U.S. patent application number 10/275446 was filed with the patent office on 2004-11-11 for shape memory device for changing shape at small temperature changes.
Invention is credited to Holemans, Thierry, Stalmans, Rudy.
Application Number | 20040221614 10/275446 |
Document ID | / |
Family ID | 8179977 |
Filed Date | 2004-11-11 |
United States Patent
Application |
20040221614 |
Kind Code |
A1 |
Holemans, Thierry ; et
al. |
November 11, 2004 |
Shape memory device for changing shape at small temperature
changes
Abstract
With the use of shape memory elements a series of devices can be
made which change shape at small changes of the environmental
temperature. The devices may be composed of a shape memory element
and a bias element, in such a way that the thermal hysteresis of
the device becomes lower than the intrinsic thermal hysteresis or
the shape memory element. The devices may be used in jewelry that
is adapted to change shape or configuration when subjected to
temperatures in the region of the body of a wearer.
Inventors: |
Holemans, Thierry;
(Bruxelles, BE) ; Stalmans, Rudy; (Aarschot,
BE) |
Correspondence
Address: |
Barnes & Thornburg
PO Box 2786
Chicago
IL
60690-2786
US
|
Family ID: |
8179977 |
Appl. No.: |
10/275446 |
Filed: |
January 29, 2003 |
PCT Filed: |
March 6, 2002 |
PCT NO: |
PCT/EP02/02457 |
Current U.S.
Class: |
63/35 |
Current CPC
Class: |
A44C 15/005 20130101;
A44C 5/0092 20130101; A44C 5/0084 20130101; A44C 15/00 20130101;
A44C 27/001 20130101; C22C 5/02 20130101; A44C 5/20 20130101; A44C
27/008 20130101; C22C 9/00 20130101; C22F 1/006 20130101; C08L
2201/12 20130101; A44C 27/003 20130101 |
Class at
Publication: |
063/035 |
International
Class: |
A44C 025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 8, 2001 |
GB |
01200868.6 |
Jan 25, 2002 |
GB |
201720.0 |
Claims
1-17. Cancelled.
18. A device comprising a shape memory element and a bias element,
the shape memory element and the bias element co-operating together
to provide the device with a thermal hysteresis having a mechanical
property activation temperature and a mechanical property
deactivation temperature, wherein the difference between the
activation temperature T.sub.A and the deactivation temperature
T.sub.D is 10.degree. C. or less, preferably 7.degree. C. or less
or most preferably 5.degree. C. or less.
19. The device according to claim 18, wherein the bias element
co-operates with the shape memory element so that the stress on the
shape memory element is lower in the activated state compared to
the deactivated state resulting in a lowering of a transformation
range of the device compared to a transformation range of the shape
memory element alone.
20. The device according to claim 19 wherein the stress is a
bending stress
21. A device according to claim 18, further comprising a blocking
element so that said shape memory element has a restricted
movement, resulting in a lowering of the thermal hysteresis of the
device.
22. A device according to claim 18, wherein said shape memory
element and the bias element are adapted so that the shape memory
element buckles under the action of the bias element, resulting in
a lowering of the transformation ranges of the device.
23. A device according to claim 18, wherein said shape memory
element and the bias element are adapted so that the bias element
buckles under the action of the shape memory element.
24. A device according to claim 18, wherein the transformation
range of the device is such that when placed in a thermal gradient
the device is repeatedly activated and deactivated.
25. A device according to claim 18, wherein the device exhibits
thermal hysteresis with a deactivation temperature in the range 20
to 32.degree. C.
26. A device according to claim 18, wherein the device exhibits
thermal hysteresis and has an activation temperature in the range
25.degree. C. to 35.degree. C.
27. A piece of jewelry comprising at least one device according to
claim 18.
28. A piece of jewelry including a plurality of parts which are
adapted for relative movement between each other so as to vary an
externally visible shape or configuration of said jewelry, said
jewelry further comprising a shape memory element and a bias
element adapted to co-operate so as to cause said relative
movement, said shape memory element exhibiting a thermal hysteresis
having upper and lower temperature transformation ranges, said
upper temperature transformation comprising a range of 10.degree.
C. or less, preferably 7.degree. C. or less and more preferably
5.degree. C. or less.
29. The jewelry piece according to claim 28, wherein the shape
memory element has an upper transformation start temperature
A.sub.S in the range 18 to 25.degree. C.
30. The jewelry piece according to claim 28, wherein the shape
memory element has an upper transformation finish temperature
A.sub.F in the range 23.degree. C. to 30.degree. C.
31. The jewelry piece according to claims 28, wherein said parts
are adapted to cycle automatically between at least two different
said externally visible shapes or configurations.
32. The jewelry piece according to claim 28, wherein said parts are
adapted to move or cycle automatically between different said
shapes or configurations when said piece of jewelry is located in
the region of a wearer's body.
33. The jewelry piece according claim 28, wherein said jewelry
piece comprises a broach or a necklace and said relative movement
is adapted to move or cycle said parts between an externally
visible closed or folded shape or configuration and an externally
visible open shape or configuration.
34. A piece of jewelry according to claim 28, wherein said shape
memory element and said bias element comprise a device comprising a
shape memory element and a bias element, the shape memory element
and the bias element co-operating together to provide the device
with a thermal hysteresis having a mechanical property activation
temperature and a mechanical property deactivation temperature,
wherein the difference between the activation temperature T.sub.A
and the deactivation temperature T.sub.D is 10.degree. C. or less,
preferably 7.degree. C. or less or most preferably 5.degree. C. or
less.
