Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
1998-10-30
2002-06-11
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C501S135000
Reexamination Certificate
active
06403053
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a process for preparing translucent Sr
x
Ba
1-x
Nb
2
O
6
, x=0.2-0.7, through the reaction sintering at temperatures in a range of 1300° C. to 1320° C., and subsequent low-temperature heat treatment at temperatures in a range of 1260° C. to 1275° C.
BACKGROUND OF THE INVENTION
General conventional solid-state reacton processes using high temperature (>1150° C.) calcination would make powders aggregated. During the sintering process, the aggregation creates nonuniform densification that results in the discontinuous grain growth, whereas pores are inclined to be trapped and remain inside the grain. Thus, high-density ceramic body cannot be obtained. According to the literature survey, there are two ways to achieve high-density strontium barium niobate ceramics, i.e., using sintering aid and applying external pressure. However, the former would deteriorate the properties and the latter is costly and would restrict the shape of ceramics.
On account of this, this invention proposes a method using reaction sintering without pressure to produce a translucent ceramic body of Sr
x
Ba
1-x
Nb
2
O
6
, x=0.2-0.7.
DESCRIPTION OF THE PRIOR ART
Sr
x
Ba
1-x
Nb
2
O
6
, 0.25≦x≦0.75, has a tetragonal tungsten bronze structure; R. R. Neurgaonkar et al., 1988
Ferroelectrics
, Vol. 87, pp.167, thought that when a substance is non-centric ferroelectricity, it can be applied to electro-optical and pyroelectric device. Linear electro-optical coefficient and pyroelectric coefficient are the reference elements in the design of electro-optical and pyroelectric components. Among all electro-optical materials, R. R. Neurgaonkar et al., 1987,
Opt. Eng.
, Vol. 26, pp.392, reported that single crystal Sr
x
Ba
1-x
Nb
2
O
6
is endowed with the highest linear electro-optical coefficient. Furthermore, in 1969, A. M. Glass in
J. Appl. Phys.
, Vol. 40, pp.4699-713 stated that single crystal Sr
x
Ba
1-x
Nb
2
O
6
possesses higher pyroelectric coefficient. Consequently, this material has became the invention subject for many scientists. Since the material was discovered in 1967, K. Megumi et al., in 1976
J. Mat. Sci.
Vol. 11, pp.1583, reported that the growing technology and property of single crystal Sr
x
Ba
1-x
Nb
2
O
6
have been widely invented, and some products are used in industries. However, in actual application, single crystal is still subjected to limits such as shape, size, mechanical strpength, and price, etc. Thus, developing Sr
x
Ba
1-x
Nb
2
O
6
ceramic is an essential trend as its preparation procedure is simpler and easier to control, and mass production of complicated shape is feasible, which would reduce product cost and adds application value.
For translucent electro-optical ceramic, the light permeable quality is closely related to its microstructure. The pores, impurity, and grain boundary will create light scattering or barrier, which would in turn affect its transparency. Consequently, manufacturing translucent sintered sample of high density and uniform grain is critical for obtaining a fine electro-optical ceramic. For Sr
x
Ba
1-x
Nb
2
O
6
of tetragonal tungsten bronze structure, on account of difficult in controlling microstructure and achieving high density, there has been little report on the manufacture of transparent Sr
x
Ba
1-x
Nb
2
O
6
as of present, and only 3 reports claimed that the test samples were able to reach 99% theoretical density.
1. In 1981, K. Nagaka et al. in
Ferroelectrics
, Vol. 38, pp.853, reported that hot-pressing method can be used to achieve density close to 99% theoretically: however, microstructure is sensitive to the pressure direction, and the thickness of the sample produced is smaller than 0.2 mm for translucency.
2. In 1988, S. I. Lee et al. in
Ferroelectrics
, Vol. 87, pp. 209, used Li or La as sintering aid to achieve high density. Despite which, additive will affect its property, and the thickness of the sample produced is smaller than 0.2 mm for translucency.
3. In 1991, N. S. Van Damme et al., in
J. Am. Ceram. Soc.,
Vol. 74, Issue 8, pp. 1785, reported two-step treatment, i. e., presureless-sintered and then hot-isostatic pressing sinter to produce Sr
x
Ba
1-x
Nb
2
O
6
ceramic of high density and translucency. Such manufacturing procedure is rather complex, and the cost of instrumentation and equipment is very expensive. However, the thickness of the samples produced is smaller than 1 mm for translucency.
In summarizing the three reports above, it can be derived that, when using conventional single-phase sintering method to produce Sr
x
Ba
1-x
Nb
2
O
6
, an external pressure or additives may be required to prevent the abnormal grain growth. Moreover, as the ratio of Sr/Ba increases, the temperature for synthesizing single-phase Sr
x
Ba
1-x
Nb
2
O
6
must also be elevated, resulting in powder aggregation, which is not good for subsequent sintering. Thus, means of reducing agglomeration as well as producing small and uniform powders remain to be the precepts of deriving a ceramic of high-density and uniform microstructure.
SUMMARY OF THE INVENTION
The invention proposes a method of reaction sintering to produce translucent Sr
x
Ba
1-x
Nb
2
O
6
. The primary intent of this invention is to present a new production technology, in which SrNb
2
O
6
and BaNb
2
O
6
powders were synthesized at lower temperature (950° C.), as an improvement comparing with the conventional signal-phase sintering which requires calcination of single-phase Sr
x
Ba
1-x
Nb
2
O
6
powders over 1150° C. that has the drawback of serious aggregation. When the two powders are mixed in appropriate ratio and sintered in oxygen, the new method is able to achieve microstructure of high density (close to 99% theoretical density) and uniform size.
The manufacturing method according to the invention comprises taking equal mole weight of SrCO
3
and BaCO
3
to mix with Nb
2
O
5
powders by ball-milling, respectively. The two types of mixed powders are ground when dry, and synthesized in air at a temperature range of 800 to 1050° C. for 1 to 4 hours to produce SrNb
2
O
6
and BaNb
2
O
6
. After mixing approximate amount of SrNb
2
O
6
and BaNb
2
O
6
, the mixture is pressed into green compact, which is then subject to reaction sintering in oxygen under 1300 to 1320° C., followed by 12 hours heat treatment in a temperature range of 1260 to 1275° C. in for the production of translucent Sr
x
Ba
1-x
Nb
2
O
6
.
ADVANTAGE OF INVENTION
1. The process can be derived from sintering in constant pressure without application of hot pressing or hot-isostatic pressing, which reduces cost.
2. High density can be achieved at a lower temperature (1300° C.), and the microstructure can be more easily controlled and less likely to have abnormal grain growth.
REFERENCES:
patent: 3640865 (1972-02-01), Burns et al.
patent: 4187109 (1980-02-01), Megumi et al.
patent: 4283752 (1981-08-01), Layton
patent: 5445806 (1995-08-01), Kinugasa et al.
patent: 5552355 (1996-09-01), Cava et al.
patent: 5954994 (1999-09-01), Kimura et al.
Fang Tsang-Tse
Lee Wen-Jiung
Chang Chi Ping
Griffin Steven P.
Ildebrando Christina
National Science Council
Pacific Law Group LLP
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