U.S. patent number 3,781,523 [Application Number 05/238,422] was granted by the patent office on 1973-12-25 for thermochromic surface heating apparatus.
This patent grant is currently assigned to General Electric Company. Invention is credited to Marcus P. Borom.
United States Patent |
3,781,523 |
Borom |
December 25, 1973 |
THERMOCHROMIC SURFACE HEATING APPARATUS
Abstract
A smooth surface heating apparatus is provided having a heat
spreader plate of high thermal conductivity coated, at least on its
upper surface, with a thermochromic glass-ceramic material
containing a predominant crystalline phase of lithium disilicate in
a glassy matrix and having a coefficient of expansion in the range
of 80-120.times.10.sup..sup.-7 per .degree.C. The thermochromic
property is obtained by the addition of cadmium sulfide and
selenium preferably with zinc oxide to the batch ingredients. An
insulated electrical resistance heating element and a reinforcing
member are attached to the underside of the heat spreader plate. A
reflector pan is provided beneath the heating element to direct the
heat in an upward direction.
Inventors: |
Borom; Marcus P. (Schenectady,
NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
22897824 |
Appl.
No.: |
05/238,422 |
Filed: |
March 27, 1972 |
Current U.S.
Class: |
219/465.1;
219/530; 219/544; 252/408.1; 252/962; 392/422; 436/2; 436/149;
501/5 |
Current CPC
Class: |
C03C
14/006 (20130101); C03C 1/10 (20130101); C03C
10/0027 (20130101); F24C 15/10 (20130101); H05B
3/746 (20130101); C03C 4/00 (20130101); H05B
3/688 (20130101); C03C 4/02 (20130101); Y10S
252/962 (20130101) |
Current International
Class: |
C03C
14/00 (20060101); C03C 10/00 (20060101); C03C
4/00 (20060101); F24C 15/10 (20060101); C03C
1/10 (20060101); C03C 1/00 (20060101); C03C
4/02 (20060101); H05B 3/68 (20060101); H05B
3/74 (20060101); H05b 003/68 () |
Field of
Search: |
;219/345,347,436,438,461,462,464,530,540,542,543,544 ;117/129,212
;99/447 ;106/39DV ;73/356 ;252/408 ;23/230 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
W A. Weyl, Coloured Glasses, Society of Glass Technology, 1967, pp.
308-313..
|
Primary Examiner: Mayewsky; Volodymyr Y.
Claims
We claim:
1. A surface heating apparatus comprising a metallic heat spreader
plate of high thermal conductivity, an insulated electrical
resistance heating element attached to the underside of the heat
spreader plate for heating thereof, and a thermochromic
glass-ceramic coating covering at least the upper surface of the
plate, said thermochromic glass-ceramic comprising a base
glass-ceramic composition consisting essentially as calculated from
the batch on the oxide basis in weight percent of 6-20% Li.sub.2 O,
0-10% Al.sub.2 O.sub.3, 70-80% SiO.sub.2, 0.5-6.0% P.sub.2 O.sub.5,
0-10% B.sub.2 O.sub.3, 0-6% K.sub.2 O and 0-5% ZnO, and 0.5-2.0%
CdS, 0.5-2.0% Se, and 0-7.0% ZnO to impart a reversible color
transformation to the coating upon exposure to temperature changes,
wherein the coefficient of expansion of the glass-ceramic is in the
range of 80-120.times.10.sup.-.sup.7 per .degree.C. and the major
crystalline phase is lithium disilicate.
2. The heating apparatus of claim 1, comprising additionally a
reinforcing member attached to the underside of said plate to
prevent warpage and a reflector pan beneath the heating element to
direct the heat in an upward direction.
3. The heating apparatus of claim 1, wherein said glass-ceramic
coating has a thickness of about 3 to 12 mils.
4. The heating apparatus of claim 2, wherein said coating covers
both sides of the heat spreader plate and the heating element.
5. The heating apparatus of claim 1, wherein the glass-ceramic
coating contains up to about 50 percent of a crystalline phase.