Description
FIELD OF THE INVENTION
[0001] The invention relates to devices incorporating shape memory
elements, and to improvements therein, especially to the use with
devices which are worn close to a living human or animal body or
are located within such a body, such as jewelry, piercings or
medical devices and implants.
BACKGROUND OF THE INVENTION
[0002] Materials, both organic and metallic, capable or possessing
shape memory and/or superelasticity are well known. An article made
of materials showing shape memory can be deformed from an original,
heat-stable configuration to a second, heat-unstable configuration.
The article is said to have shape memory for the reason that, upon
the application of the heat alone, it can be caused to revert or
attempt to revert from its heat-unstable configuration to its
original heat-stable configuration, i.e., it "remembers" its
original shape.
[0003] Among metallic alloys, the ability to possess shape memory
is a result of the fact that the alloy undergoes a reversible
transformation from an austenitic state to a martensitic state with
a change of temperature. The transformation from the austenitic
phase to the martensitic phase shows a temperature hysterisis,
illustrated in FIG. 1. It follows that the transformation cannot be
characterized by a single value of temperature, but has to be
characterized by four temperatures: M.sub.S and M.sub.F,
respectively, A.sub.S and A.sub.F to indicate the temperatures at
which the martensitic and the reverse martensitic transformation
start and finish. Thus, an article made from such an alloy is
easily deformed from its original configuration to a new
configuration when cooled to a temperature below Mf. When an
article thus deformed is warmed, the deformed object will return to
its original configuration starting at As and finishing at Af. A
shape memory material can be formed into a shape memory element
(SME) which may be used to activate depending upon temperature.
Many shape memory alloys (SMA) are known to show also
superelasticity.
[0004] When an SMA sample exhibiting superelasticity is stressed at
temperature above Af (so that the austenitic state is stable), but
below Md (the maximum temperature at which martensitic formation
can occur even under stress) it first deforms elastically and then,
at a critical stress, begins to transform by the formation of
stress-induced martensite. When the stress is released, the
martensite becomes unstable and transforms back to austenite, with
the sample returning to its original shape. Similarly, as the
thermally induced transformation, the stress induced transformation
shows a stress hysterisis.
[0005] Shape memory alloys can now be found in a wide range of
applications and products that take advantage of the shape memory
and superelastic properties. However the application of shape
memory elements for actuation of devices is accompanied by several
disadvantages, of which two are particularly relevant with respect
to the presented invention.
[0006] Firstly, in many shape memory alloys there is a large
hysteresis as the alloy is transformed between austenitic and
martensitic states. Moreover, the transformation ranges,
{A.sub.S-A.sub.F} and {M.sub.S-M.sub.F}, can be quite substantial.
As a result, reversing of the state of an SMA element may require a
temperature excursion of several tens of degrees Celsius, or a
stress excursion of several hundreds of MPa. It follows that a
temperature excursion of several tens of degrees (or hundreds of
MPa) is required for activation and deactivation of a device
incorporating shape memory elements. For example, the commercially
available near equi-atomic binary nickel-titanium alloys can have a
hysteresis width of about 40.degree. C. The total transformation
range {A.sub.F-M.sub.F} can be easily above 60.degree. C. For
example, if the activation temperature of a device to be worn such
as jewelry would be close to body temperature (say 30 to 35.degree.
C.), the deactivation temperature would be below -20.degree. C.,
which would limit the practical use of such a device. To remove the
device would require immersion of the respective body part in a
salt/ice mixture, in a deep freeze or similar which is not only
impractical but also not very pleasant for the wearer of
jewelry.
[0007] Second, it is difficult to control the transformation
temperatures of shape memory alloys with accuracy as they are
usually extremely composition- and processing-sensitive. The result
is that the activation and deactivation temperatures of devices
incorporating SMA-elements, are difficult to control.
[0008] The combination of these factors limits the practical
application of devices actuated by shape memory elements in many
fields. In many cases, it is desirable that the device can be
actuated in a small temperature range and that the actuation
temperatures can be easily controlled or more correctly accurately
set.
[0009] It would thus be desirable to develop a way for easy control
of the activation and deactivation temperatures of SMA devices and
for decreasing the hysteresis and the transformation ranges. This
is especially relevant for devices used in or close to a mammalian
body.
[0010] One attempt to vary the shape of the hysteresis of a shape
memory device is discussed in GB-2142724 in relation to Nitinol. In
most currently available commercialized Nitinol alloys, two
transformations usually occur:
[0011] an R-phase transformation with a thermal hysteresis of 2 to
5.degree. C. and a maximum shear strain of about 1.5 to 2%; and
[0012] a martenisitic transformation with a thermal hysteresis of
typically 20 to 30.degree. C. and a maximum shear strain of over
10%.
[0013] In most shape memory applications, the martensitic
transformation is used as the basis of the shape memory element.
This is because the much higher shear strains that can be obtained
enable higher deflections to be achieved. The R-phase
transformation is often considered an anomaly because it is less
useful due to the intrinsically low shear strain. In most cases,
this R-phase transformation will precede the martensitic
transformation by some way, e.g. to about 30.degree. C. during
cooling, but may coincide during heating. This proximity in
temperature makes it difficult to use only the R-phase
transformation for practical applications, although GB-2142724
proposes the use of a reversible R-phase transformation in Nitinol
for actuator applications.
[0014] Use of bias elements is known in proposals for altering the
activation temperature of devices incorporating shape memory
elements, as discussed for example in U.S. Pat. No. 5,261,597 and
U.S. Pat. No. 4,523,605. In these cases, the use of a bias element
slightly alters the hysteresis, but still leaves a difference of
some tens of degrees centigrade between activation and deactivation
temperatures. For example, in U.S. Pat. No. 4,523,605 even with the
use of a mechanical bias, the temperature hysteresis of a complete
transformation in Nitinol is still likely to exceed 20.degree.
