Image fixing apparatus and image fixing roller

An image fixing apparatus includes such an image fixing roller for thermally fixing images on an image receiving material that includes (a) a core roller member; and (b) an exothermic phase transition layer provided on the core roller member. The exothermic phase transition layer includes an exothermic phase transition material capable of performing reversible phase transition from an amorphous state to a crystalline state and vice versa, and crystallizing at a crystallization temperature which is lower than a predetermined image fixing temperature, with liberation of crystallization heat therefrom, and the exothermic phase transition material having a melting point higher than the image fixing temperature, thereby additionally increasing the temperature elevation rate before the temperature of the outer peripheral surface of the image fixing roller reaches the image fixing temperature to shorten the warm up time of the image fixing roller.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image fixing apparatus for use in an 
electrophotographic copying machine, more particularly to an image fixing 
apparatus for thermally fixing toner images on a transfer sheet. The 
present invention also relates to an image fixing roller for use in the 
image fixing apparatus. 
2. Discussion of Background 
For example, in a conventional electrophotographic copying machine provided 
with a laser printer, a rotatable photoconductor drum is provided, and 
copies are made with the following steps: A photoconductive portion of the 
photoconductive drum is uniformly charged by a charging unit, and 
information is recorded in the form of latent electrostatic images by the 
application of a laser beam thereto by a laser scanning unit. The latent 
electrostatic images are then developed with toner to toner images by a 
development unit in the electrophotographic copying machine. The developed 
toner images are then transferred to a recording sheet. The 
toner-images-bearing recording sheet is then passed through a thermal 
image fixing apparatus, in which the toner images are thermally fixed to 
the recording sheet. Thus, copies are made by the conventional 
electrophotographic copying machine. 
In the above-mentioned conventional thermal image fixing apparatus, for 
instance, an image fixing roller as illustrated in FIG. 10 is employed, 
which is composed of a hollow core cylinder 21 which is made of, for 
instance, aluminum, and a toner-releasing layer 22 which is made of, for 
instance, a fluoroplastic, and provided on the outer peripheral surface of 
the hollow core cylinder 21. The toner-releasing layer 22 is capable of 
preventing toner from adhering to the outer peripheral surface of the 
image fixing roller during the image fixing process, and releasing toner 
from the surface of the image fixing roller. 
In the image fixing roller, a heater (not shown) such as a halogen lamp is 
provided in a vacant portion within the hollow core cylinder 21 along the 
revolution axis thereof, whereby the image fixing roller is heated from 
the inside thereof by the radiation heat from the heater. 
In parallel with the image fixing roller, there is provided a pressure 
application roller (not shown) which comes into pressure contact with the 
peripheral surface of the image fixing roller. The image fixing roller and 
the pressure application roller are rotated in the same direction in the 
contact portion where the two rollers are mutually in pressure contact, 
and the toner-images-bearing recording sheet is transported so as to pass 
through the contact portion between the two rollers, whereby the toner 
images transferred to the recording sheet are softened by the heat from 
the image fixing roller and fixed to the recording sheet which is held 
between the two rollers, under the application of the pressure thereto by 
the pressure application roller. 
In such a thermal image fixing apparatus, however, a relatively long 
warm-up time is required before the outer peripheral surface of the image 
fixing roller reaches a predetermined image fixing temperature required 
for toner image fixing after the thermal image fixing apparatus is 
powered. 
Conventionally, in order to shorten the warm-up time, the main switch for 
the image fixing apparatus is designed in such a manner that when turned 
on, the preheating of the image fixing roller is started and continued. 
This method, however, has the shortcoming of wasting a significant amount 
of power. 
Further, in order to avoid the above problem, there have been proposed, for 
example, the following various methods for shortening the worm-up time for 
such an image fixing roller: 
A method of providing a resistive heat emitting layer at or near the 
peripheral surface of an image fixing roller (Japanese Laid-Open Patent 
Applications 55-164860, 56-138766 and 2-285383); a method of blackening 
the inner wall of a hollow portion of an image fixing roller to increase 
the radiant efficiency thereof, thereby increasing the heat absorption 
efficiency, and a method of increasing the surface area of the inner wall 
of a hollow portion of an image fixing roller by roughening the surface of 
the inner wall (Japanese Laid-Open Patent Applications 4-34483 and 
4-134387); a method of constructing an image fixing roller composed of a 
heat pipe (Japanese Laid-Open Patent Application 3-139684); a method of 
heating an image fixing roller by electromagnetic induction (Japanese 
Patent Laid-Open Application 4-55055); a method of constructing an image 
fixing roller by use of an electroconductive elastic material and causing 
electric current to flow therethrough, thereby directly heating the image 
fixing roller (Japanese Laid-Open Patent Application 4-186270); and a 
method of constructing an image fixing roller which includes a cylindrical 
heater in which a positive thermistor material is used (Japanese Laid-Open 
Patent Application 4-42185). 
In order to make the above-mentioned methods actually effective in 
practical use, it is required that the core roller for each of the image 
fixing rollers have good heat conductivity. However, there is a limitation 
to the reduction of the thickness of the core roller for increasing the 
heat conductivity in view of the mechanical strength required for the 
image fixing roller for use in practice. Therefore the above-mentioned 
methods are not always practical. Furthermore, a large amount of energy 
has to be applied to the heating elements such as heaters for the image 
fixing rollers in order to sufficiently shorten the warm-up time for such 
conventional image fixing rollers. 
SUMMARY OF THE INVENTION 
It is therefore a first object of the present invention to provide an image 
fixing apparatus comprising an image fixing roller, which is capable of 
sufficiently reducing the warm-up time for the image fixing roller for use 
in practice, without being restricted by the thermal conductivity of a 
core roller member for the image fixing roller. 
A second object of the present invention is to provide the image fixing 
roller for use in the above-mentioned image fixing apparatus. 
