Process for making isotropic negative thermal expansion...

Details Process for making isotropic negative thermal expansion... Process for making isotropic negative thermal expansion...

2001-04-17

2002-06-11

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Refractory

C501S103000, C501S104000, C264S653000, C264S660000, C264S681000

Reexamination Certificate

active

06403511

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to isotropic, negative thermal expansion ceramics, and to a process for preparing isotropic, negative thermal expansion ceramics which may, for instance, be used in temperature compensating grating packages.
TECHNOLOGY REVIEW
As described in U.S. Pat. No. 5,694,503, incorporated herein by reference, it is possible to package a fiber grating on or in a negative expansion material so that the package and grating dimensions decrease with an increase in temperature, resulting in minimal variation of the reflection wavelength with temperature.
In order for this approach to be practical the value for the thermal expansion of the package/substrate material must lie within an acceptable range. The ideal expansion coefficient is given by the expression
α
ideal
=
α
fiber
+
-
A
(
1
-
P
e
)
⁢
 
⁢
λ
nom
where &agr;
fiber
is the thermal expansion of the fiber grating (for instance, 0.55×10
−6
° C.
−1
), A is the temperature sensitivity of the unpackaged grating (e.g., 0.0115 nm °C.
−1
for a particular 1550 nm grating), P
e
is the photoelastic constant (typically 0.22) and &lgr;
nom
is the nominal grating wavelength (~1550 nm in many cases). For a particular 1550 nm grating an “ideal” package material would have a thermal expansion coefficient (CTE) of −8.96×10
−6
° C.
−1
. A package material will still be beneficial even if its thermal expansion is not exactly equal to the ideal value. For the above assumptions a factor of about 20 improvement in temperature sensitivity would be achieved if the package material's thermal expansion coefficient were within 0.47×10
−6
° C.
−1
of the ideal value. Such improvement in thermal stability of a fiber grating would be of commercial importance.
It is known that ZrW
2
O
8
is metastable at room temperature, with the lower limit of stability being at 1105±3° C., below which ZrW
2
O
8
decomposes into ZrO
2
and WO
3
. See, for instance, J. Graham et al.,
J. American Ceramics Society,
Volume 42, page 570 (1959), and L. L. Y. Chang et al.,
J. American Ceramics Society,
Volume 50, page 211 (1967). It is also known that ZrW
2
O
8
has a relatively high and isotropic negative coefficient of thermal expansion (CTE) over an extensive range of temperatures that includes room temperature. See, for instance, C. Martinek et al.,
J. American Ceramics Society,
Volume 51, page 227 (1968) and T. A. Mary et al.,
Science,
Volume 272, page 90 (1996). Specifically, the CTE is substantially constant from near absolute zero temperature to 150° C., with a value near −10×10
−6
° C.
−1
. The material exhibits an order-disorder transition at 150° C., after which the CTE drops to −5×10
−6
° C.
−1
. This value of the CTE is maintained until the decomposition of ZrW
2
O
8
which occurs at a relatively high rate near 800° C.
In view of this complex behavior of ZrW
2
O
8
, it is not surprising that earlier attempts to produce mechanically strong monolithic bodies of ZrW
2
O
8
, that have a predetermined negative CTE, did not yield fully satisfactory results. For instance, C. Verdon et al.,
Scripta Materialia,
Volume 36, page 1075 (1997) report that their attempts to form electrically conducting bodies from ZrW
2
O
8
and Cu with a low CTE resulted in decomposition of the ZrW
2
O
8
and formation of Cu
2
O along with other compounds. Such decomposition is generally undesirable and hinders the production of suitable bodies.
To the best of our knowledge, prior art efforts to make ZrW
2
O
8
ceramic bodies used the conventional technique of sintering ZrW
2
O
8
powder. Thus produced bodies have densities less than 90% of the theoretical density and exhibit relatively poor mechanical properties, specifically, a low modulus of rupture. Additional prior art describes the preparation of ZrW
2
O
8
