DIRECTED SYNTHESIS OF FINE-LAYER COATINGS,
HERMETICS AND NANOCOMPOSITES BY MEANS OF
CHEMICAL REACTIONS IN OXIDE MELTS
V.Zhabrev
Grebentshikov Institute of the Chemistry of Silicates RAS
St.-Petersburg. Russia
Grebentshikov Institute of the Chemistry of Silicates RAS is the
fundamental scientific center of Russia in the field of nanostate.
Scientific directions concerning of synthesis and study of nanoparticles
and nanocomposits is developed. There are the investigations of structure
and properties of high temperature oxide compounds, phase
equilibriums, nanocrystals and nanocomposites based on oxides and
hydroxides and organo-inorganic hybrid compositions. Other directions
are investigation of chemical bonds and structure of compounds in glassy
and crystal state, new methods making materials with certain
properties.(glass, piroceramics, coatings).
Now the synthesis of nanoparticles and nanocomposites is realized
by or hydrothermal or zol-gel methods. Complexity and multistage this
methods is known because it is necessary to make methods of synthesis
on nanoparticles and nanocomposites.
Two aspects of making of nanosize material is considering in this
paper – synthesis of oxide nanocompounds using the chemical reaction
in oxide melts and any kinetic questions on formation of high
temperature coatings basing infusible oxides and nanosize reagents.
1. Synthesis nanocrystal mixed titanates of barium and strontium
by means of reaction of crystal modification of titanium dioxide with
KNO3 + Ba(NO3)2 or KNO3 + Sr(NO3)2 mixed melts.
Titanates of barium and strontium are used as segnetoelectrics.
Monocrystal segnetoelectrics usually are fabricated growing from both
melt or solution and gas phase, polycrystal segnetoelectrics – under
ceramic technology, film ones – vacuum evaporation or extrusion
technology.
Usually materials in system BaTiO3-SrTiO3 are produced as solid
state reaction between carbonates and titanium dioxide at 1300-1400оС:
323
BaCO3+TiO2 = BaTiO3+CO2 
SrCO3+TiO2 = SrTiO3+CO2 
At 1400оС BaCO3, SrCO3, TiO2 are solid powders and to make
equal mixed solid solutions it is necessary too large annealing time – up
to ten hours. Solid state synthesis has very many limitation to produce
nanosize materials as dispersion of productes of reaction is increasing in
comparison with dispersion of original components. High temperature
annealing reduces to tendency to cake. This factor do not give possibility
of fabrication of equal dispersion low size materials.
At last time new methods of production of titanates are developed.
It is methods using hydrolysis of metallorganic compounds ( Si(OC2H5)4
+ Ba(OH)2) or TiCl4 + BaCl2 etc. The mainly this methods is low
temperature of reactions (up to 200оС), that guarantees well qualify of
products. However ones have a lot of difficulties - toxicity and large
price of reagents, necessary to keep technology.
Thus it is needed to work out simply methods of making equal
alloying oxide materials at more low temperature. Those methods must
guarantee possibility to regulate dispersion of samples including ultraand nanosize crystals and high degree of purity. It may be the synthesis
of materials in melting salts. Temperature of melting of nitrates of
potassium, barium, strontium is given on Table 1:
Таble 1
Melting temperature of compounds, оС
Ba(NO3)2
595
Sr(NO3)2
645
TiO2
1840
KNO3
337
Potassium nitrate is used as reaction medium. In the medium
certain portions of other reagents are adding. During reaction the
particles of titanium oxide are solid ones and they do not solve in mixed
324
salt melts. To make nanosize materials in system BaxSr1-xTiO3 it is
necessary to use high dispersion particles of titanium oxide. Preparing of
powder TiO2 may be realized by hydrolysis of sulfate solution:
Ti(SO4)2 + 4NH4OH  2(NH4)2SO4 + Ti(OH)4 
with following annealing of hydrates
Ti(OH)4  TiO2 + 2H2O 
The synthesis of titanates is carried out at 700-800оС:
Ba(NO3)2+TiO2 = BaTiO3+2NO2+1/2O2
Sr(NO3)2+TiO2 = SrTiO3+2NO2+1/2O2
After this the products of reaction are washing off water soluble
nitrates. Phase analysis of products of reaction is realized using RFA.
At pictures 1 and 2 you can see diffractogrammes of powder
materials BaTiO3 and SrTiO3 after reaction. It is seen only peaks of
products of reaction.
Fig.1. Diffractogramme of BaTiO3 prepared at 800oС from mixed salt melt
325
Fig.2. Diffractogramme SrTiO3 prepared at 825оС from mixed salt melt.