C.
[0015] It is known to use shape memory materials in jewelry and
examples of some current proposals may be found in DE-19934312,
EP-0313070 and U.S. Pat. No. 6,183,490.
[0016] In DE-19934312 a shape memory material is used in a
super-elastic state to hold decorative pieces for display. The
super elastic properties are used in particular to allow bending or
flexing of the jewelry with good quality return to original shape
having no work hardening or residual creases or similar from the
deformation. The items displayed may be varied but, after having
been configured and put on, the jewelry looks the same unless and
until manually reconfigured to hold a different display.
[0017] In EP-031070 jewelry bands such as rings or bracelets are
formed from overlapping wound sections of shape memory wire. These
bands may carry ornamentation and use shape memory material in a
super-elastic state so as to retain shape whilst being worn,
temperature changes not being necessary for fitting, wearing or
removal. Once it has been put on, in similar fashion to
DE-19934312, the jewelry of EP-0313070 acts as a static display.
Although it may be temporarily deformed during fitting, this
jewelry uses only the super-elastic properties of shape memory
material and then only for its configuration into a condition
suitable for being worn and for shape retention. It will be noted
that the gauge of wire used in this arrangement is quite thin (e.g.
0.7 mm diameter) and that the bands can be expanded for fitting
using a low force. Removal of this low force allows the bands to
return towards their original shape, which in free space they
should achieve. Since only a low force is required to open the
super elastic element, the jewelry can be worn by people having
different sizes or body part, e.g. finger or wrist. In the event
that a body part such as a finger/wrist is thicker than the inner
diameter of the ring/bracelet, then the super elastic element will
only exert a gentle pressure on that body part due to the thin
gauge of the constituent shape memory wire.
[0018] In U.S. Pat. No. 6,183,490 a stud arrangement is disclosed
for keeping a body piercing open after initial formation. In this
arrangement, a projection from the back of the stud deforms inside
a captive member behind the earlobe so as to invisibly capture the
stud in place. The shape memory material is used only to provide
mechanical holding properties and is only one embodiment amongst
the holding means discussed.
[0019] In the world of jewelry, it is a continuous problem to seek
expressions of fashion and style that are appealing to the wearer
and it is an object of the present invention to provide improved
jewelry arrangements. It is another object of the present invention
to provide improved jewelry arrangements that incorporate an
element formed from a shape memory material, such as a shape memory
alloy. It is also an object of the present invention to provide an
improved method of producing such jewelry.
SUMMARY OF THE INVENTION
[0020] In one aspect of the present invention includes a device
incorporating at least one shape memory element is provided
offering (i) a substantially decreased temperature range between
the activation and deactivation temperature and (ii) an easy method
for control of the activation and deactivation temperatures.
Specific shape memory alloys show a small hysteresis and a small
transformation range. By integrating SMA elements made of such low
hysteresis alloys into the said devices, an improved device
results. The alloys may be metal alloys.
[0021] The present invention may provide a device comprising a
shape memory element and a bias element, the shape memory element
and the bias element co-operating together to provide the device
with an activation temperature and a deactivation temperature,
wherein the difference between the activation temperature T.sub.A
and the deactivation temperature T.sub.D is 10.degree. C. or less,
preferably 7.degree. C. or less or most preferably 5.degree. C. or
less. The bias element may co-operate with the shape memory element
so that the stress on the shape memory element is lower in the
activated state compared to the deactivated state resulting in a
lowering of the transformation range of the device compared to the
transformation range of the shape memory element alone. The stress
may be a bending stress. The device may also comprise a blocking
element so that said shape memory element has a restricted
movement, resulting in a lowering of the transformation range of
the device. The shape memory element and the bias element may be
adapted so that the shape memory element buckles under the action
of the bias element, resulting in a lowering of the transformation
range of the device. The shape memory element and the bias element
may be adapted so that the bias element buckles under the action of
the shape memory element, resulting in a lowering of the
transformation range of the device. The transformation range of the
device may be such that when placed in a thermal gradient the
device is repeatedly activated and deactivated. The device
preferably exhibits a thermal hysteresis with a deactivation
temperature in the range 20 to 32.degree. C. The device may have an
activation temperature in the range 25.degree. C. to 35.degree. C.
Any such devices are preferably included in a piece of jewelry such
as a necklace or a bracelet or brooch.
[0022] The present invention also provides a piece of jewelry
including a plurality of parts which are adapted for relative
movement between each other so as to vary an externally visible
shape or configuration of said jewelry, said jewelry further
comprising a shape memory element and a bias element adapted to
co-operate so as to cause said relative movement, said shape memory
element exhibiting a thermal hysteresis having upper and lower
temperature transformation ranges, said upper temperature
transformation comprising a range of 10.degree. C. or less,
preferably 7.degree. C. or less and more preferably 5.degree. C. or
less.
[0023] Said shape memory element may have an upper transformation
start temperature A.sub.S in the range 18 to 25.degree. C. Said
shape memory element may have an upper transformation finish
temperature A.sub.F in the range 23.degree. C. to 30.degree. C.
[0024] Said parts may be adapted to cycle automatically between at
least two different said externally visible shapes or
configurations. Said parts may be adapted to move or cycle
automatically between different said shapes or configurations when
said piece of jewelry is located in the region of a wearer's
body.