The first object of the present invention can be achieved by an image 
fixing apparatus comprising: 
an image fixing roller for thermally fixing images on an image receiving 
material at a predetermined image fixing temperature, the image fixing 
roller comprising (a) a core roller member; and (b) an exothermic phase 
transition layer provided on the core roller member, comprising an 
exothermic phase transition material capable of performing reversible 
phase transition from an amorphous state to a crystalline state and vice 
versa, and crystallizing at a crystallization temperature which is lower 
than the predetermined image fixing temperature, with liberation of 
crystallization heat therefrom, and the exothermic phase transition 
material having a melting point higher than the predetermined image fixing 
temperature, thereby additionally increasing the temperature elevation 
rate before the temperature of the outer peripheral surface of the image 
fixing roller reaches the predetermined image fixing temperature; 
heating means for heating the image fixing roller so as to have the outer 
peripheral surface thereof reach and maintain the predetermined image 
fixing temperature; first phase transition means for performing phase 
transition of the exothermic phase transition material from the amorphous 
state to the crystalline state by heating the exothermic phase transition 
layer for liberation of the crystallization heat therefrom; 
second phase transition means for performing phase transition of the 
exothermic phase transition material from the crystalline state to the 
amorphous state via a melted state by cooling the exothermic phase 
transition layer for successive use of the crystallization heat thereafter 
by use of the first phase transition means; and 
a pressure application roller which is rotated in contact with the 
peripheral surface of the image fixing roller, with the application of a 
predetermined pressure to the image fixing roller. 
In the above image fixing apparatus, it is preferable that the exothermic 
phase transition material for use in the exothermic phase transition layer 
comprise at least one component selected from the group consisting of a 
chalcogen and a chalcogenide. 
The above exothermic phase transition material may further comprise at 
least one additional component selected from the group consisting of the 
elements of Groups IIIA through VIB of the Periodic Table except the 
chalcogen, and a compound comprising any of the elements of Groups IIIA 
through VIB of the Periodic Table except the chalcogenide. 
Instead of the above additional component, the exothermic phase transition 
material may further comprise an exothermic polymeric material capable of 
performing reversible phase transition from an amorphous state to a 
crystalline state and vice versa, and crystallizing at a crystallization 
temperature which is lower than said predetermined image fixing 
temperature, with liberation of crystallization heat therefrom, and said 
exothermic phase transition material having a melting point higher than 
said predetermined image fixing temperature, thereby additionally 
increasing the temperature elevation rate before the temperature of the 
outer peripheral surface of said image fixing roller reaches said 
predetermined image fixing temperature. 
Alternatively, in addition to the additional component, the exothermic 
phase transition material further comprises the above-mentioned exothermic 
polymeric material. 
Alternatively, the exothermic phase transition material for use in the 
exothermic phase transition layer may be a polymeric material having the 
same function as that of the above-mentioned exothermic polymeric 
material. 
Furthermore, in the image fixing apparatus of the present invention, there 
can be employed an exothermic phase transition material which comprises a 
chalcogen and at least one additional component selected from the group 
consisting of the elements of Groups IIIA through VIB of the Periodic 
Table except the chalcogen, and crystal nuclei with the number thereof per 
unit volume of the exothermic phase transition material being 10.sup.6 
/cm.sup.3 or more. 
In the image fixing apparatus of the present invention, the second phase 
transition means may comprise (a) melting means for melting the exothermic 
phase transition material which is in the crystalline state to change the 
crystalline state to the melted state, and (b) cooling means for cooling 
the exothermic phase transition material which is in the melted state to 
change the state to the amorphous state. 
In the image fixing apparatus of the present invention, the image fixing 
roller may further comprise a protective layer which is provided on the 
exothermic phase transition layer and seals the opposite ends thereof. 
Furthermore, in the image fixing apparatus of the present invention, the 
image fixing roller may be provided with a toner release layer on the 
outermost peripheral surface thereof. 
The above toner release layer may also be used as a protective layer for 
protecting the image fixing roller. 
The image fixing apparatus of the present invention can also be constructed 
so as to further comprise a protective layer for protecting the exothermic 
phase transition layer, which is provided on the exothermic phase 
transition layer, and wherein the exothermic phase transition material 
comprises a chalcogen and at least one additional component selected from 
the group consisting of the elements of Groups IIIA through VIB of the 
Periodic Table except the chalcogen, and crystal nuclei with the number 
thereof per unit volume of the exothermic phase transition material being 
10.sup.6 /cm.sup.3 or more, and increasing in the direction of the 
thickness of the exothermic phase transition layer toward the protective 
layer. 
In the image fixing apparatus of the present invention, the core roller 
member for the image fixing roller may comprise a resistive heating layer 
which serves as the heating means for heating the image fixing roller and 
also as the melting means for the second phase transition means, and the 
image fixing roller may further comprise an insulating layer between the 
resistive heating layer and the exothermic phase transition layer to avoid 
the electric connection between the resistive heating layer and the 
exothermic phase transition layer, when necessary. 
Instead of the above mentioned resistive heating layer, a resistive heating 
member can also be employed. More specifically, the image fixing roller 
for the image fixing apparatus of the present invention can be constructed 
so as to further comprise: 
a resistive heating member between the core roller member and the 
exothermic phase transition layer, the resistive heating layer serving as 
the heating means for heating the image fixing roller and also as the 
melting means for the second phase transition means, and 
an insulating layer between the exothermic phase transition layer and the 
resistive heating member. 
In the image fixing apparatus of the present invention, the exothermic 
phase transition material which is in the melted state may be cooled by 
the cooling means for the second phase transition means as the image 
fixing roller is rotated. 
It is preferable that in the image fixing apparatus of the present 
invention, the exothermic phase transition material which is in the melted 
state be cooled with the predetermined pressure applied to the peripheral 
surface of the image fixing roller by the pressure application roller 
being reduced. 
A second object of the present invention can be achieved by an image fixing 
roller for thermally fixing images on an image receiving material at a 
predetermined image fixing temperature, comprising: 
a core roller member; and 
an exothermic phase transition layer provided on the core roller member, 
comprising an exothermic phase transition material capable of performing 
reversible phase transition from an amorphous state to a crystalline state 
and vice versa, and crystallizing at a crystallization temperature which 
is lower than the predetermined image fixing temperature, with liberation 
of crystallization heat therefrom, and the exothermic phase transition 
material having a melting point higher than the predetermined image fixing 
temperature, thereby additionally increasing the temperature elevation 
rate before the temperature of the outer peripheral surface of the image 
fixing roller reaches the predetermined image fixing temperature. 
In the above image fixing roller of the present invention, it is preferable 
that the exothermic phase transition material for use in the exothermic 
phase transition layer comprise at least one component selected from the 
group consisting of a chalcogen and a chalcogenide. 
The above exothermic phase transition material may further comprise at 
least one additional component selected from the group consisting of the 
elements of Groups IIIA through VIB of the Periodic Table except the 
chalcogen, and a compound comprising any of the elements of Groups IIIA 
through VIB of the Periodic Table except the chalcogenide. 
