U.S. patent application number 11/243519 was filed with the patent office on 2006-06-08 for portable energy storage devices and methods.
Invention is credited to A. David Johnson.
Application Number | 20060118210 11/243519 |
Document ID | / |
Family ID | 36572873 |
Filed Date | 2006-06-08 |
United States Patent
Application |
20060118210 |
Kind Code |
A1 |
Johnson; A. David |
June 8, 2006 |
Portable energy storage devices and methods
Abstract
Devices and methods which store and selectively release
relatively substantial amounts of energy for enabling individuals
to undertake superior performance in locomotion and other physical
activities. The different embodiments include a hyperelastic SMA
element which stores and releases energy in a differential pulley
set, in a hinged knee, and in a pogo stick.
Inventors: |
Johnson; A. David; (San
Leandro, CA) |
Correspondence
Address: |
Law Offices of Richard E. Backus
887 - 28th Ave.
San Francisco
CA
94121
US
|
Family ID: |
36572873 |
Appl. No.: |
11/243519 |
Filed: |
October 4, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60615846 |
Oct 4, 2004 |
|
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|
60637741 |
Nov 22, 2004 |
|
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60658862 |
Mar 7, 2005 |
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Current U.S.
Class: |
148/404 |
Current CPC
Class: |
C30B 29/52 20130101;
C22C 9/01 20130101; C22F 1/08 20130101 |
Class at
Publication: |
148/404 |
International
Class: |
C22C 27/06 20060101
C22C027/06 |
Claims
1. An energy storage device comprising a hyperelastic shape memory
alloy element which changes to one shape while storing energy
responsive to a stress, and a structure which applies the stress to
the element, the structure being selected from the group consisting
of a differential pulley mechanism, a knee brace and a pogo
stick.
2. A device as in claim 1 in which the hyperelastic element is a
single crystal.
3. A device as in claim 1 in which the hyperelastic element is a
single crystal of copper aluminum nickel.
4. A device as in claim 3 in which the alloy contains about 80% Cu,
12% Al, and 3% Ni by weight.
5. A device as in claim 1 in which the pulley ratio of the
differential pulley is about 4/3.
6. A device as in claim 1 in which the hyperelastic element
elongates by more than 8 percent strain responsive to the
stress
7. A method of storing and selectively releasing energy in a device
that can be carried by an individual, the method comprising the
steps of providing a hyperelastic shape memory alloy element,
applying a stress to the element which stores energy in the element
which when release is sufficient to enable the individual to
undertake superior performance in locomotion and other physical
activities, and releasing the energy.
8. A method as in claim 7 in the step of applying the stress is
carried out by elongating the element while storing energy, and the
step of releasing the energy is carried out by enabling the element
to contract.
Description
CROSS-REFERENCE TO PRIOR APPLICATIONS
[0001] This application claims the benefit under 35 USC
.sctn.119(e) of U.S. provisional patent applications Ser. No.
60/615,846 filed Oct. 4, 2004; 60/637,741 filed Nov. 22, 2004; and
60/658,862 filed Mar. 7, 2005.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to the storage and release of
mechanical energy by portable devices.
[0004] 2. Description of the Related Art
[0005] A warfighter on foot must carry equipment and is often
required to move fast and over terrain that requires increased
physical effort. It is recognized that increasing the mobility of
the footsoldier makes him or her more effective in combat and other
situations.
[0006] Exoskeletons have been demonstrated that provide super
strength for individuals. Driven by electrical stepper motors,
these require a source of energy, usually batteries which are too
heavy to be practical for operational use.
[0007] The idea of storing mechanical energy to enhance the
physical capabilities is incorporated in an invention by Nicholas
Zagn of Saint Petersburg Russia in 1890. His apparatus used
conventional springs that are limited to less than one percent
strain, and have varying force according to Hooke's law. This does
not provide an optimum energy source for locomotion.
[0008] The prior art has not provided a compact, light-weight and
portable device for storing sufficient amounts of potential energy
which can be quickly released into kinetic energy for enhancing
human locomotion and other physical activities.