[0025] Said jewelry piece may comprise a broach or a necklace and
said relative movement may be adapted to move or cycle said parts
between an externally visible closed or folded shape or
configuration and an externally visible open shape or
configuration. Said shape memory element and said bias element may
comprise a device according to the present invention.
[0026] Accordingly, this invention provides improved devices with a
very low temperature difference between the activation and
deactivation temperature and with easily controllable activation
and deactivation temperatures.
[0027] The invention will now be described with reference to the
following drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 shows a schematic illustration of a property change
(such as length) versus temperature for a shape memory element. The
hysteresis, the activation temperature T.sub.A, the deactivation
temperature T.sub.D and the transformation temperatures M.sub.S,
M.sub.F, A.sub.S, A.sub.F are indicated.
[0029] FIG. 2 shows the effect of a bias force schematically. The
property P is shown versus temperature T. By applying a higher bias
force, the transformation temperatures will shift to higher
temperatures.
[0030] FIG. 3 shows an embodiment of the present invention--the use
of partial transformation to reduce hysteresis. The movement from
position 1 to position 2 and vice versa involves only a partial
transformation. The difference between the activation temperature
T.sub.A and the deactivation temperature T.sub.D becomes lower due
to this partial transformation.
[0031] FIG. 4 is an illustration of the effects of different
biasing systems on the transformation ranges.
[0032] FIGS. 5a and b show two states of a device in accordance
with an embodiment of the present invention using a snap
mechanism.
[0033] FIG. 5c show the hysteresis loop for the devices shown in
FIGS. 5a and b when placed close to a heat source indicating cyclic
behavior.
[0034] FIGS. 5d to 5i show alternative embodiments of the present
invention all having a lower stress in the shape memory element in
the activated state.
[0035] FIGS. 6a and b show two states of a device in accordance
with an embodiment of the present invention not using a snap
mechanism.
[0036] FIG. 6c show the hysteresis loop for the devices shown in
FIGS. 5a and b when placed close to a heat source showing two
semi-stable positions P1 and P2.
[0037] FIGS. 7a and b show two states of a brooch in accordance
with an embodiment of the present invention.
[0038] FIGS. 7c and d show the two states of the activating devices
for the brooch of FIGS. 7a and b.
[0039] FIG. 8a shows a necklace having moveable flowers in
accordance with an embodiment of the present invention.
[0040] FIGS. 8b and c show the two states of the activating device
for the necklace of FIG. 8a which activate cyclically when placed
in a temperature gradient.
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0041] The present invention will be described with respect to
certain embodiments and drawings but the present invention is not
limited thereto but only by the claims. Although, allowance is
granted for the publication of this patent application all rights
such as copyright or design rights especially of the jewelry shown
in the drawings is maintained with reference to other third
parties.
[0042] The present invention will be discussed by describing in
detail the hysteresis and the transformation ranges of the shape
memory element, and how the difference between the activation and
deactivation temperature of the device can be lowered. In
accordance with an aspect of the present invention the difference
between T.sub.A and T.sub.D is 10.degree. C. or less, preferably
7.degree. C. or less, most preferably 5.degree. C. or less. The
following methods, optionally in combination can achieve these
levels. An additional advantage of the presented embodiments is
that they result in a very compact design. This is of critical
importance in the design of jewelry where any moving parts must be
so small that they do not disturb the aesthetic appearance of the
piece.
[0043] The transformation Range of a Shape Memory Element: use of
Shape Memory Alloys with a Low Hysteresis.
[0044] Shape memory alloys exhibit a hysteresis between the forward
and reverse transformation, as shown in FIG. 1. Accordingly,
devices actuated by shape memory elements, will exhibit a
temperature hysteresis between a forward and reverse shape change
which may be defined as a difference of temperature, e.g. at the
center or midline (horizontal) of the hysteresis curve shown in
FIG. 1. The intrinsic hysteresis of the shape memory elements will
thus be reflected in any device making use of such materials. Many
applications of shape memory alloys nowadays are using near
equiatomic binary nickel-titanium alloys. Such nickel-titanium
alloys typically have a hysteresis width of about 40.degree. C.
Specific alloys processed properly show however a much lower
hysteresis. Examples thereof are:
[0045] Cu-based shape memory alloys, such as Cu--Zn--Al and
Cu--Al--Ni, and especially single crystals hereof
[0046] Ni--Ti--X alloys, and especially Ni--Ti--Cu--X based alloys
(X being Fe, Cr and the like), and single crystals hereof
[0047] Ni--Ti alloys and Ni--Ti--X alloys, by use of the R-phase
transition in these alloys, and especially Ni--Ti--Fe alloys
[0048] Au based shape memory alloys, and single crystals hereof
[0049] By integrating elements of such low hysteresis alloys into
devices, these devices will also show a much lower hysteresis.
[0050] Another important point is that the upper and lower
transformation ranges of the hysteresis curve, {As-Af} and {Ms-Mf},
can be quite substantial. FIG. 1 shows that in order to switch from
the `on` state at the activation temperature Ta to the `off` state
at the deactivation temperature Td and vice versa, the hysteresis
and the two transformation ranges have to be overcome. Large
transformation temperature ranges result in a slow/sluggish action
of such devices.
[0051] Accordingly, in order to minimize the difference between the
activation and deactivation temperature, the transformation ranges
should also be minimized. These transformation ranges can be
minimized, for example, in one aspect by proper alloy selection and
proper processing. This minimization can be done based on the
knowledge of the skilled person. Thus, by selecting elements of
shape memory alloys with low hysteresis and/or small transformation
ranges, a device will be obtained in which the difference between
the activation and deactivation temperature will be lower. However,
setting the exact activation temperatures still remains a
problem.