Instead of the above additional component, the exothermic phase transition 
material may further comprise an exothermic polymeric material capable of 
performing reversible phase transition from an amorphous state to a 
crystalline state and vice versa, and crystallizing at a crystallization 
temperature which is lower than said predetermined image fixing 
temperature, with liberation of crystallization heat therefrom, and said 
exothermic phase transition material having a melting point higher than 
said predetermined image fixing temperature, thereby additionally 
increasing the temperature elevation rate before the temperature of the 
outer peripheral surface of said image fixing roller reaches said 
predetermined image fixing temperature. 
Alternatively, in addition to the additional component, the exothermic 
phase transition material further comprises the above-mentioned exothermic 
polymeric material. 
Alternatively, the exothermic phase transition material for use in the 
exothermic phase transition layer may be a polymeric material having the 
same function as that of the above-mentioned exothermic polymeric 
material. 
Furthermore, in the image fixing roller of the present invention, there can 
be employed an exothermic phase transition material which comprises a 
chalcogen and at least one additional component selected from the group 
consisting of the elements of Groups IIIA through VIB of the Periodic 
Table except the chalcogen, and crystal nuclei with the number thereof per 
unit volume of the exothermic phase transition material being 10.sup.6 
/cm.sup.3 or more. 
The image fixing roller of the present invention may further comprises a 
protective layer which is provided on the exothermic phase transition 
layer and seals the opposite ends thereof. 
The image fixing roller of the present invention may further comprise a 
toner release layer which is provided on the outermost peripheral surface 
of the image fixing roller. 
The above toner release layer may also be used as a protective layer for 
protecting the image fixing roller. 
The image fixing roller of the present invention can also be constructed so 
as to further comprise a protective layer for protecting the exothermic 
phase transition layer, which is provided on the exothermic phase 
transition layer, and wherein the exothermic phase transition material 
comprises a chalcogen and at least one additional component selected from 
the group consisting of the elements of Groups IIIA through VIB of the 
Periodic Table except the chalcogen, and crystal nuclei with the number 
thereof per unit volume of the exothermic phase transition material being 
10.sup.6 /cm.sup.3 or more, and increasing in the direction of the 
thickness of the exothermic phase transition layer toward the protective 
layer. 
In the image fixing roller of the present invention, the core roller member 
may be constructed so as to comprise a resistive heating layer for heating 
the image fixing roller and for maintaining the predetermined image fixing 
temperature, and also for melting the exothermic phase material to its 
melting point, and further so as to comprise an insulating layer between 
the resistive heating layer and the exothermic phase transition layer to 
avoid the electric connection between the resistive heating layer and the 
exothermic phase transition layer, when necessary. 
Instead of the above-mentioned resistive heating layer, a resistive heating 
member can also be employed. More specifically, the image fixing roller of 
the present invention can be constructed so as to further comprise: 
a resistive heating member between the core roller member and the 
exothermic phase transition layer, the resistive heating layer being for 
heating the image fixing roller and for maintaining the predetermined 
image fixing temperature, and also for melting the exothermic phase 
material to its melting point, and 
an insulating layer between the exothermic phase transition layer and the 
resistive heating member, to avoid the electric connection between the 
resistive heating member and the exothermic phase transition layer, when 
necessary.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An image fixing apparatus of the present invention comprises: 
an image fixing roller for thermally fixing images on an image receiving 
material at a predetermined image fixing temperature, the image fixing 
roller comprising (a) a core roller member; and (b) an exothermic phase 
transition layer provided on the core roller member, comprising an 
exothermic phase transition material capable of performing reversible 
phase transition from an amorphous state to a crystalline state and vice 
versa, and crystallizing at a crystallization temperature which is lower 
than the predetermined image fixing temperature, with liberation of 
crystallization heat therefrom, and the exothermic phase transition 
material having a melting point higher than the predetermined image fixing 
temperature, thereby additionally increasing the temperature elevation 
rate before the temperature of the outer peripheral surface of the image 
fixing roller reaches the predetermined image fixing temperature; 
heating means for heating the image fixing roller so as to have the outer 
peripheral surface thereof reach and maintain the predetermined image 
fixing temperature; 
first phase transition means for performing phase transition of the 
exothermic phase transition material from the amorphous state to the 
crystalline state by heating the exothermic phase transition layer for 
liberation of the crystallization heat therefrom; 
second phase transition means for performing phase transition of the 
exothermic phase transition material from the crystalline state to the 
amorphous state via a melted state by cooling the exothermic phase 
transition layer for successive use of the crystallization heat thereafter 
by use of the first phase transition means; and 
a pressure application roller which is rotated in contact with the 
peripheral surface of the image fixing roller, with the application of a 
predetermined pressure to the image fixing roller. 
More specifically, the above image fixing apparatus will now be explained 
with reference to FIG. 1 which shows a schematic diagram of an 
electrophotographic copying machine. 
In FIG. 1, reference numeral 1 indicates an electrophotographic copying 
machine; reference numeral 2, an outer cover for the electrophotographic 
copying machine 1; reference numeral 3, a recording sheet feed unit; 
reference numeral 4, a photoconductor comprising a photoconductive layer 
4a on the surface thereof; reference numeral 5, an image transfer unit; 
and reference numeral 6, the image fixing apparatus of the present 
invention. 
The image transfer unit 5 comprises a pair of recording sheet 
transportation rollers 5a, an endless belt 5b trained over the 
transportation rollers 5a and a bias roller (not shown). 
Recording sheets 3a stacked in the recording sheet feed unit 3 are 
successively fed therefrom toward the photoconductor 4 with a 
predetermined timing by a sheet feed roller (not shown). 
Toner images are formed on the surface of the photoconductive layer 4a and 
transferred onto the recording sheet 3a. The recording sheet 3a which 
bears the toner images thereon is then transported to the image fixing 
apparatus 6 along the path shown by the broken line H in FIG. 1. 
The image fixing apparatus 6 comprises a pressure application roller 7 and 
an image fixing roller 8-1, which is an image fixing roller of the present 
invention. The pressure application roller 7 is in pressure contact with 
the image fixing roller 8-1, so that the image fixing roller 8-1 is driven 
in rotation by the rotation of the pressure application roller 7. 
Near the image fixing apparatus 6, there is provided a pair of auxiliary 
rollers 10 for guiding the recording sheet 3a toward the nip between the 
pressure application roller 7 and the image fixing roller 8-1. 
As the pressure application roller 7, there can be employed a conventional 
pressure application roller which comprises a core metal roller 7a made 
of, for example, aluminum or iron, and an elastic layer 7b made of, for 
example, rubber, which covers the entire peripheral surface of the core 
metal roller 7a. 