6. The heating apparatus of claim 1, wherein said thermochromic
glass-ceramic coating comprises about 93 parts by weight of the
base glass-ceramic composition consisting essentially as calculated
from the batch on the oxide basis in weight percent of about 73.0%
SiO.sub.2, 13.7% Li.sub.2 O, 5.6% K.sub.2 O, 4.9% Al.sub.2 O.sub.3
and 2.8% P.sub.2 O.sub.5 and the balance being said thermochromic
additive consisting essentially as calculated from the total
thermochromic glass batch in weight percent of 1% CdS, 1% Se and 5%
ZnO.
7. The heating apparatus of claim 1, wherein said heat spreader
plate is a composite metal sheet material having a core selected
from the group consisting of copper, silver and aluminum placed
between two integral outer layers of a metal or alloy selected from
the group consisting of carbon steel, stainless steel, nickel and
chromium.
Description
Conventional electric cooktops are usually provided with a
plurality of metal sheathed electrical resistance heating elements
which are each wound in the form of a spiral coil and positioned in
an opening formed in the cooktop. Each heating element is adapted
to support a cooking utensil thereon. Food soil can be
automatically cleaned from the metal sheathed heating elements by
the high temperatures reached once the elements are energized,
while any spillovers are permitted to drain through the heating
element and are accumulated in a collection pan located beneath the
cooktop from which they must be manually cleaned.
In order to simplify the cleaning process and to provide a more
esthetic appearance, entire counter cooktops or in some cases
individual solid surface heating units have been manufactured in
which the exposed surface is formed from a glass-ceramic material
such as described in U.S. Pat. No. 2,920,971. Materials of this
type are commercially available under the trademarks "PYROCERAM",
"CER-VIT", and "HERCUVIT". The opaque glass-ceramic, because of its
smooth top surface, not only presents a pleasing appearance, but is
also readily cleanable and avoids the drainage of spillovers into
the subsurface portions of the unit.
However, a problem which must be considered is that of obtaining
rapid heating rates and rapid cooling rates comparable to those of
either a standard metal sheathed electrical resistance heating
element or a gas surface burner. The rapid transfer of heat through
a thick glass-ceramic material does not occur because of its rather
poor thermal conductivity. Such materials are widely used as both
thermal and electrical insulators, rather than as thermal
conductors. Heat does not readily diffuse laterally through the
glass-ceramic plate, and during the cooking procedure heat is
transferred to the utensil primarily by means of conduction at
points of contact between the heating surface unit and the utensil.
Moreover, the glass-ceramic plate has a comparatively large heat
capacity, which further contributes to slow cooling when the
heating element is turned off. Also, this type of glass-ceramic
plate becomes more electrically conductive as the temperature is
increased, so that a safety hazard might be created when an
open-coiled heater is employed as the electrical heating means.
Some of the problems of the prior art have been overcome by
locating beneath the glass-ceramic plate a heat spreader plate of
high thermal conductivity which bears against the underside of the
glass-ceramic plate by mechanical means and thereby creates an even
temperature distribution as disclosed in U.S. Pat. No. 3,622,754.
Such a unit is capable of efficient operation when used with
conventional cooking utensils.
A further improvement has been made by Borom et al., U.S. Pat.
application Ser. No. 236,104, filed Mar. 14, 1972, (RD-4299) which
discloses a smooth surface heating apparatus having a heat spreader
plate of high thermal conductivity, coated at least on its upper
surface, with a glass-ceramic material containing a predominant
crystalline phase of lithium disilicate in a glassy matrix and
having a coefficient of expansion in the range of
80-120.times.10.sup.-.sup.7 per .degree.C. The disclosure set forth
in that patent application is hereby incorporated by reference.
Commercially available electric cooktops usually have an indicator
light to show when the heating unit is on. However, even after the
heating unit has been turned off, the cooktop may still remain
quite hot and reasonable care must be exercised to avoid burn
hazards.
Quite surprisingly, we have now discovered an improved surface
heating unit in which the glass-ceramic coating contains a
thermochromic material capable of imparting a reversible color
transformation to the coating upon exposure to temperature changes.
Thus, the glass-ceramic coating not only has chemical corrosion
resistance, high mechanical strength, good adhesion to the
substrate, high softening temperature, good thermal shock
resistance, and a coefficient of thermal expansion which
approximately matches the substrate, but also the coating provides
a visual indication whether the heating unit is hot or cold.
