D. HYDROTHERMAL PRECIPITATION OR HYDROTHERMAL HYDROLYSIS
Bạn đang xem bản rút gọn của tài liệu. Xem và tải ngay bản đầy đủ của tài liệu tại đây (24.38 MB, 737 trang )
10
Chemical Processing of Ceramics, Second Edition
TABLE 1.4
Phases Present and Crystallite Size of Products by
Hydrothermal Reaction at 100 MPa for 24 h
Average crystallite size (nm)
Mineralizer
Temperature
(°C)
Tetragonal
ZrO2 (nm)
Monoclinic
ZrO2 (nm)
KF (8 wt%)
KF (8 wt%)
NaOH (30 wt%)
H2O
LiCI (15 wt%)
KBi (10 wt%)
200
300
300
300
300
300
Not detected
Not detected
Not detected
15
15
13
16
20
40
17
19
15
Mined bauxite
Precipitate
Al(OH)3
Aluminum
hydroxide seeds
Mill
Classification
Soda digestion
45-50 psi 150200ºC
Wash
Depressurize &
cool to boiling
temperature
Calcination
1200ºC
Settle and filter
solids
Mill
Cool to 55ºC
FIGURE 1.7 Bayer process (Riman).
© 2005 by Taylor & Francis Group, LLC
Hydrothermal Synthesis of Ceramic Oxide Powders
11
TABLE 1.5
Typical Characteristics of ZrO2 Powders by Hydrothermal Homogeneous
Precipitation
Powder
Chemical composition (wt%)
ZrO2
Y 2O 3
Al2O3
SiO2
Fe2O3
Na2O
Cl–
Ignition loss
Crystallite size (nm)
Average particle size (µm)a
Specific surface area (m2/g)b
Sintered specimens
Bulk density (g/cm3)
Bending strength (Mpa)c
Fracture toughness (Mpam1/2)d
Vicker’s hardness (GPa)
Thermal expansion 20ºC–1000ºC
(10–6/ºC)
a
b
c
d
ZY30
94.7
5.2
0.01
0.01
0.005
0.001
<0.01
1.5
22
0.5
20
1400ºC × 2 h
6.05
1000
6.0
12.5
11.0
ZY80
86.0
13.9
0.01
0.01
0.005
0.001
<0.01
1.5
22
0.5
25
1500ºC × 2 h
5.85
300
2.5
11
10.6
ZP20
>99.9
—
0.005
0.005
0.005
0.001
<0.01
8.0
20
1.5
95
Photo sedimentation method.
BET method (N2).
Three-point bending method.
MI method.
2. Zirconia
Hydrothermal homogeneous precipitation is one of the best ways to produce
zirconia powders. The process, properties of the powders, and microstructure of
the sintered body are shown in Table 1.5 and Figure 1.8, Figure 1.9, and
Figure 1.10.30,31
E. HYDROTHERMAL ELECTROCHEMICAL METHOD
Figure 1.11 shows an apparatus used in the hydrothermal electrochemical method.
For preparing BaTiO3, titanium and platinum plates are used as anodes and
cathodes, respectively. A solution of barium nitrate 0.1 N or 0.5 N and temperatures up to 250°C were used for the experiment. The current density was
100 mA/cm2. Under these conditions we were able to produce BaTiO3 powder.
The BaTiO3 powder produced by this process is shown in Figure 1.12. ZrO2 was
also produced by this method. In the case of ZrO2, Zr plates were used.32,33
© 2005 by Taylor & Francis Group, LLC
12
Chemical Processing of Ceramics, Second Edition
Yttrium chloride, YCl36H2O
Zirconium oxychloride
ZrOCl2-8H2O
Urea, CO(NH2)2
Dissolution
Hydrothermal treatment
Crystalline sol
Washing
Drying
Fine particle
Calcination
Calcined particle
Milling
FIGURE 1.8 ZrO2 produced by the hydrothermal homogeneous precipitation process.
ZY 30
100 nn
ZY 80
100 nn
ZP 20
100 nn
FIGURE 1.9 TEM of different grades of zirconia powder using hydrothermal homogeneous precipitation (Chichibu Onoda Cement Corp.) (see Table 1.5.)
© 2005 by Taylor & Francis Group, LLC
Hydrothermal Synthesis of Ceramic Oxide Powders
13
10 µm
FIGURE 1.10 TEM of zirconia sintered at 1400°C for 2 h.
DC Power
supply
Ammeter
V
Potentiometer
A
Electrolytic
autoclave
A
Potentiostat
Glass
beaker
B
D E
C
FIGURE 1.11 Schematic of the electrochemical cell and circuit arrangements for anodic
oxidation of a titanium metal plate under hydrothermal conditions. (A) Counter electrode
(platinum plate), cathode; (B) thermocouple; (C) stirrer; (D) reference electrode (platinum
plate); (E) working electrode (titanium plate), anode.
© 2005 by Taylor & Francis Group, LLC
14
Chemical Processing of Ceramics, Second Edition
FIGURE 1.12 TEM of BaTiO3 powders prepared by the hydrothermal electrochemical
method (250°C, 0.5 N Ba(NO3)2, titanium plate).
F. REACTIVE ELECTRODE SUBMERGED ARC
Reactive electrode submerged arc (RESA) is a totally new process for making powders.34,35 RESA produces extremely high temperatures (approximately 10,000 K) with
a pressure of 1 atm H2O (possibly more in the nanoenvironment). It allows one to
change liquids very easily. Figure 1.13 shows the apparatus to produce powders.
G. HYDROTHERMAL MECHANOCHEMICAL PROCESS
Ba(OH)2 and FeCl3 were used as starting materials. The precipitate was crystallized hydrothermally in an apparatus (Figure 1.14) combined with an attritor and
Gas inlet
Linear
actuator
Dielectric fluid
Power load
Current
sensor
Power
load
Metal
electrodes
AC
power
225 A
Computer
interface
Printer
FIGURE 1.13 Schematic of microprocessor-controlled RESA apparatus for fine powder
preparation (A. Kumar and R. Roy).
© 2005 by Taylor & Francis Group, LLC
Hydrothermal Synthesis of Ceramic Oxide Powders
15
Teflon propeller
Heater
Heater
Autoclave
Balls
Starting materials
Teflon beaker
FIGURE 1.14 Experimental apparatus for hydrothermal mechanochemical reactions.
ambient water pressure. The starting solutions with the precipitate and stainless
steel balls (5 mm diameter) were placed in Teflon beakers. A Teflon propeller
was rotated in the beaker under 200°C and 2 MPa. The speed of the propeller
was from 0 to 107 rpm. The number of stainless steel balls was 200, 500, and
700. X-ray diffraction profiles are shown in Figure 1.15.36
BaO . 6Fe2O3
BaO . Fe2O3
(c)
(b)
(a)
25
30
35
40
45
50
20 Cuka
FIGURE 1.15 X-ray diffraction profiles of (a) starting materials, (b) material fabricated
at 200°C under 2 MPa for 4 h without rotation, and (c) material fabricated at 200°C for
4 h using 200 balls at 37 rpm.
© 2005 by Taylor & Francis Group, LLC