As mentioned previously, the image fixing roller of the present invention 
comprises (a) a core roller member; and (b) an exothermic phase transition 
layer comprising an exothermic phase transition material capable of 
performing reversible phase transition from an amorphous state to a 
crystalline state and vice versa, and crystallizing at a crystallization 
temperature which is lower than the predetermined image fixing 
temperature, with liberation of crystallization heat therefrom, and the 
exothermic phase transition material having a melting point higher than 
the predetermined image fixing temperature, thereby additionally 
increasing the temperature elevation rate before the temperature of the 
outer peripheral surface of the image fixing roller reaches the 
predetermined image fixing temperature. 
FIG. 2 schematically shows a cross-sectional view of the image fixing 
roller 8-1 for use in the image fixing apparatus 6 according to the 
present invention. 
As the core roller member for use in the image fixing roller 8-1, for 
example, there can be a hollow cylindrical core metal 8a as illustrated in 
FIG. 2. As the material for the hollow cylindrical core metal 8a, 
conventionally employed materials with excellent thermal conductivity such 
as aluminum, aluminum alloys, and SUS, can be employed, but are not 
limited to such particular materials since the material for the core 
roller member is not restricted by the thermal conductivity thereof in the 
present invention. 
On the outer peripheral surface of the hollow cylindrical core metal 8a, 
there is provided an exothermic phase transition layer 8b, which comprises 
the previously mentioned exothermic phase transition material. 
In the present invention, it is required that the exothermic phase 
transition material be capable of performing reversible phase transition 
from an amorphous state to a crystalline state and vice versa, and 
crystallize at a crystallization temperature which is lower than the 
predetermined image fixing temperature, with liberation of crystallization 
heat therefrom, and that the exothermic phase transition material have a 
melting point higher than the predetermined image fixing temperature, in 
order to additionally increase the temperature elevation rate before the 
temperature of the outer peripheral surface of the image fixing roller 
reaches the predetermined image fixing temperature. 
Currently the image fixing temperature is generally in the range of 
180.degree. to 200.degree. C., so that in the case where the image fixing 
temperature is in the range of 180.degree. to 200.degree. C., it is 
preferable that the exothermic phase transition material crystallize at a 
temperature, for instance, in the range of 80.degree. C. to 180.degree. 
C., and that the exothermic phase transition material have a melting point 
higher than 200.degree. C. 
It is also preferable that the exothermic phase transition material be 
capable of repeatedly and easily performing reversible phase transition 
from an amorphous state to a crystalline state and vice versa, with 
liberation of crystallization heat at the crystallization temperature. 
Examples of the exothermic phase transition material for use in the 
exothermic phase transition layer 8b are materials comprising a chalcogen 
such as O, S, Se or Te, or a chalcogenide. 
Specific examples of the chalcogenide are alloys such as Si-S, Si-S-Sb, 
Si-Se-As, Si-Se-Sb, Si-Te, Si-Te-P, Si-Te-As, Si-As-Te, Si-Ge-As-Te, 
Si-Ge-As-Te, Ge-S, Ge-S-In, Ge-S-P, Ge-S-As, Ge-Se, Ge-Se-Tl, Ge-Se-P, 
Ge-Se-As, Ge-Se-Sb, Ge-Te-P, Ge-Te-As, Ge-As-Te, Ge-P-S, Ge-S, Ge-Sb-Se, 
Ge-As-Se, Ge-P-S, As-S-Se, As-S-Tl, As-S-Sb, As-S-Te, As-S-Br, As-S-I, 
As-S-Bi, As-S-Ge, As-S-Se-Te, As-Sb-Tl-S-Se-Te, As-Sb-P-S-Se-Te, As-Se-Cu, 
As-Se-Ag, As-Se-Au, As-Se-Zn, As-Se-Cd, As-Se-Hg, As-Se-Ga, As-Se-B, 
As-Se-Tl, As-Se-P, As-Se-Sb, As-Se-Te, As-Se-I, As-Se-In, As-Se-Sn, 
As-Se-Pb, As-Se-Ge, As-Se-Bi, As-Te-Tl, As-Te-I, As-Te-Ge, Sb-S, and C-S; 
oxides such as SeO.sub.2 ; sulfides containing any of B, Ga, In, Ge, Sn, 
N, P, As, Sb, Bi, O, or Se; selenium compounds containing any of Tl, Si, 
Sn, Pb, P, As, Sb, Bi, O, Se, or Te; and tellurium compounds containing 
any of Tl, Sn, Pb, Sn, Bi, O, Se, As, or Ge. 
The above-mentioned chalcogens and chalcogenides may also be used in 
combination. 
Of the above-mentioned chalcogens and chalcogenide alloys, selenium and 
selenium-tellurium alloys are particularly preferable for use in the 
present invention. This is because selenium and selenium-tellurium alloys 
become amorphous from a melted state when cooled; and crystallize, with 
conspicuous and rapid liberation of crystallization heat, when heated up 
to a crystallization temperature in the range of 80.degree. to 200.degree. 
C. 
The exothermic phase transition layer 8a may further comprise at least one 
additional component selected from the group consisting of the elements of 
Groups IIIA through VIB of the Periodic Table except the chalcogen, and a 
compound comprising any of the elements of Groups IIIA through VIB of the 
Periodic Table except the chalcogenide. 
Specific examples of such an additional component are alloys such as Ge-As; 
oxides such as P.sub.2 O.sub.5, B.sub.2 O.sub.3, As.sub.2 O.sub.3, 
SiO.sub.2, GeO.sub.2, In.sub.2 O.sub.3, Tl.sub.2 O.sub.3, SnO.sub.2, 
PbO.sub.2, K.sub.2 B.sub.4 O.sub.7 NaPO.sub.3, Na.sub.2 Si.sub.2 O.sub.5, 
PbSiO.sub.3 ; and halogenides such as BeF.sub.2 AlF.sub.3, ZnCl.sub.2, 
AgCl, AgBr, AgI, PbCl.sub.2, and PbI.sub.2. 
The exothermic phase transition material for use in the exothermic phase 
transition layer may also be a polymeric material capable of repeatedly 
and easily performing reversible phase transition from an amorphous state 
to a crystalline state and vice versa, with liberation of crystallization 
heat at the crystallization temperature. 