In accordance with the present invention, we have discovered a
smooth surface electric heating apparatus comprised of a heat
spreader plate of high thermal conductivity, an insulated
electrical resistance heating element attached to the underside of
the plate, a reinforcing member also attached to the underside of
the plate to prevent warpage, a reflector pan beneath the heating
element to direct the heat in an upward direction, and a
thermochromic glass-ceramic coating bonded directly to at least the
upper surface of the heat spreader plate.
The base glass-ceramic coating material consists essentially as
calculated from the batch on the oxide basis in weight percent of
the following:
Ingredient w/o Li.sub.2 O 6-20 Al.sub.2 O.sub.3 0-10 SiO.sub.2
70-80 P.sub.2 O.sub.5 0.5-6.0 B.sub.2 O.sub.3 0-10 K.sub.2 O 0-6
ZnO 0-5
wherein the coefficient of expansion of the glass-ceramic is in the
range of 80-120.times.10.sup.-.sup.7 per .degree.C. The thickness
of the thermochromic glass-ceramic coating should be sufficient to
form a protective coating on the surface of the heat spreader plate
and typically is about 3 to 12 mils. It is comprised of a
crystalline phase of predominantly lithium disilicate (Li.sub.2
O.sup.. 2SiO.sub.2) in a glassy matrix. In order to obtain the
desired strength properties the amount of lithium metasilicate
(Li.sub.2 O.sup.. SiO.sub.2) should be kept to a minimum. The
percent crystallinity may vary to some extent and is preferably up
to about 50 percent and may be somewhat higher as determined by
X-ray diffraction techniques.
The thermochromic property is imparted to the base glass-ceramic
composition by the presence of a sufficient amount of a
thermochromic additive to impart a reversible color transformation
to the coating upon exposure to temperature change. Such a color
transformation is obtained by the addition of a mixture of cadmium
sulfide and selenium to the batch ingredients. Preferably the
thermochromic additive also contains a certain percentage of zinc,
usually in the form of ZnO or ZnS, which is believed to stabilize
the sulfur during smelting of the glass. The ranges of the amounts
of ingredients as calculated from the total thermochromic
glass-ceramic batch in weight percent is generally 0.5-2.0% CdS,
0.5-2.0% Se, and 0-7.0% ZnO. It is not fully understood how the
thermochromic transformation occurs or exactly how the additive is
combined in the glass-ceramic composition and, consequently, I have
set forth a simple way of describing the additive by calculating
the ingredients in weight percent from the total batch. However,
the thermochromic system may be substantially more complicated and
the cadmium sulfide may be formed from CdO reacted with an excess
of sulfur, the selenium may be added as Se or as a selenide, and
the zinc may be added as ZnO or ZnS. In smelting of the glasses
containing sulfides the batch should be selected with a low water
content. Also, since even traces of iron may affect the resulting
colors, only high purity raw materials should be used.
Smelting of the initial glassy material should be under reducing
conditions. When using a gas fired furnace the air/gas ratio should
be adjusted to yield a glass which when cast on a plate develops a
transparent red color. A rapid water quenching of the melt results
in a transparent colorless glass which can be struck to develop a
red coloration upon heating to 500-600.degree.C. for 5-10 minutes.
After the glassy material has been prepared, it is ball milled to a
particle size of -100 mesh U.S. standard and used in the process
described hereinbelow.
The invention is more clearly understood from the following
description taken in conjunction with the accompanying drawing in
which:
FIG. 1 is a partially broken-away plan view of a smooth surface
electrical heating apparatus employing the present invention;
FIG. 2 is a fragmentary cross sectional view of the surface
electrical heating unit of FIG. 1 taken along line 2--2 with parts
broken away to show the various portions of the assembly; and
FIG. 3 is another fragmentary cross sectional view taken along line
2--2 of a modification of FIG. 2 illustrating a further embodiment
of the invention.
Turning now to a consideration of the drawings and in particular to
FIG. 2, there is shown a fragmentary cross sectional view of a
smooth surface electrical heating unit 10 which has a heat spreader
plate 12 of high thermal conductivity, preferably of a thin
composite metal sheet material with a thin center core 13 for
distributing the heat rapidly over the entire plate so as to obtain
a generally uniform temperature distribution. Such a core would be
selected from metals and alloys such as copper, silver and
aluminum. Copper has very low strength at temperatures ranging in
the vicinity of 700.degree.C., and also it oxidizes very readily.
