A.A. Malkov1, A.A. Malygin2, I.V Egorova3,
S.V. Vikhman4, G.L. Brusilovskii5, V.A. Silin6, and N.A. Kulikov7
EFFECT OF CHROMIUM
OXIDES NANODOPANTS
ON THERMAL
TRANSFORMATIONS
OF CERAMIC MASS
St. Petersburg State Institute of Technology (Technical University), Moskovskii pr. 26, St. Petersburg, 190013 Russia
Svetlana−Rentgen ZAO, Promyshlennaya ul. 5, St. Petersburg,
198099 Russia
Abstract—Molecular layering nanotechnology was used to synthesize ceramic mixtures containing chromium oxide nanodopants. Comparative study
of the structural chemical transformations and shrinkage of the initial and
modified samples under the conditions of linear heating (10°С/min) from
room temperature to 1000°С. Doped samples were found to have lower
shrinkage initiation temperatures and larger relative changes of linear dimensions than the original ceramic mass at comparative temperatures.
Key words: nanotechnology, molecular layering, chromium oxide nanodopants, thermal transformations, ceramic mass, sintering.
The state of surface (chemical composition, structure, topography) is quite an important issue in many processes associated with the production and use of materials,
on account of the fact that the surface is the first to be exposed to chemical, thermal, mechanical, and other effects. It
is not uncommon that by forming functional surface layers
with preset characteristics one obtains a tool for controlling
the properties of the entire material
The X-ray tube manufacturing technology at Svetlana−X-Ray CJSC involves use of ceramic cathode insulators
which are manufactured by sintering ceramic masses at
1050°С. One of the practically important tasks to be solved
for optimizing the latter process is to decrease the sintering
temperature by 70−100°С to avoid embrittlement of molybdenum electrodes. This task can be approached, among other
ways, by modifying the surface of the intermediate product
particles with nanodopants capable of decreasing the sintering temperatures compared to those characteristic of the
starting ceramic mass (hereinafter referred to as CM). In this
connection, precision synthesis of surface nanostructures by
molecular layering (ML) seems quite a promising technology.
The essence of the ML consists in the implementation of
nonequilibrium cyclic chemical reactions on the surface of a
solid body between reagents fed from the outside and surface
functional groups`(FG) [1, 2]. The ML ensures a strong
chemical contact between CM and active dopant and a uniform deposition of dopants on each individual grain, whatever
small it is, over the whole bulk of the original material. As
shown previously on an example of aluminum oxide ceramics
[3−5], the latter circumstance favors a much lower sintering
initiation temperature and more intense solid-phase process
as a whole.
With the aim of identifying the composition of CM
used at Svetlana−X-Ray CJSC for fabrication of cathode insu1
lators, choosing methods ML nanodoping conditions, and
assessing the nature of thermal transformations, we performed a physicochemical research of both CM and its components by X-ray diffraction (XRD) analysis, IR spectroscopy,
UV/VIS diffuse reflectance spectroscopy (DRS), and differential thermal analysis (DTA).
In the existing technology, CM contains 92 wt % of
fine alumina, 5 wt % of clay from the Veselovskое deposit
(hereinafter referred to as Veselovskaya clay), and 3 wt % of
chalk.
The X-ray patterns of CM and its components were
obtained on a Difray DNR-401 diffractometer (CuK radiation,
λCuK 1.5412 nm, 2θ range 20−65°).
The IR spectra were measured in KBr on a FSM1201 FTIR spectrometer in the range 400−4000 cm−1 with a
resolution of 4 cm−1 in the transmission mode. The optional
software which provides automated measurement of spectra
and their graphical representation was used.
The structural and chemical transformations in CM
and its components during their high-temperature sintering
were traced by DTA on a Paulik−Paulik−Erdey derivatograph
(Hungary) in the temperature range 20−1000°С at a heating
rate of 10°C/min under argon. The resulting products were
then studied by IR spectroscopy and XRF analysis.
The specific surface of the samples was measured
by low-temperature adsorption (77 К) on a SORBI N.4.1 instrument.
The DR-spectra were measured on a Specord-М40
instrument equipped with an integrating sphere unit (reference MgO, wavelength range 250−900 nm).