Thus the synthesis of nanosize materials of BaxSr1-xTiO3 into
mixed nitrate melts permits to make ones at 700-900оС, that lower on
200-400оС then we use solid state reaction from carbonates. Processes of
migration into melt are intensified and reaction time is smaller. This
method permits to regulate the dispersion of compounds up to nanosize
value. Picture 3 shows the results of study of preparing particles by
AMS set.
Fig.3. Structure of nanoparticles of barium titanate.
Next step of our investigation is the synthesis of new
segnetoelectric compositions (table 2). To prepare of film carriers of
326
information those nanoparticles are deposited on hybrid organoinorganic coatings making by zol-gel method.
Table 2.
Some reactions in KNO3 melt
Reaction
t, С
о
Time, min
2Bi2O3+3TiO2Bi4Ti3O12
900
15-60
—
8Bi2O3+4LaCl3+15TiO2
5(Bi0.8La0.2)4Ti3O12
900
15-60
—
900
15-60
1:1
15-180
1:2
240
1:2
15-120
1:0.5
15-120
1:1
15-120
1:2
8Bi2O3+4LaCl3+15TiO2
5(Bi0.8La0.2)4Ti3O12
1000
8Bi2O3+4LaCl3+15TiO2
5(Bi0.8La0.2)4Ti3O12
2.The kinetics of heterophase interactions under formation of
temperature stably coatings.
In this part of paper influence of composition and condition
termoannealing in air to prepare temperature stably porousless coating
based on ZrB2 – SiC is discussed. The original reagent are powders with
size of particles near 63 mkm. As binder water solution of
carbometilcellulose or zol of silica is applied. Particles of silica is 20 nm.
For comparing the compositions with addition of glass SiO2 –80, B2O3 –
17.5, Al2O3 – 2.5 (mаss.%) is used. Study cоmposition is given in table
3. Porosity coating termoannealing for one hour at 1400˚С is 30%.
The velocity of all reactions under formation of coating is
determined as increase or decrease of mass of samples ( ∆m, mg/cm2)
during experiment at 800,1200 и 1400оС.
327
Таble 3
Study compositions
Name
1а
2а
3а
4а
1б
2б
3б
4б
1в
ZrB2
100
95
90
70
100
95
90
70
90
Соmposition, mаss.%
SiC
SiO2
5
10
30
7.5
5
7.5
10
7.5
30
7.5
-
АБС
10
In table 4 the results of RFA of surface of some composition after
thermoannealing at 1400С and different time of treatment are shown. It
is seen that new phases are formed on surface of samples namely ZrO2
monoclinic modification, ZrSiO4 и -crystabolit.
Table 4
Crystal phases on surface of samples after annealing at 1400С in air
Name
Phase composition after annealing for different time, min.
5
60
1а
ZrB2,ZrO2
ZrB2,ZrO2
2а
4а
ZrO2
ZrO2
1б
ZrB2,ZrO2
3б
ZrB2,ZrO2,
ZrSiO4
1в
ZrO2
ZrO2
ZrO2
ZrB2,ZrO2,
ZrSiO4
ZrO2,
ZrSiO4
ZrO2, -
crystabolit
328
300
ZrB2(следы),
ZrO2
1020
ZrO2,
ZrSiO4
ZrO2,
ZrSiO4
ZrO2,
ZrSiO4
ZrO2,
ZrSiO4
ZrO2,
ZrSiO4
ZrO2,
ZrSiO4
ZrO2
This results show point of chemical reactions passing under
oxidation zirconium diboride and silicon carbide. It is possible to
suppose that at high temperature oxidation this composition passing next
reactions:
- oxidation zirconium diboride and silicon carbide in air at 8001400оС as:
ZrB2(s) + 2.5O2 = ZrO2(т) + B2O3(l)
SiC(s) +2 O2=SiO2(s) + CO2 ↑
-
-
solution in liquid B2O3 zirconium dioxide and silica: B2O3(ж) +
ZrO2(т) + SiO2(т)  к ZrO2 mB2O3.nSiO2 and resulting product
is anionic matrix of melt.
crystalisation of melt with precipitating of zircon ZrSiO4 и crystabalit
Kinetic curves of oxidation compositions 1а, 2а, 3а и 4а is shown
Fig.1 at 800оС (1а),1200оС (1б) и 1400оС (1в).Alone ZrB2 is oxidized
at 1400оС very strong. It is possible to suppose that produced melt of
B2O3 is not protected layer because its viscosity is small. Introduction in
composition of increasing portions of SiC leads to
Fig.3 shows kinetic curves of oxidation of compositions 3а, 3б, и
1в during long time. For 17 hours at 1400С ∆m of samples is not more
then 25 mg/sm2.