[0052] Adjusting the Transformation Temperatures and Decreased
Hysteresis by using a Bias Element
[0053] The transformation temperatures of shape memory alloys are
composition and processing dependent. It follows that the
transformation temperatures can be controlled to some extent by
controlling the composition and the processing. However, it is
difficult to control the transformation temperatures of shape
memory alloys in this way with sufficient accuracy.
[0054] In most devices actuated by shape memory elements, the shape
memory elements work against a bias element. The SMA element
therefore generates force and motion, and thus work, on heating.
The simplest mode of operation is an SMA wire acting against a
constant force. It is known that the transformation temperatures
will increase when the bias force is increased, as schematically
illustrated in FIG. 2. It follows immediately that by proper
selection of the bias element, and/or installing an adjustable bias
element, the activation and deactivation temperature of the device
can be further adjusted.
[0055] An accurate control of the transformation temperatures in
accordance with embodiments of the present invention is thus
obtained by a proper selection of composition and processing of the
shape memory element, with a final adjustment via a proper
selection of the bias element and/or installing an adjustable bias
element, e.g. a bias element such as a compression spring and a
screw for adjusting the initial bias of the spring. It has been
found that the hysteresis (in .degree. C. difference) in a biased
system will in general be lower than in a stress-free shape memory
element. Application of a bias element can thus be used also for
lowering the hysteresis of the device.
[0056] Reducing the Hysteresis and Reducing the Transformation
Ranges by Use of a Partial Transformation
[0057] In a first approximation, the hysteresis (in .degree. C.
difference) is proportional to the amount of involved
transformation. A cyclic partial transformation will thus show a
smaller hysteresis and a partial transformation range, as
illustrated in FIG. 3. The difference between T.sub.D and T.sub.A
is also reduced. A partial transformation can be imposed basically
in two ways.
[0058] 1. The device can be designed in such a way that a further
shape change is impeded during heating and/or during cooling by a
blocking element. Accordingly, the difference between activation
and deactivation temperature will be lowered.
[0059] 2. The external conditions can also cause a partial
transformation. For example, a shape memory element can be put onto
or into or onto a warm object, e.g. the mammalian body. In case
heating, respectively cooling is used, body temperature will be the
minimum, respectively maximum temperature of the SMA element. In
case body temperature is located between M.sub.S and M.sub.F,
respectively A.sub.S and A.sub.F, a partial transformation will be
obtained involving a smaller difference between activation and
deactivation temperatures.
[0060] Reduced Transformation Range by Applying a Decreasing Bias
Stress
[0061] In most devices incorporating SMA-elements, the bias element
exerts (i) constant or (ii) increasing bias stresses on the SMA
element as the SME element is activated, as illustrated in FIG. 4.
In the case of a constant bias, the transformation temperature
ranges {A.sub.F-A.sub.S} and {M.sub.S-M.sub.F} will nearly not be
affected. In the second case, the transformation range increases.
Accordingly, the temperature difference between activation
temperature and deactivation temperature also increases.
[0062] In accordance with an embodiment of the present invention,
bias systems are provided in which the bias element co-operates
with shape memory element so that there is a decreased biasing
stress on the SMA-element at a higher temperature. In such cases,
transformation temperature ranges {A.sub.F-A.sub.S} and
{M.sub.S-M.sub.F} will decrease. Accordingly, the temperature
difference between activation temperature T.sub.A and the
deactivation temperature T.sub.D also decreases. The reduction in
the transformation ranges results in an activation action which is
closer to a "snap-action" as well as reducing the temperature
change required to change states of the SME.
[0063] Reduction of the Transformation Range by Buckling:
Snap-Action Mechanisms
[0064] A further embodiment of the present invention includes
devices in which one of the biasing element and the SMA element
will buckle during cooling and will `reverse-buckle` during
heating. The shape change of the SMA-element during cooling will
occur nearly at constant temperature (the deactivation temperature)
and the reverse shape change during heating will also occur nearly
at a constant temperature (the activation temperature), as shown in
FIG. 5c when referring to devices schematically shown in FIGS. 5a
and b for the example of a shape memory plate which is compressed
by a bias element which may be a pre-biased compression spring. The
stable state of the SME is a flat plate as shown in FIG. 5a. A
support 2 is provided to which is attached a compression biasing
element 4 such as a spring which acts on a coplanar elongate SME 6.
Above the activation temperature, this elongate SME 6 such as a
plate or rod will be straight. During cooling, because of the
compressive stresses applied by the biasing element 4 the straight
shape will become unstable due to weakening of the SME 6. Hence, at
a critical temperature the SME 6 will buckle to a curved shape as
shown in FIG. 5b. During subsequent heating, the reverse will occur
at a higher temperature and the curved SME 6 gains mechanical
strength and `reverse-buckles` to the original straight shape. In
summary, because of this snap-action mechanism both transformation
ranges are reduced to nearly a constant single temperature--as
indicated in FIG. 5c.
[0065] Included within the present invention are also ways to
enhance or adjust the buckling effect. For example, for the case of
the elongate SMA-element 6 (e.g. plate or rod shaped), the buckling
can be enhanced and adjusted by weakening, e.g. thinning, the
middle part of the SME 6. The application of this snap-action
mechanism to devices incorporating SMA elements is not limited to
the presented example, but can be applied in many different devices
and the involved SMA-elements are not limited to plate- or
rod-shaped SMA-elements.