The high-temperature linear shrinkage was determined by dilatometry, using prism-shaped samples with a
height of 30 mm and an edge length (diameter) of 3 mm. The
temperature was raised to 950°С at a rate of 300оС/h. The
Anatolii Alekseevich Malkov, PhD (Chem.), Associate professor, Department of Chemical Nanotechnology and Electronic Engineering Materials,
[SPbSIT(TU)], е-mail: [email protected]
2
Anatolii Alekseevich Malygin, Dr. Sci. (Chem.), Professor, Head of Department of Chemical Nanotechnology and Electronic Engineering Materials,
SPbSIT(TU), е-mail: [email protected]
3
Irina Valentinovna Egorova, Researcher, Department of Chemical Nanotechnology and Electronic Engineering Materials, SPbSIT(TU)
4
Sergei Valer'evich Vikhman, PhD (Eng.), Associate professor, Department of Chemical Technology of Ceramics, SPbSIT(TU), е-mail: [email protected]
5
Gennadii L'vovich Brusilovskii, PhD (Eng.), Head of Department, Svetlana− Rentgen ZAO, е-mail: [email protected]
6
Vladimir Alekseevich Silin, Deputy Director, Chief Technologist, Svetlana− Rentgen ZAO, е-mail: [email protected]
7
Nikolai Aleksandrovich Kulikov, PhD (Eng.), Director, Svetlana− Rentgen ZAO, е-mail: [email protected]
I. CHEMISTRY AND CHEMICAL TECHNOLOGY • CHEMISTRY OF SOLID SUBSTANCES AND NANOTECHNOLOGY
linear dimensions were continuously measured by means of
an inductive motion sensor and reduced to relative units using
a Dilatometer software.
Cathode insulators were fabricated from ramming
ceramics. The insulating ceramic layer was stuffed on an
SPMZ special-purpose machine. Sintering was performed in a
hydrogen tunnel furnace at 950оС, i.e. lower by 100оС compared to the conditions of thermal treatment of CM.
The electrical resistance of insulators was measured
with a F4102/1 mega ohmmeter in air at room temperature
for 3 min. The first reading was taken immediately after voltage had been applied and then in 1 min and 3 min. The time
required to reach a resistance of 50 МOhm after voltage application was also measured.
The physical and mechanical properties of sintered
CM and modified CM samples were studied using the singlegrain crush test and flexural strength measurements.
As seen from the X-ray patterns shown in Fig. 1, the
main component of CM is alumina: Its reflections at 2θ angles
of 25.66, 35.25, 37.89, 43.46, 52.67, 57.59, and 61.48° (Fig.
1b) are the strongest (Fig. 1а). Search in the JCPDS database
(card 10-173 Al2O3, corundum) showed that the main component of CM is α-Al2O3. Even though the fractions of chalk and
Veselovskaya clay are much smaller, their strongest reflections are seen in the X-ray pattern of the sample at 2θ 26.66
and 29.44°, respectively. Furthermore, analysis of the X-ray
pattern of Veselovskaya clay shows that it contains characteristic of quartz (Fig. 1c), along with reflections of a clay mineral.
The IR spectrum of alumina (Fig. 2) shows absorption bands at 460, 486, 614, 650, and 785 cm−1, characteristic of vibration modes of α-Al2O3 [6, 7]. These bands all are
present in the IR spectrum of CM. This spectrum also contains absorption bands corresponding to chalk (870 and 1450
cm−1) and Veselovskaya clay (1090 cm−1).
Fig. 2. IR spectra of CM (1) and its components: alumina (2), Veselovskaya
clay (3), and chalk (4)
Fig. 3. Results of differential thermal analysis of CM (a) and its components: alumina (b), chalk (c), and Veselovskaya clay (d). Curves: 1 weight loss, 2 - DTG, and 3 - DTA
Fig. 1. X-ray patterns of the CM (a), alumina (b), chalk (c), and
Veselovskaya clay (d)
Thus, the results of XRF analysis and IR spectroscopy established that CM mostly consists of α-Al2O3 and also
contains small fractions of chalk (as calcite and aragonite)
and Veselovskaya clay with a quartz admixture.