The exothermic phase transition material for use in the exothermic phase 
transition layer may also comprise the above-mentioned exothermic 
polymeric material and the previously mentioned chalcogen or chalcogenide, 
optionally with further addition of at least one component selected from 
the group consisting of the elements of Groups IIIA through VIB of the 
Periodic Table except the chalcogen, and a compound comprising any of the 
elements of Groups IIIA through VIB of the Periodic Table except the 
chalcogenide. 
Specific examples of the exothermic polymeric material for use in the 
exothermic phase transition layer in the present invention are 
polyethylene, polypropylene, polybutene, polyvinylidene fluoride, 
polyoxymethylene, polyoxyethylene, polyoxytetramethylene, 
polyoxyteramethylene, polyoxybischloromethyltrimethylene, polyethylene 
diadipate, polyethylene terephthalate, nylon-6, nylon-7, nylon-8, 
nylon-10, nylon-11, nylon-12, nylon-66, nylon-77, nylon-610, polybutylene 
terephthalate, polychlorotrifluoroethylene, polyvinyl alcohol, polyvinyl 
fluoride, polyvinylidene chloride, polychloroprene, polyethylene oxide, 
polytrifluorochloroethylene, polyvinyl methyl ether, polyacetal, 
polyphenylene sulfide, polyether ether ketone, thermoplastic 
fluoroplastics, aromatic polyester, polyisotactic butadiene, and 
polyteremethylene terephthalate. 
In the image fixing apparatus of the present invention, the image fixing 
roller may further comprise a protective layer which is provided on the 
exothermic phase transition layer and seals the opposite ends thereof. 
To be more specific, with reference to FIG. 2, a protective layer 8c made 
of, for example, fluoroplastic, is provided on the outer peripheral 
surface of the exothermic phase transition layer 8b and seals the opposite 
ends of the exothermic phase transition layer 8b, so that even when the 
exothermic phase transition material in the exothermic phase transition 
layer 8b is melted, the exothermic phase transition material is prevented 
from flowing out of the exothermic phase transition layer 8b. 
The protective layer 8c may be composed of a material such as 
fluoroplastic, which prevents toner from adhering to the protective layer 
8c. In this case, the protective layer 8c can also be used as a toner 
releasing layer. 
Instead of the protective layer 8c, a toner releasing layer may be 
provided, which also may function as the above-mentioned protective layer. 
Alternatively, as such a protective layer or a toner releasing layer, a 
heat-shrinkable tube made of, for example, tetrafluoroethylene - 
perfluoroalkylvinyl ether copolymer (PFA resin), may also be used so as to 
cover the exothermic phase transition layer 8b, with application of heat 
to the heat-shrinkable tube. 
In the image fixing roller 8-1 shown in FIG. 2, a halogen lamp 8d is 
provided within the hollow cylindrical core metal 8a as heating means for 
heating the image fixing roller 8-1 so as to have the outer peripheral 
surface thereof reach and maintain the predetermined image fixing 
temperature. 
The halogen lamp 8d also has the function of heating the exothermic phase 
transition layer 8b to perform phase transition of the exothermic phase 
transition material from the amorphous state to the crystalline state for 
liberation of the crystallization heat therefrom; and has the function of 
heating the exothermic phase transition material to change the crystalline 
phase of the exothermic phase transition material to a melted state. 
On each of the opposite ends of the image fixing roller 8-1, there is 
formed an axial end portion 8e. Furthermore, a cylindrical support portion 
8f is mounted on each of the axial end portion 8e in such a manner that 
the axial end portion 8e is rotatable on the cylindrical support portion 
8f. 
As shown in FIG. 2, inside the cylindrical support portion 8f, there is 
provided an air fan 11-1 as cooling means for rapidly cooling the 
exothermic phase transition layer 8b when performing the phase transition 
of the exothermic phase transition layer 8b from the crystalline state to 
the amorphous state via the melted state. 
The pair of the cylindrical support portions 8f serves as the path for 
guiding cool air through the inside of the image fixing roller 8-1, 
whereby the exothermic phase transition layer 8b is efficiently cooled for 
the phase transition thereof from the crystalline state to the amorphous 
state via the melted state. 
FIG. 3 schematically shows another image fixing roller 8-2 for use in the 
image fixing apparatus of the present invention. The image fixing roller 
8-2 is the same as the image fixing roller 8-1 shown in FIG. 2 except that 
cool air is not passed through the inside of the image fixing roller 8-2, 
but is directly blown against the outer peripheral surface of the image 
fixing roller 8-2 to cool the exothermic phase transition layer 8b by the 
cool air from an air fan 11-2 which is disposed outside, whereby the phase 
transition thereof from the crystalline state to the amorphous state is 
performed via the melted state. 
In the above image fixing apparatus, in order to minimize the deformation 
of the exothermic phase transition layer 8b which is in indirectly 
pressure contact with the pressure application roller 7 during the cooling 
of the exothermic phase transition layer 8b, it is preferable that the 
cool air be blown against the nip 9 between the image fixing roller 8-2 
and the pressure application roller 7 while the image fixing roller 8-2 
and the pressure application roller 7 are rotated. 
Furthermore, it is more preferable to reduce the pressure applied between 
the exothermic phase transition layer 8b and the pressure application 
roller 7 during the above-mentioned cooling of the exothermic phase 
transition layer 8b for preventing the deformation of the exothermic phase 
transition layer 8b. 
In an image fixing roller comprising the core roller member and the 
previously mentioned exothermic phase transition layer provided on the 
core roller member for use in the present invention, the core roller 
member itself may be a resistive heating element which is capable of 
emitting heat when energized by causing an electric current to flow 
through the core roller member, and serves as the heating means for 
heating the image fixing roller and also as the melting means for the 
second phase transition means, optionally with the provision of an 
insulating layer between the core roller member and the exothermic phase 
transition layer in order to avoid the electric connection between the 
core roller member and the exothermic phase transition layer when 
necessary. 
Alternatively, in the image fixing apparatus of the present invention, the 
core roller member for the image fixing roller may comprise a resistive 
heating layer having the same functions as those of the above-mentioned 
resistive heating element, namely, which serves as the heating means for 
heating the image fixing roller and also as the melting means for the 
second phase transition means, and the image fixing roller may further 
comprise an insulating layer between the resistive heating layer and the 
exothermic phase transition layer to avoid the electric connection between 
the resistive heating layer and the exothermic phase transition layer, 
when necessary. 
Instead of the above mentioned resistive heating layer, a resistive heating 
member can also be employed. More specifically, the image fixing roller 
for the image fixing apparatus of the present invention can be constructed 
so as to further comprise: 
a resistive heating member between the core roller member and the 
exothermic phase transition layer, the resistive heating layer serving as 
the heating means for heating the image fixing roller and also as the 
melting means for the second phase transition means, and 
an insulating layer between the exothermic phase transition layer and the 
resistive heating member. 