It is needs to know that kinetic curves show summary result of
different chemical reactions spring from annealing of samples in air –
oxidation of initial components, interactions between oxides, forming of
liquid borosilicate glassy melt, evaporation of small part of B2O3 and
full evaporation of CO2.
Therefore it needs to sort out them with point of view of
interphase interactions. In common mechanism of interphase reaction of
oxidizing of study compositions may be describe by means of forming
of reaction boundary of division of phases and future processes of
interaction of solid phase with products of dissociation of melt:
migration ionic associates of oxide melt to surface of separation and
insertion of products of reactions into structure of melt.
329
1а
m
200
150
2а
100
3а
50
4а
0
0
20
40
60
80
100
t,min
m (mg/sm2)
1б
50
40
2а
30
3а
20
4а
10
0
0
20
40
60
80
100
t,min
1в
m (mg/sm2)
40
1а
30
2а
20
3а
10
4а
0
0
20
40
60
80
t, min
Fig.1 Dependence of increasing of mass ∆m (mg/sm2) samples 1а-4а after
annealing in air at 800оС (а), 1200оС (б) и 1400оС (в) vs time t (min).
330
20
2б
18
16
3б
m мг/см
привес,
2
14
4б
1в
12
10
8
6
4
2
0
-2
0
20
40
60
80
100
t, время,
min мин
Fig.2. ∆m of samples 2б-4б,1в vs t after annealing at 1400 ºС in air.
35
mмассы
привес
30
25
1в
20
3а
15
3б
10
5
0
0
5
10
15
20
25
t,ч
t, hours
Fig.3. Dependence of ∆m (mg/sm2) of samples 3а, 3б и 1в after annealing in
air at 1400оС for t (min).
Processing rate is determines by diffusion in drowing layer of
reaction products and three-dimensional decreasing of volume of
reagents as result of moving of reaction boundary of separation.
331
Oxidation of any studied compositions on base of ZrB2 and SiC is
complexity process and there is no possibility to fixe of variation of
comcentration of each from all component of composition. To avoid
indefiniteness in determination of rate of chemical interphase reactions (
without certain component at which determined common rate) It is
needs to give the definition of degree of conversion as value that
indicates the quantity reacted or formed substance (without to origin or
final ones). Farther this value will mark  and race of reaction will be d/dt. The degree of conversion is unsize relation of maximal
increasing of mass on kinetic curves (Fig.1 and 2) to current increasing
of mass. ( = mi/ mmax). We also admit the possibility of parallel (
А+В  С и А+В  D) and consecutive ( А + В С  D ) reactions.
Kinetic equations for reaction with advance of boundary of
division of phases are taken out by summation the volumes of all nuclei
of new phases:
dα
V(t) = o V(titj)  ----- ) dtj
dt
t
(1)
where: V(t) – volume of product of reaction for time t. As V(t) is
proportional to degree of conversion, integration of (1) gives:
α = (kt)n
(2)
where: к – constant of race of reaction, n – order of the reaction.
The equation (2) is named by power law of growth of product of
new phase and its is right for any combination of laws of nucleation and
growth of new phase.
Estimation of regularity of variation of degree of conversion as
function of time lets to predict the rate of reaction under certain
composition of reagents and certain value of constant of rate. In this
prediction analyzing the order of reaction it is possible to make
conclusion about mechanism of reaction. Constant of rate is equal the
rate of reaction if concentrations are 1 and depends from temperature as
Arrenius equation:
к= A exp(-E/RT)
332
(3)
where: А – preexponential member; Е – activation energy; R –gas
constant; Т- temperature.
Fig.4-6 show the dependence of degree of conversion (a) and rate
of reaction (б) as function of time of interaction at 800,1200 и 1400оС
for compositions 1а,2а,3а,4а. For all compositions and at all
temperatures rate of reaction falls sharply for first 10 minutes and after
this we can see low decreasing of rate of reaction. Also introduction
rising quantity of SiC leads to gradual decreasing of oxidation rate.
For kinetic analysis of topochemical reactions are used large
quantity of equations α = f(t). Those equations are take into account
different points of view about formation and moving of boundary of
separation, diffusion of reagents in layer of products of reactions.
First treatment of experimental data are maken by AvraamiErofeev equation:
α = 1- exp (-ktn)
(4)
This equation describes common dependence of degrre of
conversion from time.
Transformation of (4) gives:
lg[-ln(1- α)] = lgk +nlgt
(5)
1,2
1
0,8
 0,6
0,4
0,2
0
0
20
40
60
t, min
Fig.4а. Dependence of degree of conversion from time at 1400 оС
for compositions: ● – 1а, - 2а,  - 3а, ▲- 4а.