Analysis of the DTA results (Fig. 3) allowed us to
identify three stages of CM decomposition, differing from in
other in the character of weight loss (Fig. 3а). The characteristic features of this process clearly reveal themselves in the
DTG and DTA as corresponding peaks at 100, 525, and
800°C. From a comparison of the DTG, DTA, and TG curves
of CM and its components (Figs. 3a−3d) we can suggest that
the first stage of thermal transformations of CM (100−400°С)
involves loss of adsorbed water, primarily from clay and corundum. The second stage (400−600°С), which is accompanied by an endothermic effect and has a maximum rate с
weight loss at 525°С, corresponds to loss of coordinated water from Veselovskaya clay (Fig. 3d). The maximum rate of
weight loss at the third stage (600−900°С) occurs at 800°С
(Fig. 3а). The weight loss at the start of the third stage at
600−700°С is obviously associated of the structurally bound
I. CHEMISTRY AND CHEMICAL TECHNOLOGY • CHEMISTRY OF SOLID SUBSTANCES AND NANOTECHNOLOGY
water from Veselovskaya clay (Fig. 3d) and beginning with
700°С, with thermal decomposition of chalk stone (СаСО3)
(Fig. 3c), involving formation of a solid product (CaO) and
release of СО2 to the gas phase. Taking into account the
above assumptions, we calculated weight losses for all the
materials in focus and all the temperature ranges (Table 1).
Analysis of these data showed that, except for the first stage
(<400°С), the CM weight losses are equal to the total weight
losses of the individual components estimated from their fractions in CM. The lack of the observed correlation at the first
stage is probably associated with a higher water content of
the individual alumina compared to the respective that of CM.
Table 1. Weight losses in the course of thermal transformations of CM and
its components
Temperature range,
ºС
25-400
400-600
600-1000
26-1000
Weight loss, m, % (with account for the percentage in CM)
mCM
0,28
0,41
1,20
1,90
malumina
0,38(0,35)
0,06(0,06)
0(0)
0,44(0,405)
mclay
mchalk
*mΣ
3,44(0,170)
6,14(0,31)
0,77(0,038)
10,35(0,518)
0(0)
0(0)
36,12(1,08)
36,12(1,08)
0,52
0,37
1,12
2,01
mΣ/mCM
1,84
0,90
0,93
1.06
*(mΣ) Total weight loss of the individual components with account for their fractions in CM.
Fig. 4. X-ray patterns of the CM (a), alumina (b), chalk (c),
and Veselovskaya clay (d) after DTA
Fig. 5. IR spectra of CM (1) and its components after DTA: alumina (2);
Veselovskaya clay (3); and chalk (4)
As seen from the X-ray patterns of CM and its components after DTA (Fig. 4), the main reflections of CM and
corundum remain unchanged. At the same time, the positions
and intensities of reflections of Veselovskaya clay and chalk
point to essential structural and chemical transformations of
these components. According to [8], in the temperature
range 550−700°С, kaolin minerals and clays undergo dehydration followed by amorphization on further heating, which is
indeed observed in the X-ray pattern of Veselovskaya clay.
The two fairly intense reflections at 2θ 27.76 and 36.63° (d/n
= 0.333 and 0.245 nm), as well as two weak reflections at
50.5 and 60.0° (d/n = 0.182 and 0.154 nm) are assignable to
the quartz admixture in the clay [9]. In the X-ray pattern of
chalk after thermal treatment of primary notice is the disappearance or substantial attenuation of the calcite and aragonite reflections. However, no reflections of СаО, an expected
thermolysis product, are also observed. Obviously, the reflections at 2θ 28.80, 34.07, 47.30, 50.75, and 54.39° (d/n =
0.310, 0.263, 0.192, 0.180, and 0.169 nm) belong to Ca(OH)2
formed by hydration of the СаО formed [9]. The IR spectra of
the products of thermal transformations provide further evidence for the occurrence of structural and chemical changes
in chalk (Fig. 5). The observation of a narrow band with its
maximum at 3465 cm−1 in these spectra (Fig. 5, spectrum 4)
is a convincing evidence in favor of dehydration of the the
СаО formed.