Since a copper core sheet 13 is of small thickness, on the order of
0.040 inches, it would tend to warp or deform easily under normal
use conditions due to thermal stresses caused by temporary uneven
temperature distribution during the preheat period and also due to
the high temperatures to which it is exposed. Hence, the core 13 is
sandwiched or sealed between two thin, integral skins 15 and 17,
each of the thickness of about 0.017 inches. Such skins would be
selected from metals and alloys such as stainless steel, nickel and
chromium. In any selection of materials it should be borne in mind
that the core and skin materials should have matched coefficients
of thermal expansion or that the skin materials be of sufficient
strength to support the stresses arising from any thermal mismatch
without distortion. In order to avoid exposure of the copper on the
peripheral edge of the plate, the two stainless steel skins 15 and
17 are sealed over the edge of the core with a pinching action to
protect against corrosion and oxidation. The stainless steel skins
15 and 17 being on the outer surface of the composite plate 12
provide strength to the plate and resists warpage because it
combines a high strength with high heat diffusivity, which no
single material plate can provide. This thin composite sheet
material 12 is illustrated by a central copper core 13 and two
outer stainless steel skins 15 and 17, and it may be formed of
individual sheets which are "area welded" by a process such as
explosive welding, which causes a bonding of the metal sheets along
their mating surfaces.
A metal sheathed resistance heating element 14 is brazed to the
underside of the heat spreader plate 12. As is well understood by
those skilled in this art, such a metal sheathed heating element 14
would include a central electrical resistance, nichrome heating
wire of helical formation that is inserted into a thin metal tube
or sheath of Inconel, stainless steel or the like. Then the sheath
is filled with a suitable electrically insulating and thermally
conducting material such as magnesium oxide (MgO) or the like to
separate the heater wire from the metal sheath. The top surface of
the heating element 14 is flattened so as to obtain a good contact
area of the metal sheath with the heat spreader plate 12. Two
terminals 19 of the heating element are shown in FIG. 1 extending
down in a vertical direction beneath the heating element 14, and
are adapted for receiving a slip-on connector (not shown) for
making an electrical connection therewith as is conventional in
this art.
In order to strengthen the heat spreader plate 12, the edge of the
plate is provided with a downturned flange 22 thereby giving the
heat spreader plate a configuration similar to an inverted shallow
pan. Another means of reinforcing the heat spreader plate 12 is to
provide a series of diagonal or radial struts 16 which are arranged
edgewise and fastened to the underside of the heat spreader plate
and possibly to the sheath of the heating element 14 as by brazing
or similar methods. Such strut members 16 may be of many different
configurations as would be obvious to a person of ordinary skill in
the art. The purpose is to give the heat spreader plate sufficient
depth or beam action so that it does not deflect readily under
thermal or mechanical stresses.
As shown in FIG. 2, a reflector pan 18 is provided beneath the
heating unit 10 and separated therefrom by an annular heat
resistant spacer 20, so as to direct the heat from the heating
element 14 in an upward direction. This reflector may be used as a
hold down means for the heating unit. An adjustable tension member
in the form of an inverted J-bolt 39 is adapted to be connected
between a reinforcing member 38 and the reflector pan 18. The
reinforcing member 38 is provided with an aperature 41 through
which the head of the J-bolt is inserted. The reflector pan 18 has
a central opening 43 for receiving the lower end of the J-bolt
therethrough. The lower end of the bolt has a threaded portion for
receiving an adjusting nut 45 thereon.
Turning now to the unique improvement of the present invention, the
thermochromic glass-ceramic coating 30 discussed hereinabove is
bonded directly to the surface of the heat spreader plate 12. In
the embodiment shown in FIG. 2 the thermochromic glass-ceramic
coating is applied only to the upper surface and covers the portion
exposed to view on the surface of the heating apparatus. On the
other hand, it may be advantageous to coat both the top surface and
the bottom surface of the heat spreader plate 12 together with the
heating means 14 as illustrated by FIG. 3.
