile structure, short-range and long

range magnetic ordering. Under some special conditions, two other binary compounds,

having a higher copper content, have been prepared: Cu

SbO

4,5

and Cu

[Bystrom et

al.,1944; Shimada et al.,1982; 1985;1988].Stan et al., have studied the (10-n)CuO + nSb

mol

(n=0;1;….10) mixtures belonging to the Sb

-CuO binary system at room temperature.

According to the thermal analysis results, the following solid state reactions occur

[Stan et

al.,1998] :

773 K Sb

+ 1/2 O

=Sb

(1)

>1023 K Sb

+ CuO + 1/2 O

= CuSb

(2)

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

106

>1223 K CuSb

+7CuO=(1-r) CuSb

+7(1-r)CuO+ 2rCu

SbO

4.5

+2rO

(3)

Where rє (0,1) is the fraction of CuSb

that transforms into Cu

SbO

4.5

In order to emphasize an intermediate stage in the compound formation, Fig. 1 shows the

relative intensities of the X-ray characteristic lines for the mixtures thermally treated for only

one hour.

The same compounds have been obtained in both isothermal treatment (one hour and three

hours) and non-isothermal conditions. This points to the high reactivity of the oxides in this

system. There is no evidence of Cu

compound formation. This compound as

reported only for high pressures (see Table 2 ). Then, in the range of 293-1273 K at normal

atmospheric pressure, the following compounds can be found in equilibrium: CuO, Cu

, Sb

, CuSb

and Cu

SbO

4.5

Fig. 1. Relative intensities of the X-ray characteristics lines for the binary mixtures

thetmally treated at 773, 823 and 1273 K, one hour

All the experimental evidences show that equations (1), (2) and (3) are suficient to describe

the reactions occuring in the Sb

-CuO binary system with increasing temperature. If the

Advanced SnO

-Based Ceramics: Synthesis, Structure, Properties

107

reactions are considered as completed, including r=1 in equation (3), then the quatity of

oxygen that is gained or lost can be calculated for each sample.

The atomic composition modifications during the thermal treatments for the four

representative samples is given Fig.2. At room temperature the compositions lay on the

-CuO line as shown in Fig.2. The composition line moves to Sb

-CuO at 773 K and

changes to Sb

-CuSb

-CuO over 823 K. Finally, if the reaction (3) is completed at 1273

K, the compositions lay on the CuO- Cu

SbO

4.5

-CuSb

- Sb

Fig. 2. Atomic compositions after the thermal treatment for the following initial mixtures: S

the mixture of 90 mol% of CuO and 10% mol of Sb

; S

the mixture of 70 mol% of CuO

and 30% mol of Sb

; S

the mixture of 50 mol% of CuO and 50% mol of Sb

; S

the

mixture of 30 mol% of CuO and 70% mol of Sb

[Stan et al.,1998]

2.2 Solid state reactions in the SnO

-Sb

-CuO ternary system

The SnO

-Sb

-CuO ternary system have been investigated in the whole concentrations

range isothermal and non-isothermal conditions in[Zaharescu et al.,1991;1993; Mihaiu et

al.,1995] .In the Fig.3 experimental ternary compositions are graphically presented.

Equimolecular mixture of CuO and Sb

(which leads to CuSb

formation) determine the

next evolution of the mixtures in the system and this ratio affords the classification of sample

in three categories:

(I) CuO:Sb

=1 and the samples lie on SnO

-equimolecular mixture;

(II) CuO:Sb

>1 and the samples are belonging to the CuO-CuO.Sb

–SnO

pseudo-

ternary system;

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

108

(III) CuO:Sb

<1 and the samples are belonging to the Sb

-CuO.Sb

–SnO

pseudo-

ternary system [Zaharescu et al., 1993]

Fig. 3. Domain of the studied compositions in the SnO

-Sb

– CuO system [Zaharescu

et al., 1999]

The oxidation of Sb

to Sb

(and Sb

), as well as the high temperature interactions of

with SnO

and respectively with CuO have been discussed in the section 2.1.1 and

2.1.3. The thermal behaviour of Sb

in the presence of both mentioned oxides in non-

isothermal conditions (up to 1273 K) is summarized in the Table 2 .For the mixtures from (I)

categorie (CuO:Sb

=1) on can note that the oxidation of Sb

to Sb

is total and

take

place in two steps. The experimental weight increase is in good agreement with the

theoretical one. The resulted phase composition of the samples corresponds to a mixture of

SnO

and CuSb

. Accordigly, in the presence of both SnO

and CuO there is a higher

tendency of Sb

to react with CuO than to form solid solution with SnO

In the ternary mixtures of molar ratio CuO:Sb

>1, a total oxidation of Sb

to Sb

obtained; above 1233 K; a weight loss is also observed which is assigned to the reduction of

CuO to Cu

O. After thermal treatment the phase composition consists in SnO

, CuSb

and

SbO

4.5

mixtures.

In the ternary mixtures of molar ratio CuO:Sb

<1, in the first step Sb

entirely oxidize to

. In the second step the transformation of Sb

to Sb

is limited to the quantity

required by CuSb

formation.The unreacted Sb

dissolves partially into SnO

with the

formation of a solid solution with SnO

crystal structure.