333
80
1400
0,04
0,035
0,03
0,025
da/dt 0,02
0,015
0,01
0,005
0
0
10
20
30
40
50
60
t
Fig.4б. Dependence of rate of reaction from time at 1400 оС for compositions:
 – 1а, - 2а,  - 3а, ▲- 4а.
1200
1
0,8
0,6

0,4
0,2
0
0
10
20
30
40
50
60
70
80
t, min
Рис.5а. Dependence of degree of conversion from time at 1200 оС
334
90
for compositions: - 2а,  - 3а, ▲- 4а.
1200
0,04
0,035
0,03
0,025
da/dt 0,02
0,015
0,01
0,005
0
0
20
40
60
80
t
Fig.5б. Dependence of rate of reaction from time at 1200 оС for compositions:
- 2а,  - 3а, ▲- 4а.
1
0,9
0,8
0,7
0,6
 0,5
0,4
0,3
0,2
0,1
0
0
10
20
30
40
50
60
70
80
min
Fig.6а. Dependence of degree of conversion from time at 800 оС
for compositions: - 3а,  - 2а, ▲- 4а.
335
90
0,035
0,03
0,025
da/dt
0,02
0,015
0,01
0,005
0
0
10
20
30
40
50
60
70
80
t
Рис.6б. Dependence of rate of reaction from time at 800 оС for compositions:
- 2а,  - 3а, ▲- 4а.
The value of angular coefficient of linear dependence lg[-ln(1- α)]
= lg(t), gives n (order of reaction).
In Fig. 7a,b,в are shown the dependences lg[-ln(1- α)] vs lg (t) at
800,1200 и 1400оС for compositions 1а-4а. At 800оС for 2а и 3а linear
part of dependence is observed only at small time of interaction, FOR
4а equation ( 4 ) is applicated in all times. The value of angular
coefficient of linear part of dependence is 0.65±0.10. This value of n
answers by model of diffusion – controlled reaction.
0,6
lg|-ln(1-
)
0,4
0,2
3a
0
2a
-0,2
4a
-0,4
-0,6
0,9
1,1
1,3
1,5
1,7
1,9
2,1
lgt
Fig.7а Dependence lg[-ln(1-)] vs lgt at 800оС for compositions: ▲- 4а,  - 2а,
■- 3а.
336
In first steps of oxidation 2а и 3а quantity of silica is not
sufficiently to prevent motion of oxygen. Forever there are two
reactions: oxidation of components and forming glassy film and we can
see deflecton from ( 4 ). For composition ( 4a ) only diffusion –
controlled process is observed.
0,8
0,6
0,4
2a
0,2
4a
3a
0
-0,2
-0,4
0,9
1,1
1,3
1,5
1,7
1,9
2,1
Fig.7б Dependence lg[-ln(1-)] vs lgt at 1200оС, composition: ■- 3а, ▲ - 4а,
- 2а.
0,8
0,6
0,4
1a
3a
0,2
2a
4a
0
-0,2
-0,4
1
1,25
1,5
1,75
Fig.7в Dependence lg[-ln(1-)] vs lgt at 1400оС:–1а, - 2а,  - 3а, ▲- 4а.
Now it is needs to consider the chemical aspect of reactions.
Zirconium in structure of borate melt may form groups [ZrO4/2] and
those groups enter in ionic associates of melt:
337
ZrO2 + 2[ВO3/2]  [ZrO4/2] 2[ВO3/2]
(6)
The same reaction for silica:
SiO2 + 2[ВO3/2]  [SiO4/2] 2[ВO3/2]
(7)
As result of two parallel reactions common matrix of melt is
formed [SiO4/2] [ZrO4/2] 2[ВO3/2]. Those reactions determine common
kinetics of process. In Table 3 the values of lgk is given for composition
2а-4а at all temperature range.
Table 3
Values of lgk for compositions 2а-4а at 800, 1200 и 1400оС
Состав
2а
3а
4а
о
800 С
-1.15
-1.05
-0.90
1200оС
-0.95
-0.70
-0.85
1400оС
-0.75
-0.70
-.0.95
Fig.8 show results of kinetic analysis for composition 1б – 3б.
0,7
lg[-ln(1-a)
0,6
0,5
1б
0,4
2б
0,3
3б
0,2
0,1
0
1
1,2
1,4
1,6
1,8
2
lgt
Рис.8. Dependence lg[-ln(1-)] vs lgt at 1400оС for composition: ♦–1б,  2б, ▲- 3б.
In conclusion it is fact that formation of glassy matrix of melt is
important part of protection of material.
338