[0066] Various designs of biasing elements are shown in FIGS. 5d to
i. In FIG. 5d a biasing element 4 e.g. of a tension spring is shown
attached to both ends of an elongate SME 6. The tension spring
exerts a compressive force on the SME 6 causing it to buckle below
the critical temperature as shown in FIG. 5e. In FIG. 5f a weight 8
is placed on the elongate SME 6 as a biasing element. As shown in
FIG. 5g the SME 6 buckles below the critical temperature. Despite
the fact the force exerted by the weight 8 is constant, the bending
moment (and therefore stress) on the SME 6 at its center is larger
at the lower temperature.
[0067] Instead of the SME buckling, additional useful effects can
be produced by the biasing element being made to buckle or lower
its biasing force. An embodiment is shown in FIGS. 5h and i. The
biasing element 4 is a spring strip which is buckled when the
elongate SME 6 (e.g. rod, strip or plate) is in the activated state
(high temperature, stiff properties) as shown in FIG. 5h. As the
temperature drops the SME 6 becomes weaker and buckles as shown in
FIG. 5i. By attaching mechanical linkages to the SME 6 and the bias
element 4, movements can be actuated in both the activated and
deactivated states. This results in a double action switch which is
very compact and efficient. Where a heat source 8 is provided close
to the SME 6, a cyclic action can be achieved (see below).
[0068] The snap-action mechanism may be also used in so-called
on-off actuators.
[0069] Action of the Device in a Temperature Gradient: a Device
Acting/Reacting at Constant Thermal Conditions
[0070] Embodiments of the present invention include devices in
which the SMA element moves in the opposite direction to a
decreasing thermal gradient, i.e. it moves towards a heat source
whose temperature is higher than the environment. There is a
temperature gradient in which the temperature drops with increasing
distance from the heat source. The behavior of the device is
especially relevant in combination with the above presented
snap-action mechanism. The embodiment of FIG. 5, placed close to a
heat source 8, as illustrated in FIGS. 5a and b shows this
behavior. In the deactivated state, the elongate SME 6 is curved
(FIG. 5b) and is close to the heat source 8. As a result of heat
transfer from the heat source 8 the temperature of the SME 6 will
increase. At the critical activation temperature, the SME 6 will
`buckle back` and moves away from the heat source 8 (FIG. 5a). Due
to the temperature gradient, the SME 6 will now start to cool. At
the critical deactivation temperature, the SME 6 will buckle again.
This cyclic movement (ABDC of FIG. 5c, the temperature gradient
experienced by the SME is shown as the curve from Te to temperature
Ts) will be repeated as long as the temperature gradient exists.
The periodic time of the cycle will depend on the thermal gradient,
but can also be controlled via the design of the device and the
selection and processing of the shape memory elements, as explained
above, as well as the thermal mass of the complete device.
Evidently, in case the thermal gradient is relatively small, shape
memory elements with a very low hysteresis will have to be
selected, optionally in combination with the methods presented
above for a farther lowering of the hysteresis and of the
transformation ranges.
[0071] As FIGS. 6a to c show, the snap-action mechanism (small
transformation ranges) is preferable in order to obtain a sharp and
definite cyclic movement of the device in a thermal gradient. In
FIGS. 6a and b a shape memory element 6 is constrained within
cavity in a block support 2, the block 2 having a groove 3 near its
upper surface into which the SME 6 fits. The SME 6 is biased by a
biasing device 4 such as a compression spring acting
perpendicularly to the SME 6 at its center. The transformation
ranges are larger than in FIG. 5c (no snap action). In this
situation the SME 6 may find one of two semi-stable positions:
either the position of SME 6 determined by position B of the
hysterisis loop (position P2 in a bent position). Otherwise the SME
6 takes a position determined by D on the hysteresis loop position
P1). Note that the force exerted by the spring 4 is lower in the
deactivated state (low temperature state) and the bending stress in
the SME 6 is lower despite the large amount of bend. Such a device
can make a movement to one of the two semi-stable positions
depending upon whether device starts at position C or A. When the
environmental conditions change again the device can take on the
other semi-stable position. The device is a mechanical bi-stable
flip-flop or switch.
[0072] Brooch Reacting on Small Changes of the Environmental
Temperature
[0073] Conventional brooches have a fixed shape. In accordance with
the present embodiment a brooch is provided with thermally
activated moving parts. These parts should move when there are
changes of the local temperature. An example is that the brooch is
positioned on a dress but below a coat (or the like). In that case,
the temperature of the brooch will be relatively close to body
temperature. When the coat is removed, the local temperature at the
brooch will decrease to a temperature closer to room
temperature--this will result in activation of the moving parts of
the brooch, either closing or opening.
[0074] Other sources of such small temperature changes can also be
used. For example, when the person is walking around the local
temperature will be closer to room temperature than when the person
is sitting quietly on a chair. Another example is that the person
goes from a warmer room to a colder place and vice versa or if the
temperature of the room itself changes.
[0075] The moving parts of the brooch should react on small
temperature differences. As an embodiment of the present invention,
a brooch 12 has a decorative design, e.g. the shape of a peacock 14
with moving feathers 16 as shown in FIGS. 7a and b. These feathers
16 are actuated by a device in accordance with an embodiment of the
present invention--a combination of a bias element 4 such as a bent
wire acting against small elongate, e.g. beam-shaped, shape memory
element 6 as shown schematically in FIGS. 7c and d. The brooch 12
is a mechanical bi-stable flip-flop device similar, in principle,
but not in detail, to that shown in FIG. 6. As shown in FIG. 7c,
the wire spring 4 is bent and exerts a large bending moment on the
shape memory element 6 when the temperature is high (activated
state of the shape memory element). The SME 6 is mounted on a
support 2. In this state the shape memory element 6 is very stiff
and is almost straight. As the temperature is reduced the shape
memory element 6 becomes weak and bends easily (FIG. 7d). This
movement is used to move one of the feathers 16 by a linkage 7. To
limit the movement (partial transformation) a blocking element 9 is
used which is forced against a stop 11 which may be part of support
2.