In forming the thermochromic glass-ceramic coating 30, the batch
ingredients are initially weighed and mixed as, for example, by
ball milling. Then the batch is melted at elevated temperatures of
about 1,200-1,250.degree.C. to form a homogeneous melt, quenched in
cold water, and ball milled to a particle size of about -100 mesh
U.S. Standard. The glass particles are thereafter combined with
about 3-4 percent by weight of a suspending agent, e.g. clay,
calcined clay, or colloidal silica and minor amounts of other
conventional additives, e.g. electrolytes such as sodium
pyrophosphate, sodium nitrite, to form an aqueous slip. The metal
substrate is prepared for forming an adherent coating by
sandblasting or oxidizing the metal surface and then the aqueous
slip is applied to the metal substrate by conventional means such
as spraying, dipping or coating. The coated metal substrate is now
dried to remove the vehicle and the enamel is matured at a
sufficient temperature of about 1,000.degree. C. for about 1-3
minutes. While the exact heat treatment for nucleation and crystal
growth will vary to some extent with the initial glass composition
within the ranges described, we have found that generally the
optimum conditions for nucleation are about 500-650.degree.C. for
about 0.25-1 hour, while the crystal growth temperatures are
preferably about 750-900.degree.C. for about 0.5-4 hours. When the
growth temperature is below about 750.degree.C., the predominant
crystal phase is lithium metasilicate and additionally when the
growth temperature exceeds 950.degree.C. the crystal phase is
converted to the lithium metasilicate.
Coatings of our novel glass-ceramic materials, on a surface heating
unit as shown in FIG. 1, had excellent mechanical, thermal and
chemical properties. Thus, the coatings showed excellent stain
resistance to mild organic acids as found in lemon juice, ketchup,
barbecue sauce, etc. The thermal properties of the novel
glass-ceramic coated heating unit are illustrated by the fact that
when sodium chloride (m.p. 801.degree.C.) was sprinkled on the
surface of an energized heating unit, the salt becomes molten while
the glass-ceramic coating still remained rigid, maintained its
adhesion to the metal substrate, and was not attacked by the molten
salt. It should be noted that the glass-ceramic coatings useful in
the present invention may be distinguished from those described in
U.S. Pat. No. 2,920,971 in that the latter materials have a
coefficient of expansion of about 0.times.10.sup.-.sup.7 per
.degree.C. whereas our materials are substantially higher and more
closely match the thermal expansion of the metallic substrates used
in the heat spreader plate. Other commercially available enamels
fail to meet the mechanical, thermal or chemical requirements for
making the high temperature surface heating unit of our
invention.
Our invention is further illustrated by the following examples. The
compositions, unless otherwise noted, are given in weight percent
and mole percent as calculated from the batch on the oxide basis.
Initially glasses were prepared by melting the batch ingredients
under standard conditions at temperatures of 1,200-1,600.degree.C.
for about 4-20 hours in platinum crucibles.
Example i
a preferred glass composition was prepared and melted from batch
ingredients to yield the following formulation on the oxide
basis:
Constituent Weight % Mole % SiO.sub.2 73.0 67.5 Li.sub.2 O 13.7
25.4 K.sub.2 O 5.6 3.3 Al.sub.2 O.sub.3 4.9 2.7 P.sub.2 O.sub.5 2.8
1.1
the batch ingredients were weighed and mixed by ball milling. The
batch was then placed in a platinum crucible and melted at a
temperature of 1,200-1,250.degree.C. overnight. The hot melt was
quenched in cold water and ball milled to a particle size of -100
mesh U.S. Standard.
A slip for application onto a metal plate was prepared from the
following formulation:
Ingredient Parts by Weight Glass frit (-100 mesh) 1000 Ferro No. 55
Clay (calcined) 40 Sodium aluminate 2.1 Bentonite 2.1 Gum
tragacanth 0.2 Potassium carbonate 2.5 Distilled water 460
The mixture was ball milled for about 1 hour to form a homogeneous
dispersion. The slip was then applied to yield a fired thickness of
0.006 inch onto a metal plate by the following technique. The slip
was adjusted to a specific gravity of 1.68 gm/cm.sup.3 by the
addition of water. Electrolytes such as tetrasodium pyrophosphate
or sodium nitrite were added to adjust the consistency of the slip
to the point that a metal sheet dipped into the slip would retain
about 36 grams of slip per square foot of metal area on removal and
drainage of the metal part.
