Method of making silicon clathrates

Details Method of making silicon clathrates Method of making silicon clathrates

2000-05-24

2002-07-23

Nguyen, Ngoc-Yen (Department: 1754)

Chemistry of inorganic compounds

Silicon or compound thereof

Binary compound

C423S348000, C423S349000, C136S236100

Reexamination Certificate

active

06423286

ABSTRACT:

The present invention relates to a method of making a substantially homogenous, silicon clathrate composition, and more particularly, to a method of making a substantially homogenous, alkali-metal free Si
136
clathrate composition.
BACKGROUND OF THE INVENTION
Thermoelectric materials are solid-state materials that can reversibly convert electricity to heat. Key properties for any thermoelectric material are thermoelectric power, electrical conductivity and thermal conductivity. These three parameters are often combined into a dimensionless figure of merit ZT that characterizes efficiency of conversion of thermal energy into electrical energy (or vice versa) (see, e.g. Mahan, G., Sales, B., and Sharp, J., Physics Today, p. 42, (March 1997). Most thermoelectrical materials used presently have ZT~1. However, for greater efficiency, thermoelectric materials with a ZT value approaching 2 or greater are preferred. In order to meet these requirements, thermoelectric materials must exhibit large thermoelectric powers and electrical conductivities while at the same minimizing thermal conductivities. One attempt to synthesize thermoelectric materials with larger ZT values has been the development of semiconductor clathrates (i.e., metal-doped semiconductors).
Semiconductor clathrates such as alkali-metal silicon clathrates having the formula M
x
Si
136
with x~3 to 6 have been found to exhibit large thermoelectric powers. As a result, M
x
Si
136
clathrates appear promising as new thermoelectric materials. Unfortunately, current methods of synthesizing these clathrates by thermal decomposition of alkali-metal silicides produce a mixture of M
x
Si
136
and M
y
Si
46
phases (see, e.g., Cross, C., Pouchard, M., and Hagenmuller, P., C. R. Acad. Sc. Paris, vol. 260, p. 4764 (1965); Cross, C., Pouchard, M., and Hagenmuller, P., J. Solid State Chem. Phys., vol. 2, p. 570, (1970); Roy, S. B., Sim., K. E., and Caplin, A. D., Phil. Mag. B, vol. 65, p. 1445 (1992)). As known to those skilled in the art, M
y
Si
46
has metallic character. As a result, mixtures of M
x
Si
136
and M
y
Si
46
phases exhibit significantly lower thermoelectric powers than homogenous M
x
Si
136
phases. To overcome this deficiency, extensive purification steps are required to obtain a homogenous M
x
Si
136
sample which increases the cost of production. Therefore, there is a need in the art for a simple and efficient method of synthesizing homogenous M
x
Si
136
clathrates, which will facilitate large scale commercial applications.
Accordingly, it is an object of the present invention is to provide a simple and effective of making homogenous M
x
Si
136
clathrates. It also an object of the present invention is to provide a method of making homogenous M
x
Si
136
clathrates that will allow the number of alkali-metal atoms per unit crystallographic cell “x” to be varied from a maximum value 24 to as low as 0.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a simple and effective method of making a substantially homogenous silicon clathrate composition. In one embodiment, the method entails rapidly heating, under a vacuum, an alkali-metal silicide to a decomposition temperature of at least 365° C. thereby forming a silicon clathrate having the formula M
x
Si
136
where M is the alkali-metal and 3≦x≦24. Preferably, the silicide is heated to the decomposition temperature within 2 minutes, and more preferably within 1 minute. Preferably, the alkali silicide is heated for at least 30 minutes with at least 60 minutes being more preferred. Examples of alkali-metals to be used for the alkali-metal suicides are sodium, potassium, cesium, rubidium and mixtures thereof. Heating is preferably conducted under a vacuum of at least 10
−5
Torr, with at least 10
−6
Torr being more preferred.
Advantageously, the method provides a silicon clathrate composition where at least 90 weight percent of the silicon clathrate present in the composition is M
x
Si
136
, with at least 95 weight percent M
x
Si
136
being more preferred and at least 98 weight percent M
x
Si
136
being more preferable. Preferably, the silicon clathrate formed by the above-described method has 3≦x≦6.
In another embodiment, the present invention provides a method of reducing the alkali-metal content of an alkali-metal silicon clathrate having the formula M
x
Si
136
where M is the alkali-metal and x>0, with x>3 being more preferred. This method entails a two-step process of: (a) treating the silicon clathrate with an acid, preferably strong acid; and (b) rapidly heating, under a vacuum, the treated silicon clathrate to a decomposition temperature of at least 400° C. The two steps are repeated, as necessary, to reduce the alkali-metal content to a desired number of atoms. Acids to be used include sulfuric acid, hydrochloric acid, hydrofluoric acid, nitric acid and mixtures thereof. If desired the alkali-metal content “x” can be reduced to zero to provide an alkali-metal free silicon clathrate having the formula Si
136
.


REFERENCES:
patent: 5800794 (1998-09-01), Tanigaki et al.
patent: 6103403 (2000-08-01), Grigorian et al.
patent: 6188011 (2001-02-01), Nolas et al.
Smelyansky et al, The electronic structure of metallo-silicon clathrates NaxSi136(x=0, 4, 8, 16 and 24), chemical Physics Letters 264, pp. 459-465, Jan. 1997.*
Translation of Cros et al, Journal of solid state chemistry, 2, pp. 570-581, 1970 (no month).*
Kasper et al., “Clathrate Structure of Silicon Na8Si46and NaxSi136(x<11),” Science, American Association for the Advancement of Science, vol. 150, pp. 1713-1714 (1965) No month.*
Ramachandran et al., “Synthesis and X-Ray Characterization of Silicon Clathrates,” Journal of Solid State Chemistry, vol. 145, No. 2, pp. 716-730 Jul. (1999).*
Ramachandran et al., “Silicon Clathrates: Synthesis and Characterization,” Mat. Res. Soc. Symp. Proc., vol. 507, pp. 483-486 Apr. (1998).*
Yamanaka et al., “Preparation of Barium-Containing Silicon Clathrate Compound,” Fullerene Science & Technology, vol. 3, No. 1, pp. 21-28 (1995). No month.

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