[0009] The need has therefore been recognized for compact,
light-weight and portable devices that obviate the foregoing and
other limitations and disadvantages of prior art devices of the
type described. Despite the various devices of the type in the
prior art, there has heretofore not been provided a suitable and
attractive solution to these problems.
OBJECTS AND SUMMARY OF THE INVENTION
[0010] A general object of this invention is to provide devices and
methods that store substantial amounts of potential energy which
can be quickly released into kinetic energy sufficient for
enhancing human locomotion and other physical activities.
[0011] Another object is to provide a portable `passive` device
which can store sufficient energy to lift individuals to a
specified height, or alternatively to absorb the equivalent
potential energy of descent from such a height.
[0012] The invention in summary provides devices and methods which
store and selectively release relatively substantial amounts of
energy for enabling individuals to undertake superior performance
in locomotion and other physical activities.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a chart showing the stress-strain isotherm of a
hyperelastic SMA for use in the invention.
[0014] FIG. 2 is a chart showing the stress-strain cycle for a
free-falling mass that is stopped by a hyperelastic spring in
accordance with the invention.
[0015] FIG. 3 is a chart showing the stress, strain and temperature
variables of the theoretical paths for TiNi SMA.
[0016] FIG. 4 is a schematic diagram showing a pulley arrangement
using hyperelastic SMA in accordance with one embodiment of the
invention.
[0017] FIG. 5 is a chart comparing the stress-strain cycles of
polycrystalline TiNi SMA with single crystal CuAlNi SMA in the
martensite crystalline phase.
[0018] FIG. 6 is a chart comparing the stress-strain cycles of
polycrystalline TiNi SMA with single crystal CuAlNi SMA in the
austenite crystalline phase.
[0019] FIG. 7 is a partially cut-away perspective view of a
crucible structure with a hot zone for growing single crystal SMA
of round cross section for use in the invention.
[0020] FIG. 8 is a side elevation view of a knee hinge device
having hyperelastic SMA elements for human locomotion.
[0021] FIGS. 9A, 9B, 9C, 9D, 9E and 9F comprises side elevation
views shown in sequential positions during vaulting of a human
wearing on each knee the hinge devices of FIG. 8.
[0022] FIG. 10 is a table listing the physical data for the
hyperelastic SMA material in one preferred embodiment of the
invention.
[0023] FIG. 11 is a table listing the technical parameters in one
preferred embodiment of the invention.
[0024] FIG. 12 is a vertical section view of a pogo stick device in
accordance with another embodiment showing the device in one
operating mode.
[0025] FIG. 13 is a vertical section view showing the device of
FIG. 12 in another operating mode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
General Description
[0026] The invention provides devices and methods which enable an
individual, such as a warfighter, to carry heavier loads and run
faster without increasing demands on physical stamina; and to
enable a person of ordinary ability to leap high enough to clear a
high obstacle and thus to escape from a dangerous situation. When
combined with suitable exoskeleton components, the devices and
methods will enable a person to jump from a building or make a
rapid descent by parachute without suffering injury due to sudden
deceleration. It is particularly desirable that the devices not
require electrical power, i.e. they are a `passive` system that
uses and amplifies human effort. The individual is enabled to
function on the ground with the equipment in place. The devices are
neither too heavy or bulky nor too restrictive or awkward.
Otherwise it would have negative impact on the individual's
mobility and effectiveness.
[0027] The invention provides an individual/warfighter with
extraordinary abilities to run, carry heavy loads, and leap onto
and from structures. The devices of the different embodiments are
characterized in being compact while enabling the release of a
sufficient quantity of mechanical energy to boost a human to
heights not achievable by human muscle alone. Acceleration is
constant during boost, and limited to tolerable levels. One
embodiment described in detail in connection with FIGS. 8-9
comprises a differential pulley ensemble with a hyperelastic shape
memory alloy actuator. Calculations demonstrate that 1-4 kilograms
of CuAlNi hyperelastic single crystal material can store and
release the requisite amount of energy by means of the formation
and reversion of stress-induced martensite as the crystal elongates
and contracts as much as nine percent strain. Use of the
differential pulley ensemble converts this linear displacement to
rotary motion, providing a compact mechanical energy storage
system.