FIG. 4 is a schematic cross-sectional view of a further example of the 
image fixing roller for use in the image fixing apparatus, which is 
referred to as the image fixing roller 8-3. 
In the image fixing roller 8-3, the hollow cylindrical core metal 8a 
serving as the core roller member itself is a resistive heating element 
having the previously mentioned functions, for instance, a Peltier effect 
type device, and an insulating layer 8g is interposed between the hollow 
cylindrical core metal 8a and the exothermic phase transition layer 8b. 
When the Peltier effect type device is employed as mentioned above, the 
exothermic phase transition layer 8b can also be cooled by reversing the 
direction of the flow of the electric current for energizing the Peltier 
effect type device. 
FIG. 5 is a schematic cross-sectional view of a pressure application roller 
7-1 which also serves as a cooling roller by use of the above-mentioned 
Peltier effect type device for cooling the exothermic phase transition 
layer 8b which is in a melted state to change the state to an amorphous 
state. 
More specifically, in this pressure application roller 7-1, a Peltier 
effect type device 7c is provided between a core metal 7a and an elastic 
layer 7b which covers the core metal 7a as illustrated in FIG. 5. 
When the pressure application roller 7-1 is brought into pressure contact 
with the surface of the image fixing roller 8-3, for instance, and the 
Peltier effect type device 7c is energized so as to cool the pressure 
application roller 7-1, the exothermic phase transition layer 8b is 
cooled, while the pressure applied to the exothermic phase transition 
layer 8b by the pressure application roller 7-1 is appropriately adjusted 
so as to maintain the thickness of the exothermic phase transition layer 
8b appropriately even if the exothermic phase transition layer 8b is 
heated and softened. 
As mentioned previously, in the image fixing apparatus of the present 
invention, there can be employed an exothermic phase transition material 
which comprises a chalcogen and at least one additional component selected 
from the group consisting of the elements of Groups IIIA through VIB of 
the Periodic Table except the chalcogen, and crystal nuclei with the 
number thereof per unit volume of the exothermic phase transition material 
being 10.sup.6 /cm.sup.3 or more. 
An exothermic phase transition layer comprising the above-mentioned 
exothermic phase transition material can be prepared, for example, by 
melting selenium with high purity (99.999%) and tellurium to prepare a 
SeTe alloy with the concentration of tellurium being 5 wt. % or more; or 
by melting a mixture of SeO.sub.2 and selenium with high purity (99.999%) 
with application of heat thereto to prepare a selenium solid solution with 
the amount of dissolved oxygen therein being 1 ppm or more, and depositing 
the thus prepared SeTe alloy or Se solid solution in vacuum on the core 
roller member. 
In the image fixing roller of the present invention, as mentioned 
previously, when the exothermic phase transition layer is heated and the 
state of the exothermic phase transition material therein is changed from 
an amorphous state to a crystalline state, crystallization heat is 
liberated from the exothermic phase transition material, so that the 
exothermic phase transition layer is rapidly heated and therefore the 
surface of the image fixing roller speedily reaches the image fixing 
temperature. Thus, the warm-up time for the image fixing roller can be 
sufficiently shortened. 
After the image fixing temperature is reached, the surface temperature of 
the image fixing roller is controlled by heating means for heating the 
image fixing roller. 
When the exothermic phase transition material in the exothermic phase 
transition layer has been crystallized, the heat conductivity of the 
exothermic phase transition layer is increased, so that the control of the 
image fixing temperature is further facilitated. 
When a series of copying processes have been finished, the exothermic phase 
transition material in the exothermic phase transition layer is 
temporarily heated to a temperature above the melting point thereof and is 
then cooled or allowed to stand to be cooled, whereby the exothermic phase 
transition material changes its phase back to the initial amorphous phase 
so as to be ready to liberate crystallization heat therefrom in the next 
step when heated to its crystallization temperature. 
The crystallization heat is liberated by the crystallization of the 
amorphous exothermic phase transition material, so that the liberation of 
heat of solidification at the melting point of the exothermic phase 
transition material is prevented and the liberation of the accumulated 
internal energy is utilized at the elevation of the temperature thereof. 
Therefore, it is preferable that the exothermic phase transition material 
have great heat of fusion, and perform clear-cut and complete phase 
transition between an amorphous state and a crystalline state. 
Furthermore, it is preferable that the exothermic phase transition 
material have high crystallization rate because if the crystallization 
rate is low and therefore the heat liberation rate is low, the temperature 
of the surface of the image fixing roller cannot be rapidly elevated with 
high efficiency due to the diffusion of heat. 
Generally, the crystallization rate of an amorphous material by the 
elevation of the temperature thereof depends upon the product of the 
number of crystal nuclei per unit volume of the amorphous material 
(crystal nucleus concentration) and the growth rate of crystal thereof at 
the interfaces of crystallites thereof. 
The growth rate of crystal is a specific characteristic of each material 
and therefore cannot be controlled as desired, but the crystal nucleus 
concentration can be controlled by forming specific cites such as 
structural strain in the material or by containing foreign molecules such 
as impurities serving as crystal nuclei in the material. 
The exothermic phase transition material, which comprises a chalcogen and 
at least one additional component selected from the group consisting of 
the elements of Groups IIIA through VIB of the Periodic Table except the 
chalcogen, and crystal nuclei with the number thereof per unit volume of 
the exothermic phase transition material being 10.sup.6 /cm.sup.3 or more, 
has sufficiently great heat of fusing, and can perform complete phase 
transition between an amorphous sate and a crystalline state, with high 
crystallization rate, and therefore can efficiently and rapidly elevate 
the temperature of the surface of the image fixing temperature. 
Furthermore, for use in practice, it is preferable that the exothermic 
phase transition layer for use in the present invention have a glass 
transition temperature (Tg) above room temperature, and a melting point 
which is above the image fixing temperature, but is as close to the image 
fixing temperature as possible, and do not change its properties during 
the repeated crystallization and melting operations. 
In this sense, an exothermic phase transition layer comprising as the main 
component selenium or a selenium-tellurium alloy is particularly 
preferable since such an exothermic phase transition layer has the 
above-mentioned properties. 