[0076] This embodiment makes use of the following features to
obtain a difference of T.sub.A-T.sub.D of 10.degree. C. or less,
more preferably 7.degree. C. or less or most preferably 5.degree.
C. or less:
[0077] Use of a shape memory element with a very low
hysteresis.
[0078] Cu--Zn--Al single crystals were obtained from `Centro
Atomico Bariloche` (Av. Bustillo 9,500; 8400 San Carlos de
Bariloche, Argentina, contact person: Marcos Sade). Small beam
shaped elements were cut from these Cu--Zn--Al single crystals.
Appropriate heat treatments were carried out, e.g. 825.degree. C.
for 30 minutes followed by 120.degree. C. for 50 minutes).
[0079] Adjusting the transformation range by applying a bias
stress. This is achieved by selecting the pre-biasing of the
spring. By adjusting the initial bias strain, the activation and
deactivation temperature can be shifted to appropriate values.
[0080] Reducing the hysteresis and the transformation ranges by use
of a partial transformation. The shape change of the shape memory
plates is limited since the wing movement only allows a limited
displacement. For this purpose the movement is limited by a
blocking element. By adjusting the size of the displacement, and by
adjusting the dimensions and the exact positioning of the shape
memory beams, the degree of partial transformation can be
controlled. Optionally, a snap mechanism can be built into this
devices.
[0081] Necklace Actuator Moving Cyclically in a Small Temperature
Gradient
[0082] An embodiment of the present invention provides a necklace
as shown schematically in FIG. 8a containing at least one flower 18
whose petals 19 should be opened and closed by a mechanism. In this
embodiment the flower 18 opens and closes repeatedly, in a cyclic
manner, due to a slight thermal gradient close to the human body of
the wearer of the necklace even when the global room temperature is
constant. The biasing element is one or more tension springs 4 in
parallel with the elongate shape memory element 6 as shown in FIGS.
8b and c. The SME 6 is mounted on a support 2. In the activated
state (high temperature state), the shape memory material is stiff
and the SME 6 is straight. The bending stress on the shape memory
element 6 is small (FIG. 8b) as the tension spring 4 is hardly
offset from the shape memory element 6. In this state the SME 6 is
furthest away from the human body or other heat source 8. As the
temperature drops, the shape memory material weakens and it bends
resulting in a high bending stress at the lower temperature (FIG.
8c). This movement opens and closes the petals 19 of the flowers
18. The SME 6 moves closer to the heat source 8 and begins to warm
again thus resulting in a cyclic action. A blocking element 9 may
be used which interferes with a stop 9 (e.g. part of support 2) to
restrict the movement and thus reduce the hysteresis.
[0083] In this embodiment, the following features of the present
invention have been integrated in the device to obtain a difference
of T.sub.A-T.sub.D of 10.degree. C. or less, more preferably
7.degree. C. or less or most preferably 5.degree. C. or less:
[0084] Use of a shape memory element with a very low
hysteresis.
[0085] Cu--Zn--Al single crystals were obtained from `Centro
Atomico Bariloche` (Av. Bustillo 9,500; 8400 San Carlos de
Bariloche, Argentina, contact person: Marcos Sade). Plate shaped
elements were cut from these Cu--Zn--Al single crystals.
Appropriate heat treatments have been given afterwards (825.degree.
C. for 30 minutes followed by 120.degree. C. for 50 minutes).
[0086] Reduced transformation range by applying a decreasing bias
stress. The bias element is placed in such a way that the bias
stresses decreases as the SME element becomes activated (comparable
to the design presented in FIGS. 5a and b). This results in a
smaller difference between activation and deactivation temperature.
By adjusting the initial bias strain, the activation and
deactivation temperature can be shifted to appropriate values.
[0087] Reducing the hysteresis and the transformation ranges by use
of a partial transformation. The shape change of the shape memory
plates is limited since the petal movement is restricted to a
limited displacement. By adjusting the size of the displacement,
e.g. by appropriate blocking or restraining elements, and by
adjusting the dimensions and the exact positioning of the shape
memory plate, the degree of partial transformation can be
controlled.
[0088] Snap mechanism in combination with a temperature gradient.
The shape memory plate is shaped in such a way (preferably with
some thinning in the middle) and the bias springs are placed
appropriately (comparable to the design presented in FIGS. 5a and
b) in order to obtain the snap mechanism. The device is placed in
such a way that the shape memory plate is positioned close to the
body in order to use the temperature gradient from body temperature
to room temperature. Buckling (as a result of cooling) will result
in a movement of the shape memory elements towards the body. Thus,
the shape memory plate comes closer to the body and the temperature
of the shape memory element will increase due to the body heat. At
the critical activation temperature, the shape memory plate will
`buckle back` and move away from the body and thus to an
environment at lower temperatures. Accordingly, the temperature of
the shape memory plate will decrease. At the critical deactivation
temperature, the shape memory plate will buckle and move towards
the body and thus to an environment at higher temperatures. This
sequence of `buckling back` and budding will continue.
[0089] Further Embodiments of the Present Invention
[0090] The features of the present invention described above can be
used to provide opening and closing actuators for many medical
applications. Repeated actuation can be achieved using very small
temperature differences, e.g. body heat, as well as providing a
snap actuation.