[0028] CuAlNi single crystal provides a dense storage for kinetic
energy in the form of crystal phase change. The force exerted is
constant during excursion of a hyperelastic plateau during
extension and contraction. The devices of the invention are
characterized in that they: a) of modest weight, b) are of a size
and shape practical for an individual/warfighter to carry and c)
store enough energy to significantly boost jumping ability.
Physical Considerations: Mass (of the Individual and of the Device)
and Energy Requirements
[0029] One example for employing a device in accordance with the
invention is: (a) for use with a payload of 180 lbs (80 kg) and (b)
for a jump height of 12 feet (4 meters). There is to be no external
heating or cooling during the cycle. The design is for an energy
storage device only. It is assumed that this device will be
combined with additional specialized mechanical components for
performance of specific tasks. The design may be scaled to larger
or smaller dimensions, for example to provide a projectile
launcher.
[0030] Assuming the 80-kilogram load and a jump height of 4 meters
it is necessary that the mechanism store at least 3200 joules of
energy. Shape memory alloys, which transform from one solid crystal
structure to another, are capable of energy storage at greater
densities than elastic materials. In hyperelastic transformations,
the energy is absorbed and released at nearly constant force, so
that constant acceleration is attainable.
Materials for Storing and Releasing the Required Energy
[0031] The preferred material for use as the energy store/release
element in devices the invention is copper-aluminum-nickel single
crystal hyperelastic alloy. CuAlNi sustains repeated strains of
more than 9% as demonstrated in the stress-strain isotherm of FIG.
1. The plateau in the curve results from conversion of austenite
high-temperature crystal structure to stress-induced martensite.
The stress at which this plateau occurs increases with increasing
temperature difference between the ambient temperature and the
temperature austenite finish temperature (the temperature at which
zero-stress thermal conversion from martensite to austenite is
complete.) The rate is about 2 megapascals per degree C. Transition
temperature is determined by composition. The ultimate tensile
strength is sufficiently great that large strains are tolerated by
an alloy having sub-zero (even cryogenic) transition temperature,
allowing one to tune the mechanism characteristics to the
application by varying the relative proportions of Cu, Al, and
Ni.
[0032] Physical data for the preferred hyperelastic CuAlNi SMA are
listed in Table I of FIG. 10
[0033] An example of one cycle of operation is as follows: a mass
falls under influence of gravity and lands. Deceleration compresses
a spring made of hyperelastic CuAlNi alloy. The alloy releases heat
as the stress increases to the plateau and stress-induced
martensite is formed. The alloy is then cooled (by external means)
to its austenite finish temperature as the stress diminishes to
zero. The spring is then returned to its original configuration.
This cycle is depicted in FIG. 2.
[0034] During the deceleration phase of the cycle, i.e. Path (a) of
FIG. 2), the temperature rises about 16 degrees C. as martensite is
formed, latent heat is released, and 4 joules per gram of
mechanical energy is stored. In Path (b) approximately 25 joules
per gram of Cp-deltaT heat is removed from the crystal, driving its
temperature to -10 degrees C., at which temperature the stress
becomes negligible.
[0035] FIG. 1 shows a stress-strain isotherm for a sample of CuAlNi
having a transition temperature near 0 degrees C. From these data
it can be shown that mechanical energy capacity is more than 4
joules per gram of CuAlNi alloy. Thus one kilogram of CuAlNi single
crystal alloy can store enough energy to lift an 80 kg load to a
height of 4 meters, or absorb the equivalent energy of descent.