The above results were used to choose the temperature of pretreatment of CM before its modification
(≤300−400°С). The choice of a chromium oxide nanodopant
was based on the available published information on the intensification of sintering of alumina ceramics [10, 11].
Nanodoping CM with chromium oxide by the methods ML was performed by the following engineering protocol:
1. Drying CM at 300°С to remove physically adsorbed and coordinated water and stabilize the
hydroxyl coating of solid particles, i.e. before
start of chalk thermolysis.
2. Exposure of the CM dried according to item 1 to
CrO2Cl2 vapors under a dry carrier gas at 150°С
to exclude the possible thermal degradation of
chromium oxochloride.
3. Purging with the carrier gas of the reactor with
the sample treated according to item 2 to remove unreacted reagent and gaseous reaction
product (HCl) at the same temperature.
4. Exposure of the CM modified according to items
2 and 3 to ethanol vapors at 150оС to reduce
Cr(VI) to Cr(III).
5. Vapor-phase hydrolysis of the modified product
for 0.5 h at 150оС, and then the temperature is
raised to 300°С. The process was continued
until HCl no longer evolved.
6. Purging with the carrier gas of the reactor with
the sample at 300°С to dry the sample after
hydrolysis.
The synthesis was performed in a laboratory setup
with a flow-through reactor; the carrier gas was dry air [12]
which was used to deliver to the reaction chamber chromium(VI) oxochloride, ethanol, and distilled water vapors in a
preset sequence.
The experimental setup presented in Fig. 6 includes
a flow-through reactor with a partition and indirect heating
(6), a source of carrier gas (1) whose rate was varied by
means of a fine control valve and measured with a RM-1 rotameter (3), a system of gas preparation (2), and a dispenser
for CrO2Cl2 (5). The temperature in the reaction chamber was
maintained with a TRM-202 temperature controller (11) with
an accuracy of ±5°С. In the synthesis of thermally stable
Cr(III) oxide surface structures we made use of ethanol vapors to supply oxygen and reduce Cr(VI) to Cr(III) [13]. To
remove the possible traces of hydrocarbon groups, we performed additional hydrolysis of the modified sample in a
stream of air, saturated with steam in a bubbler unit (8). The
I. CHEMISTRY AND CHEMICAL TECHNOLOGY • CHEMISTRY OF SOLID SUBSTANCES AND NANOTECHNOLOGY
products released during vapor-phase hydrolysis
trapped at the reactor outlet in an absorber flask (9).
were
Fig. 7. DR spectra of CM samples with chromium
oxide nanodopants: 1 - 1Сr; 2 - 2Cr; 3 - 3Cr; and 4 - 4Cr
Analysis of the DR spectra (Fig. 7) shows that already after the first CM treatment cycle absorption bands at
280 and 390 nm appear and grow with increasing thickness
of the chromium oxide layer. The presence of these absorption bands in the spectrum provides evidence for the formation Cr(III) oxide nanostructures on the surface of the
starting matrix [14, 15].
DTA analysis (Fig. 8) revealed three temperature
ranges for which we performed comparative analysis of the
effect of dopant concentration on weight loss (Table 3).
Fig. 6. Scheme of the experimental setup: 1, 7 - flow boosters for carrier
gas P-2; 2 - gas drying unit; 3 - fine control valve; 4 - RM-04 rotameter; 5
- chromium oxochloride container; 6 - reactor with indirect heating and
porous partition; 8 - H2O or С2Н5ОН bubbler unit; 9 - absorber flask; 10 thermocouple; 11 - TRM-202 temperature controller; 12 - vacuum gauge;
and (13−22) - valves
25-400
0.37
2Cr
25-400
0.36
3Cr
25-400
0.42
4Cr
25-400
0.54
0.37
0.48
0.5
0.51
0.65
600825
550800
550750
550750
550700
Weight loss,
%
1Cr
400600
400550
400550
400550
400550
Range,
о
С
0.21
1 substep
Weight loss,
%
25-400
CM
Range,
ºС
4,9
4,7
4,6
3,3
3,2
III
1 substep
Weight loss,
%
CM
1Cr
2Cr
3Cr
4Cr
Concentration
Cr(III),
per Cr2O3,
wt %
mmol/g


0,019
0,14
0,030
0,23
0,044
0,34
0,056
0,43
II
Range,
ºС
Specific surface
area, m2/g
Stages
I
Weight loss,%
Sample
Table 3. Temperature ranges and weight losses of CM and chromiumdoped CM samples at different stages of thermal transformations
Range, ºС
Table 2. Effect of the chromium concentration on the specific surface area
of CM
Fig. 8. Results of the differential thermal analysis of the ceramic mass with
chromium oxide nanodopants after 1 (а), 2 (b), 3(c), and 4 (d) cycles of
exposure to CrO2Cl2, C2H5OH, and H2O vapors. Curves: 1 - weight loss; 2 DTG; and 3- DTA
Sample
The implementation of the above protocol allowed
to form on the surface of each CM particle a chromium oxidedoped nanolayer.