powder from oxide precursors to be an incomplete reaction.
In view of the importance, for instance, a reduction of the temperature dependence of the reflection wavelength of optical fiber gratings, it would be highly desirable to be able to reproducibly make mechanically strong ceramic bodies having tunable negative CTE values, the ceramic bodies being useful, for example, for packaging of fiber gratings. This application discloses a method for making such bodies.
SUMMARY OF THE INVENTION
We have made the surprising discovery that reactive sintering of appropriate percursor powders (e.g., ZrO
2
and WO
3
) can result in (negative CTE) bodies (e.g., ZrW
2
O
8
) with substantially improved properties, as compared to analogous bodies produced by the prior art sintering techniques.
To the best of our knowledge, the prior art does not provide any suggestion that the use of reactive sintering could provide the observed improved results. By “reactive sintering” we mean herein compacting the unsintered body of the precursor oxides, rather than the powder of the desired final phase, and then forming the desired phase and densifying the body in a single heat treatment step.
In a broad aspect of the invention is embodied in a method of making negative CTE ceramic bodies, and in bodies produced by the method.
More specifically, the invention is embodied in a method of making a ceramic body having isotropic negative thermal expansion, the body having a major constituent selected from the group consisting of ZrW
2
O
8
, HfW
2
O
8
, ZrV
2
O
7
and HfV
2
O
7
. The method comprises the steps of providing a powder mixture, forming a “green” body that comprises the powder mixture, and heat treating the green body. The powder mixture comprises a first and a second oxide precursor powder, selected respectively from the group consisting of ZrO
2
powder and HfO
2
powder, and the group consisting of WO
3
powder and V
2
O
5
powder. The heat treatment of the green body includes heating the body to a temperature below the melting temperature of the selected constituent, such that the selected major constituent is formed from the green body by reactive sintering.
By a “green” body we mean herein the compacted precursor powders as a monolithic body prior to sintering.
In a preferred embodiment the powder mixture has a non-stoichiometric composition (e.g., excess ZrO
2
), and the heat treatment results in formation of a 2-phase material, e.g., ZrW
2
O
8
majority phase, with ZrO
2
inclusions dispersed in the majority phase. This embodiment allows tailoring of the CTE of the body.
In a further preferred embodiment the powder mixture comprises a minor amount of a sintering aid (e.g., Y
2
O
3
, Bi
2
O
3
, Al
2
O
3
, ZnO, TiO
2
, SnO
2
), whereby the density of the sintered body is substantially increased.
Important note for processing
Due to the unusually narrow stability region of ZrW
2
O
8
, standard techniques for synthesizing and densifying composite ceramics containing ZrW
2
O
8
produce monoliths with inadequate physical properties. High purity ZrW
2
O
8
is thermally stable between 1105 and 1260° C. Above 1260° C. ZrW
2
O
8
peritectically decomposes into a Liquid phase, and below 1105° C. it decomposes into ZrO
2
and WO
3
. Additives significantly alter the stability region of ZrW
2
O
8
; this study demonstrates a working temperature range of 1140 to 1180° C. for Y
2
O
3
doped ZrW
2
O
8
.


REFERENCES:
patent: 4303447 (1981-12-01), Buchanan et al.
patent: 5322559 (1994-06-01), Sleight
patent: 5334562 (1994-08-01), Newkirk et al.
patent: 5433778 (1995-07-01), Sleight
patent: 5694503 (1997-12-01), Fleming et al.
CA 129:18685, Evens et al, “Negative Thermal Expansion Materials” 1997.*
CA 68:82042, Trunov et al, “X-ray Diffracion Study of Zirconium Molybdate, Hafnium Molybdate, Zirconium Tungstenate and Hafnium Tungstenate” 1967.*
CA 130:84659, Holzer et al, “Metal-Matrix Composites with Dispersed Oxide Particles for Low Thermal Expansion and High Thermal Conductivity”. Jun. 1998.*
J. Graham et al., “A New Ternary Oxide, ZrW2O8”,Journal American Ceramics Society, vol. 42, No. 11, p. 570 (1959).
Luke L.Y.

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