[0036] FIG. 2. shows the stress-strain cycle for a free-falling
mass that is stopped by a hyperelastic spring. At Path (a), stress
increases to the plateau and strain increases as mechanical energy
is absorbed and heat is released. During Path (b) the temperature
is decreased to the Af temperature at which point the stress drops
to zero. Finally, at (c) the material is returned to its original
un-deformed state. These three thermodynamic path segments can be
plotted on a three-dimensional model of the SMA as depicted in FIG.
3.
[0037] In FIG. 3. the schematic representation is of all possible
thermodynamic paths in Nitinol titanium-nickel shape memory alloy.
The three independent thermodynamic variables are force (or
stress), length (or strain) and temperature.
[0038] In the chart of FIG. 5, polycrystalline TiNi 26 as shown by
plot 26 is compared with single crystal CuAlNi as shown by plot 22,
both in the martensite phase.
[0039] In the chart of FIG. 6, polycrystalline TiNi 26 as shown by
plot 24 is compared with single crystal CuAlN as shown by plot 20,
both in the austenite phase.
Differential Pulley Mechanism
[0040] The embodiment of FIG. 4 provides a spring-loaded
differential pulley mechanism. In its general concept, two pulleys
having different radii are fixed to the same shaft. A single
crystal SMA wire spring is anchored at the ends tangentially to the
larger and smaller pulleys and form an arc over an idler pulley in
such a way that as the pulleys rotate the wires are stretched in
linear tension. The length of the spring and the diameters of the
pulleys are selected so that as the pulleys are rotated through a
specified angle the wires in the spring are strained nine percent.
Energy stored in the spring is proportional to its mass, which is
calculated to provide sufficient force and stroke for a
pre-determined task. During the boost phase, as the spring
contracts, the output torque is constant, providing a limited and
constant acceleration. This output torque is conveniently converted
to linear motion by a cable wound on a drum.
[0041] The differential pulley embodiment of FIG. 4 comprises a
dense energy storage device 80. This device can be provided as a
back-pack transportable unit employing the passive energy storing
concept of the invention. Device 80 comprises a differential pulley
set having a large diameter pulley 82 and small diameter pulley 84
keyed together for rotation on a common axle 86. An idler pulley 88
is mounted for rotation on a second axle 90 which is positioned at
a fixed distance from axle 86. A length of wire 92 of
single-crystal CuAlNi hyperelastic SMA has its proximal end 94
reeved on and affixed to the large pulley, its mid portion 96
reeved about the idler pulley, its distal end 98 reeved back and
about the small pulley and then attached to a mass 99.
[0042] In operation of device 80, energy is stored by applying a
downward stress on distal end 98. The figure shows this step by
using gravity on mass 99 to create the stress. Other means could be
used to apply the stress, such as a small hand crank, not shown.
The stress causes the paired pulleys 82, 84 to rotate anticlockwise
as viewed in the figure. The differential diameters of the two
pulleys causes the SMA wire to elastically stretch and absorb
energy. After stretching, the gear set could be locked against
rotation by a suitable latch or detent, not shown, until energy
release is desired. When the gear set is unlocked by the user, the
wire contracts and releases the energy as the gear set rotates
clockwise. The released energy could be employed to do work, such
as by using the rotating axles 86 and/or 90 to rotate a cutting
tool, drill or the like.
Technical Design: Calculations, Dimensions, Forces, Strength of
Materials.
[0043] Table II of FIG. 11 shows calculations for the technical
parameters in one example of the differential pulley mechanism of
FIG. 4.
[0044] This table uses the equations of motion of a differential
pulley system to calculate physical sizes of the components, and to
keep these within reasonable limits. Many possible solutions can be
calculated that meet the stated goals, but not all are
practical.
[0045] Two cases are presented, each capable of delivering 3200
joules of mechanical energy. Each has been derived to produce a
half turn of the pulley assembly. In the first case, having a
stroke of 4 meters and a lifting force of 80 kg, the cable is wound
on a drum that is too large to be practical. In the second case,
the stroke has been reduced by one half, and the lifting force
doubled. This results in a drum radius that is reasonable for being
carried in a back-pack. This design presumes that the application
will contain a lever arm to increase the stroke by a factor of
two.