Multiple repetition of the above stages allows to increase the dopant concentration. We obtained experimental
samples after 1, 2, 3, and 4 cycles of consecutive treatment
of CM with CrO2Cl2, С2Н5ОН, and Н2О vapors (in what follows,
the samples are referred to as 1Cr, 2Cr, 3Cr, and 4Cr, respectively).
With increasing number of cycles, the concentration
of chromium in the resulting sample tends to increase but the
specific surface area of the sample tends to decrease (Table
2). The latter finding is apparently associated with the fact
that particle surface irregularities and defects, and, consequently, surface roughness of CM component grains are
smoothed during synthesis.
1.17
-
0.00
1.09
800-900
0.03
0.86
0.13
0.8
7501000
750-950
0.34
700-900
0.25
0.11
Analysis of the DTG curves (Fig. 8) and the weight
losses with temperature (Table 3) shows that the chromium
oxide dopant shifts the temperature ranges where the corre-
I. CHEMISTRY AND CHEMICAL TECHNOLOGY • CHEMISTRY OF SOLID SUBSTANCES AND NANOTECHNOLOGY
sponding thermal transformation occur. This is already in the
1Сr sample (i.e. already after one cycle ML) that the second
stage completes at a lower temperature than with CM (550оС
vs. 600°С, respectively). As the content of the chromium oxide dopant increases, the weight losses at the first and second stages of thermal transformations increase. At the third
stage, vice versa, the weight loss decreases with increasing
dopant content (Table 3). Furthermore, unlike what is observed with CM, the thermal transformations of modified
samples at the third stage involve two stages. Therewith, if at
the first stage of stage III the weight loss decreases with
increasing nanodopant content, at the second stage the situation is reversed. Thus, the DTA results show that chromium
oxide dopants strongly affect the character of thermal transformations of CM during high-temperature heating.
The DTG results are in agreement with the dilatometry data. Fig. 9 shows the temperature dependences of
linear shrinkage for sintered CM and chromium-containing CM
samples. It should be noted that both CM shrinkage and
thermal transformations (Table 3) involve several stages. In
the temperature ranges 25−550°С and 550−750°С (800°С),
no essential difference in the behavior of the original and
modifies samples is observed. However, at the third, final
stage in the high-temperature range, the shrinkage start
temperature decreases from 800оС for CM (Fig. 9, curve 1) to
750°С for 1Cr (Fig. 9, curve 2). Increasing multiplicity of alternate and consecutive treatments of CM with Cr2O2Cl2,
C2H5OH, and Н2О vapors slightly further decreases the
shrinkage start temperature. However, as the chromium content of CM increases, the intensity of shrinkage in the considered temperature range (750−950оС) increases. Therewith,
the total shrinkage of 4Cr at 950°С is 1.05% versus 0.65%
and 0.75% for the CM and 1Cr samples, respectively.
trical resistances in view of the engineering requirements to
fabrication of ceramic insulators.