In applying the slip onto the metal substrate, the surface of the
metal was initially prepared for obtaining an adherent coating by
sandblasting and/or oxidizing the metal. The slip was loaded in a
spray gun container and applied to the substrate. Thereafter the
sprayed substrate was dried at a temperature of about
100.degree.C., the dried coated substrate was fired at a
temperature of 1,000.degree.C. for 1 minute and cooled to room
temperature. Then the glassy material was nucleated at a
temperature of 645.degree.C. for 1 hour and subjected to a crystal
growth treatment of 830.degree.C. for 4 hours. The predominant
crystalline phase obtained was lithium disilicate (Li.sub.2 O.sup..
2SiO.sub.2). The base glass-ceramic coating is not
thermochromic.
Metal substrates coated by the above technique or modifications
thereof included the following:
316L Stainless steel clad copper
321 Stainless steel.sup.(a)
430 Stainless steel.sup.(a) clad copper
Inconel 600 (International Nickel Co., 72% min. Ni, 14-17% Cr, 6-8%
Fe, 1.75-2.75% Nb, hot rolled and heat treated for high temperature
applications)
Inconel 625 (International Nickel Co.)
Rene 41 (General Electric Co., a precipitation hardened Ni based,
high temperature alloy)
Ti-Namel (Inland Steel Co., 0.06 C, 0.30 Mn, 0.12 max. Cu, 0.05 Al,
0.30 Ti, bal. Fe, hot rolled, for sheets for enamelling, specially
prepared)
Ti-Namel clad copper
Enamelling steel
Example ii
following the procedure of Example I, 93.0 percent by weight of the
preferred glass, on the basis of batch ingredients expressed as
oxides, was mixed in a ball mill with 1.0 percent by weight of CdS,
1 percent of Se, and 5.0 percent by weight of ZnO. The batch
ingredients were placed in a mullite crucible and melted in a gas
fired smelter for about 4 hours until the molten mass was
homogeneous as determined by pulling a fiber from the melt. The gas
fired smelter was maintained under a reducing atmosphere by
adjusting the air/gas ratio to control the valence state of the
thermochromic additives and to suppress the loss of sulphur as
SO.sub.2.
When the glass was cast onto a steel plate, it developed a
transparent red color. Another portion of the glass was rapidly
water quenched from the melt. It was initially transparent and
colorless. Upon striking the colorless glass at a temperature of
about 500-600.degree.C. for about 5 minutes, it developed a
transparent red coloration.
Thereafter the glass, prepared either by air cooling or by water
quencing and heat striking, was ball milled to a particle size of
-100 mesh. The step for application to the metal plate was prepared
from the same formulation as Example I and ball milled to form a
homogeneous dispersion. Following the procedure of Example I, the
slip was applied to a metal substrate and dried at a temperature of
about 100.degree.C. The dried coating was fired at a temperature of
about 1,000.degree.C. for 1 minute and cooled to room temperature.
Then the glassy material was nucleated at a temperature of about
645.degree.C. for 1 hour and subjected to a crystal growth
treatment of about 830.degree.C. for 4 hours.
After the crystallized material was cooled to room temperature, it
had a yellow appearance. The coated metal was then subjected to
numerous heating cycles up to 600.degree.C. A reversible
thermochromic color transformation is observed whereby the color of
the coating changes upon heating as follows:
Color Temperature Bright yellow room temperature Mustard yellow
130.degree.C. Bright Orange 230.degree.C. Red Orange
450.degree.C.
upon cooling the color change is observed to proceed in the reverse
order.
Example iii
following the procedure of Example II, a glass composition was
prepared by mixing 98.5 percent by weight of the preferred glass
batch of Example I expressed as oxides together with 0.85 percent
by weight of CdS and 0.65 percent by weight of Se. After the
coating was fired onto a metal plate in accordance with the firing
schedule of Example II, similar thermochromic color transformations
are observed.
It will be appreciated that the invention is not limited to the
specific details shown in the examples and illustrations and that
various modifications may be made within the ordinary skill in the
art without departing from the spirit and scope of the
invention.
* * * * *