A particularly suitable substance for forming crystal nucleus for selenium 
is oxygen. This is because oxygen can form a solid solution with selenium 
in any ratio, and can be bonded to chains of selenium atoms at any 
position thereof, and has a different electronegativity from that of 
selenium, which is considered to be caused by a different atomic radius 
from that of selenium, a spatial strain and a different bonding force 
between oxygen and selenium, so that the rearrangement of the oxygen and 
selenium atoms in the alloy during the recrystallization thereof can be 
facilitated. 
It is further preferable that the image fixing roller for use in the 
present invention comprise a protective layer for protecting the 
exothermic phase transition layer, which is provided on the exothermic 
phase transition layer, and wherein the exothermic phase transition 
material comprises a chalcogen and at least one additional component 
selected from the group consisting of the elements of Groups IIIA through 
VIB of the Periodic Table except the chalcogen, and crystal nuclei with 
the number thereof per unit volume of the exothermic phase transition 
material being 10.sup.6 /cm.sup.3 or more, and increasing in the direction 
of the thickness of the exothermic phase transition layer toward the 
protective layer. 
By increasing the number of the crystal nuclei per unit volume of the 
exothermic phase transition material in the direction of the thickness of 
the exothermic phase transition layer toward the protective layer, 
crystallization heat is liberated more speedily near the protective layer 
so that the crystallization heat liberated from the exothermic phase 
transition layer is transmitted more speedily to the surface of the image 
fixing roller. 
For instance, when the exothermic phase transition layer comprises a SeTe 
alloy with the content of Te being 5 wt. % or more, the concentration of 
Te is increased toward the protective layer to increase the number of 
crystal nuclei near the protective layer. 
The features of this invention will become apparent in the course of the 
following description of exemplary embodiments which are given for 
illustration of the invention and are not intended to be limiting thereof. 
EXAMPLE 1 
There was formed a double cylindrical core roller member 18a which was made 
of aluminum as shown in FIG. 7, with an outer diameter of 40 mm, including 
an inner cylindrical vacant portion corresponding to a portion with 
reference number 18b. 
A fused selenium was injected into the inner cylindrical vacant portion, 
and the inner cylindrical portion was sealed, whereby an exothermic phase 
transition layer 18b composed of selenium, serving as an exothermic phase 
transition material, was formed. 
A commercially available fluoroplastic resin (Trademark "857-305" made by 
DuPont de Nemours, E.I., Co.) was then sprayed onto the outer peripheral 
surface of the double cylindrical core roller member 18a and sintered at 
380.degree. C., whereby a toner releasing layer 18c with a thickness of 
about 20 .mu.m was provided on the outer peripheral surface of the double 
cylindrical core roller member 18a. 
Thus, an image fixing roller No. 1 of the present invention as shown in 
FIG. 6 was fabricated. 
EXAMPLE 2 
An outer peripheral portion with a depth of 0.1 mm was uniformly cut off a 
cylindrical core roller member made of aluminum with an outer diameter of 
40 mm, with the opposite end portions with a length of about 5 mm near the 
opposite bearings therefor being remained and uncut, as shown in FIG. 7, 
whereby a cylindrical core roller member 28a was made. 
With the opposite end portions being masked, a selenium-tellurium alloy 
with a tellurium content of 8 wt. % was deposited in vacuum with a 
thickness of 0.1 mm on the cut outer peripheral surface of the cylindrical 
core roller member 28a, whereby an exothermic phase transition layer 28b 
composed of the selenium-tellurium alloy serving as an exothermic phase 
transition material was formed, with the same level as that of each of the 
opposite end portions of the cylindrical core roller member 28a. 
The cylindrical core roller member 28 was then covered with a 
heat-shrinkable tube made of electroconductive PFA resin and heated to 
300.degree. C., whereby a toner releasing layer 28c with a thickness of 
about 20 .mu.m was formed on the cylindrical core roller member 28. 
Thus, an image fixing roller No. 2 of the present invention as shown in 
FIG. 7 was fabricated. 
EXAMPLE 3 
With reference to FIG. 8, an outer peripheral surface of a cylindrical core 
roller made of aluminum with an outer diameter of 40 mm was subjected to 
chemical etching, whereby a rough surface with undulations of about 0.05 
mm was formed. On this rough surface of the cylindrical core roller member 
38a, a selenium-tellurium alloy with a tellurium content of 30 wt. % was 
deposited in vacuum with a thickness of 0.06 mm, and the 
selenium-tellurium alloy deposited surface was abraded to make the surface 
smooth in such a manner that the aluminum-exposed surface ratio was about 
40%, whereby an exothermic phase transition layer 38b composed of the 
selenium-tellurium alloy serving as an exothermic phase transition 
material was formed. 
On the exothermic phase transition layer 38b, finely-divided particles of a 
commercially available electroconductive fluoroplastic resin (Trademark 
"MP611" made by Du Pont-Mitsui Fluorochemcials Co., Ltd.) were 
electrostatically deposited and then sintered at 380.degree. C., whereby a 
toner releasing layer 38c with a thickness of about 20 .mu.m was formed. 
Thus, an image fixing roller No. 3 of the present invention as shown in 
FIG. 8 was fabricated. 
EXAMPLE 4 
With reference to FIG. 9, on an outer peripheral surface of a cylindrical 
core roller 48a made of stainless steel with an outer diameter of 40 mm, a 
mixture of finely-divided particles of a commercially available 
electroconductive fluoroplastic resin (Trademark "MP611" made by Du 
Pont-Mitsui Fluorochemcials Co., Ltd.) and finely-divided particles of 
selenium with a content of 50 wt. % was electrostatically deposited and 
then sintered at 250.degree. C., whereby an exothermic phase transition 
layer 48b was formed. 
This cylindrical core roller with the exothermic phase transition layer 48b 
was then covered with a heat-shrinkable tube made of electroconductive PFA 
resin and heated to 300.degree. C., whereby a toner releasing layer 48c 
with a thickness of 10 .mu.m was formed on the exothermic phase transition 
layer 48b. 
Thus, an image fixing roller No. 4 of the present invention as shown in 
FIG. 9 was fabricated. 
Comparative Example 1 
With reference to FIG. 10, an inner side of a cylindrical core roller 21 
made of aluminum with an outer diameter of 40 mm was coated with a black 
paint comprising graphite for blackening treatment. 
On the outer surface of the cylindrical core roller member 21, 
finely-divided particles of a commercially available electroconductive 
fluoroplastic resin (Trademark "MP611" made by Du Pont-Mitsui 
Fluorochemcials Co., Ltd.) were electrostatically deposited and then 
sintered at 380.degree. C., whereby a toner releasing layer 22 with a 
thickness of 20 .mu.m was formed. 