Table 4. Physicomechanical characteristics of the CM and nanodoped CM
samples sintered at 950°С
Sample
Density,
kg/m3
CM
1Cr
2Cr
3Cr
4Cr
2,17
2,27
2,11
2,21
2,15
Load in the
single-grain
crush test,
MPa
4,7±0,4
4,1±0,2
6,2±0,7
4,5±1,1
4,3±0,9
Flexural
Flexural
strength,
MPa;
23,5±1,7
35,1±3,6
28,6±3,0
36,8±0,6
35,6±1,4
As seen from Table 4, the chromium oxide nanodopant only slightly affects the mechanical strength as measured by the single-grain crush test. The absence of a positive
effect is evidently associated with a high brittleness and porosity of the obtained ceramics and high uncertainties involved in values obtained when test samples are crushed
under such low loads. The flexure tests revealed considerable
enhancement of the flexural strength of the sintered chromium oxide−doped samples. The testing results showed that
even one ML cycle would suffice to enhance the flexural
strength of the sintered material by a factor of 1.5. However,
for a denser material, in view of the results of linear shrinkage tests and the final properties of the target material (electrical insulator), three-four consecutive treatments with
CrO2Cl2, C2H5OH, and Н2О vapors are preferable.
The electrical resistance measurements (Table 5) of
the fabricated cathode insulators showed that they all meet
the outgoing inspection standards for this parameter (50
МOhm). Moreover, the insulators fabricated from modified CM
showed a lower tendency for breakout of the insulating layer
compared to CM.
Table 5. Effect of chromium oxide nanodopants on the electrical resistance of condensers
Sample
CM
1Cr
2Cr
3Cr
4Cr
Fig. 9. Temperature dependences of linear shrinkage for CM (1), 1Cr (2);
2Cr (3); and 4Cr (4). Heating rate 10оС/min
Thus, we can conclude from the resulting data that
the modification with chromium oxide nanodopants has a
dramatic impact on thermal transformations of CM at the
high-temperature stage, i.e. on the transformation of the
Veselovskaya clay and the thermal degradation of chalk.
Therewith, the dopant decreases the shrinkage start temperature of the material and the final shrinkage at 950°С compared to the unmodified matrix. The observed effect of chromium oxide dopants on sintering is largely explained by a
high degree of spacial integration of the deposited dopant
structures into the carrier surface, which is attained by the
modification of ceramic powders by the ML technology.
To assess the performance characteristics of the CM
and modified CM samples sintered under isothermal conditions for 1 h at 950°С, we performed comparative tests of
their physical and mechanical properties properties and elec-
0 min
5,0±0,1
5,0±0,1
5,0±0,1
4,7±0,4
5,1±0,8
Electrical resistance, МОhm
1min
3 min
50±2
109±4
56±5
104±5
49±2
99±4
42±2
90±5
50±1
120±9
The results of the present research were used to
develop process instructions (TI 02068474.25000.00136) for
the fabrication of chromium oxide−nanodoped ceramic mass
(MKNKhOD) by the ML nanotechnology and technical specifications (TU 5759−428−02068474−2007) for a new material
for fabrication of X-ray tube insulators. The developed process was implemented on a pilot plant scale at the Chair of
Chemical Nanotechnology and Electronic Engineering Materials, SPbSIT(TU), and nanodoped CM materials are presently
produced and supplied to Svetlana−X-Ray CJSC under the
contact for supply of high technology.
1.
2.
Conclusions
The molecular layering nanotechnology was used to
synthesize CM samples doped with chromium oxide
nanoadditives after one, two, three, and four ML cycles.
Comparative DTA and dilatometry research of the structural chemical transformations and shrinkage of the
starting and modified samples was performed under the
conditions of linear heating (10°С/min) in the range
from room temperature to 1000°С.
It was established that the shrinkage start temperature
of the sintered doped samples is lower compared with
that of CM (750°С vs. 800°С, respectively), whereas the
I. CHEMISTRY AND CHEMICAL TECHNOLOGY • CHEMISTRY OF SOLID SUBSTANCES AND NANOTECHNOLOGY
3.
4.
5.
sintering temperature of doped CM decreases from 1050
to 950°С.
The doped CM samples sintered at 950°С featured
higher densities and flexural strengths compared undoped CM samples.
The ceramic mass nanodoped with chromium oxide by
the molecular layer deposition nanotechnology was
commercialized at the Svetlana−X-Ray CJSC for the
production of ceramic cathode insulators.
This research was financially supported in part by the
Russian Foundation for Basic Research (project №
11−03−00397).
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