[0091] These features provide:
[0092] Actuator which reacts on very small temperature
differences
[0093] Actuator which acts/reacts repeatedly at a constant
environmental temperature, by using local temperature differences,
e.g. close to the body
[0094] Actuator which acts/reacts repeatedly at a constant
environmental temperature, by using local temperature differences,
e.g. close to the body, in combination with an integrated
snap-mechanism.
[0095] Bands
[0096] Embodiments of the present invention include bands such as
bracelets and necklaces to be worn by mammals such as dogs or
humans. These bands can be opened to place around the respective
body part when subjected to a low temperature and then close when
they are heated by the body warmth of the mammal.
[0097] The opening and closing mechanism can be applied also for
any band-like product (inclusive medical applications) of which the
following is non-exhaustive list: bracelets, rings (inclusive
earrings, piercings, medical implants, etc), necklaces, couplings
(industrial, medical), sealing and closing mechanisms, etc.
[0098] Such bands should ideally have the following features
[0099] the band (which might be integrated in another element) may
be fitted nicely around or inside an object, without any hinges,
locks, screws and the like.
[0100] the band will close or expand at high temperatures and can
be opened or shrunk at lower temperatures.
[0101] the band's shape changes with temperature differences.
[0102] The shape memory alloy and the processing of the shape
memory wire is performed in order to have the transformation
temperatures A.sub.S, A.sub.F, M.sub.S and M.sub.F, in the
necessary range (see below). For the example of the bracelet, the
following is considered as ideal.
[0103] 1. The ideal solution is that the bracelet can be opened at
lower temperatures, and keeps the open shape at temperatures at or
slightly above room temperature. In this way the bracelet with the
open shape can be stored and transported easily. The technical
corresponding request is that A.sub.S and A.sub.F should be
sufficiently high.
[0104] 2. The larger the temperature difference between body
temperature and AF, the faster will be the shape change from the
cold shape to the hot shape. A sufficiently fast action is required
for practical and aesthetic reasons. It follows that A.sub.F has to
be sufficiently below body temperature.
[0105] 3. For practical reasons, it should be possible to open the
bracelet at sufficiently high temperatures. It follows that As
should be sufficiently high.
[0106] 4. Combining the requests above, it also follows that a
small temperature range for the reverse transformation is
preferred, as well for aesthetic reasons and for practical
reasons.
[0107] 5. As an example, an A.sub.S close to 22.degree. C. (between
118.degree. C. and 25.degree. C.) and an A.sub.F close to
28.degree. C. (between 23.degree. C. and 30.degree. C.) is
preferred.
[0108] 6. The difference between A.sub.F and A.sub.S is preferably
10.degree. C. or less, more preferably 7.degree. C. or less and
most preferably 5.degree. C. or less. The reason for this is that
such a bracelet can be opened at some .degree. C. below A.sub.S.
Hence, A.sub.S should not be too low otherwise a low temperature
will have to be applied which may be inconvenient for the
wearer.
[0109] The shape memory element can be made of many shape memory
materials. Some examples:
[0110] binary Nickel Titanium (abbreviated to Ni--Ti)
[0111] Nickel Titanium with other alloying elements
[0112] Nickel Titanium Copper with other alloying elements
[0113] Nickel Titanium Copper Chromium
[0114] Cu based shape memory alloys, such as CuZnAl and CuAlNi with
further alloying elements
[0115] Au based shape memory alloys
[0116] The transformation temperatures of most shape memory alloys
are composition and processing dependent. In order to obtain the
appropriate transformation temperatures, alloys of appropriate
composition and which underwent appropriate processing have to be
selected. This can be done by persons skilled in the art.
[0117] Production of the Bands
[0118] The starting material is wire, strip and the like with
correct dimensions and made of appropriate alloys which have
undergone appropriate processing (see above). The further
production of the bands can involve several steps, such as (i)
shape setting treatments, (ii) extra heat treatments, (iii)
polishing and coloring processes, and (iv) adding other objects.
The sequence given here is only indicative; other sequences can be
followed. All the processes and procedures can be done by persons
experienced in developing and making of products of shape memory
alloys.
[0119] Shape Setting Treatment
[0120] In this step, the hot shape is given to the shape memory
element. It typically involves (i) the production of a mandrel with
the appropriate shape, (ii) fixing the shape memory element on or
around the mandrel, (iii) putting the mandrel with the fixed shape
memory element in a furnace or the like (molten salt bath or
similar) for appropriate times and at appropriate temperatures, and
(iv) cooling and removing the shape memory element from the
mandrel. Afterwards, the shape memory element will have the
required hot shape. Taking the example of Ni--Ti based elements,
the heat treatment temperature and time will be typically
400-550.degree. C. for 2 to 30 minutes, although other procedures
can be followed.
[0121] Additional Heat Treatments
[0122] Additional heat treatments might be required to shift the
transformation temperatures or to change the ductility of the shape
memory elements.
[0123] Polishing and Coloring
[0124] For aesthetic reasons, polishing and coloring treatments
might be required. Grinding and polishing (electropolishing or
mechanical polishing) will give a metal color. For the case of
Ni--Ti based elements, yellow, blue, black and intermediate surface
colors can be obtained by appropriate heat treatment. Different
colors can be also obtained via coatings and paints.
[0125] Adding Other Objects
[0126] Other objects can be connected to the shape memory elements.
For example diamonds, golden balls and the like can be put on the
shape memory element for aesthetic reasons.
[0127] While the invention has been shown and described with
reference to preferred embodiments, it will be understood by those
skilled in the art that various changes or modifications in form
and detail may be made without departing from the scope and spirit
of this invention.
* * * * *