Thus, a comparative image fixing roller No. 1 of a conventional type as 
shown in FIG. 10 was fabricated. 
Each of the thus fabricated image fixing rollers Nos. 1 to 4 of the present 
invention and comparative image fixing roller No. 1 was incorporated into 
the image fixing apparatus of a commercially available electrophotographic 
copying machine (Trademark "M210" made by Ricoh Company, Ltd.), and the 
elevation of the temperature of the surface of each of the image fixing 
rollers was measured while each image fixing roller was heated with a 
heater with a power of 960 W. 
The results are shown in FIG. 11, which indicates that the warm-up time of 
any of the image fixing rollers of the present invention is significantly 
shortened in comparison with the warm-up time of the comparative image 
fixing roller No. 1. 
The power applied to the heater for each image fixing roller was increased 
by 40% and cut off when the surface temperature reached 250.degree. C. 30 
minutes after that, the above-mentioned tests were repeated. The results 
were exactly the same as shown in FIG. 11. 
Differential Thermal Analysis of Exothermic Phase Transition Material 
In order to further specifically investigate the exothermic effect of the 
selenium-tellurium alloy with a tellurium content of 8 wt. % employed in 
the exothermic phase transition layer 28b in Example 2, the 
selenium-tellurium alloy was subjected to a differential thermal analysis. 
More specifically, 50 mg of the selenium-tellurium alloy with a tellurium 
content of 8 wt. % was set in a commercially available differential 
thermal analyzer (Trademark "DT-30B" made by Shimadzu Corporation) with a 
temperature elevation rate of 10.degree. C./min. 
The results are as shown in FIG. 12. In FIG. 12, Tci indicates the 
crystallization initiation temperature of the selenium-tellurium alloy, 
which was 131.degree. C.; Tcp, the crystallization peak temperature 
thereof, which was 168.degree. C.; Tcf, the crystallization finalization 
temperature thereof at which the crystallization was finalized, which was 
185.degree. C.; Tm, the melting point thereof, which was 219.degree. C.; 
and Tmf, the temperature at which the endothermic transition was 
finalized, which was 253.degree. C. 
The graph in FIG. 12 indicates that exothermic heat which was generated 
from the crystallization initiation at Tci through the crystallization 
finalization at Tcf was used for shortening the warm up of the surface of 
each image fixing roller of the present invention. 
Relationship between the number of crystal nuclei per unit volume of a SeTe 
alloy serving as an exothermic phase transition material and the 
concentration of Te in the SeTe alloy 
Reference Example 1 
SeTe alloys with the concentrations of Te being 3, 5, 10, 15, 20, 25, 30, 
35, 40 and 50 wt. % were respectively prepared by melting selenium with 
high purity (99.999%) and tellurium in the respectively corresponding 
amounts. 
Each of the SeTe alloys was heated by use of the previously mentioned 
differential thermal analyzer with a temperature elevation rate of 
10.degree. C./min until the crystallization thereof was completely 
finalized with reference to each differential thermal analysis curve, for 
example, as shown in FIG. 12. 
Each of the thus crystallized SeTe alloys was then subjected to a cleavage 
analysis and the number of spherical crystallites observed per unit area 
of a cross section thereof was counted by a scanning electron microscope 
(SEM). With the thus counted number of the crystallites per unit area of 
the cross section of the SeTe alloy being regarded as the number of 
crystal nuclei before the formation of the crystallites, the number of 
crystal nuclei per unit volume of the SeTe alloy serving as an exothermic 
phase transition material was determined. 
FIG. 13 shows the relationship between the number of crystal nuclei per 
unit volume of the SeTe alloy serving as an exothermic phase transition 
material and the concentration of Te in the SeTe alloy. 
FIG. 14 shows the relationship between the crystallization time of the SeTe 
alloy shown in FIG. 13 and the concentration of Te in the SeTe alloy. 
FIG. 14 indicates that when the concentration of Te is 5 wt. % or more, the 
crystallization time, that is, a time period from the initiation of the 
crystallization through the termination thereof, is sufficiently short for 
use in practice. 
With reference to FIG. 13, the concentration of Te as being 5 wt. % or more 
corresponds to the number of crystal nuclei in the SeTe alloy, with which 
the sufficiently short crystallization time can be obtained. 
Reference Example 2 
Selenium solid solutions, with the amounts of dissolved oxygen therein 
being 0.1, 0.5, 1.0, 5.0, 10.0, 50.0, 100, 500, 1000 ppm and a not 
detective amount of less than 0.01 ppm, were prepared by melting a mixture 
of the respectively corresponding amounts of SeO.sub.2 and selenium with 
high purity (99.999%) with application of heat thereto. 
Each of the solid solutions was heated by use of the previously mentioned 
differential thermal analyzer with a temperature elevation rate of 
10.degree. C./min until the crystallization thereof was completely 
finalized with reference to each differential thermal analysis curve, for 
instance, as shown in FIG. 12. 
Each of the thus crystallized solid solutions was then subjected to a 
cleavage analysis and the number of spherical crystallites observed per 
unit area of a cross section thereof was counted by a scanning electron 
microscope (SEM). With the thus counted number of the crystallites per 
unit area of the cross section of the solid solution being regarded as the 
number of crystal nuclei before the formation of the crystallites, the 
number of crystal nuclei per unit volume of the solid solution serving as 
an exothermic phase transition material was determined. 
FIG. 15 shows the relationship between the number of crystal nuclei per 
unit volume of the solid solution serving as an exothermic phase 
transition material and the amount of dissolved oxygen in the solid 
solution. 
FIG. 16 shows the relationship between the crystallization time of the 
solid solution shown in FIG. 15 and the amount of dissolved oxygen in the 
solid solution. 
FIG. 16 indicates that when the amount of dissolved oxygen in the solid 
solution is 1 ppm or more, the crystallization time is sufficiently short 
for use in practice. 
With reference to FIG. 15, the amount of dissolved oxygen in the solid 
solution being 1 ppm or more corresponds to the number of crystal nuclei 
in the Se solid solution, with which the sufficiently short 
crystallization time can be obtained. Japanese Patent Applications Nos. 
07-116286 and 07-116288 filed Apr. 18, 1995, Japenese Patent Application 
No. 07-144130 filed May 18, 1995, Japenese Patent Application No. 
07-157282 filed Jun. 23, 1995 and Japanese Patent Application No. 
07-281315 filed Oct. 30,1995 are hereby incorporated by reference.