Knee Hinge Device
[0046] In the embodiment of FIG. 8 and FIGS. 9A-9B knee hinge
device 100 is provided for human locomotion, such as vaulting. On
each leg of the user pairs of braces 102 and 104 are secured on
each side of the knee by respective bands 106 and 108 which are
wrapped around the respective upper portion of the calf and lower
portion of the upper leg. A length of hyperelastic SMA wire 110 is
secured at its opposite end 112 and 114 to respective braces 102
and 104. The mid portion of the wire is reeved about the curved
sector 116 of brace 104 to serve as a pulley for directing the
pulling forces of the wire along the lengths of the respective
braces. This geometry results in sequential stretching and
contraction of the SMA wire as the knee flexes
[0047] FIGS. 9A, 9B, 9C, 9D, 9E and 9F show the human wearing a
pair of the hinge devices 100, 101 with the elements in sequential
positions throughout a vaulting cycle.
[0048] During operation, as the person's legs stride forward before
a launch, one knee flexes by bending so that the thigh and calf
pivot toward each other. The weight and forward momentum of the
person causes SMA wire 110 to stretch during the flexing and store
energy. As the person's weight is shifted to the other leg and
momentum is lost, the wire contracts and releases most of the
stored energy, causing the knee to flex back and straighten the leg
while forcefully propelling the person forward and/or up.
Pogo Stick Device
[0049] FIGS. 12 and 13 illustrate a pogo stick device 210 in
accordance with another embodiment. Device 210 comprises a
cylindrical body 212 having a lower end 214 that is formed with an
opening 216 which slidably carries a strut 218. A pair of foot
pedals 220. 222 are mounted on the lower end of the strut. A handle
224 is mounted on the upper end of the body. A wire or rod 226 of
single crystal hyperelastic SMA is mounted at one end with the
upper end of the strut. The wire's other end is mounted to the
lower end 214 of the body.
[0050] FIG. 12 shows the relative positions of the body and strut
when not subjected to axial forces with wire 226 unstressed. FIG.
13 shows the positions when the strut has moved up relative to the
body while stretching the wire and storing energy during one phase
of the pogo cycle of operation. The wire releases the energy as it
contracts and moves the elements (as the human riding the pogo)
back to the positions of FIG. 12.
Technical discussion of CuAlNi Single Crystal Hyperelastic Alloy:
Why it is Superior, How it Works, and How it is Made.
[0051] Hyperelastic shape memory alloys comprise an enabling
technology providing dense energy storage, constant force, and
simple implementation. Conventional elastic spring material may be
strained about 0.5 percent without permanent deformation. In
contrast, mechanical energy is stored by a crystalline phase change
in hyperelastic SMA, allowing complete recovery of more than 9
percent, a twenty-fold increase. Deformation and recovery take
place at constant stress so that forces and hence accelerations are
limited and predictable.
[0052] Conversion of high-temperature crystalline phase to
stress-induced martensite permits a very large amount of mechanical
energy to be stored, and, as the single crystal is not plastically
deformed, complete shape recovery takes place at nearly the same
level of stress.
[0053] This material is created by pulling single crystals from
melt in a process known as the Stepanov method, similar to the
Czochralski method of fabricating silicon boules for
microelectronics manufacture.
Background of Hyperelastic Shape Memory Alloys
[0054] SMA materials have become popular for use as actuators due
to their ability to generate substantial stress during shape
recovery of large strains during temperature-induced phase
transformation. The energy density of such actuators is high
compared to other alternatives, such as electromagnetic,
electrostatic, bimetals, piezoelectric, and linear
[0055] and volume thermal expansion effects of ordinary materials.
The operating cycle of an SMA actuator includes deformation during
or after cooling, and subsequent heating which results in a
temperature-induced phase transformation and recovery of the
deformation. SMA actuation is favored where relatively large force
and small displacements are required in a device that is small in
size and low in mass.
[0056] Shape memory is the ability of certain alloys to recover
plastic deformation, which is based on a diffusionless solid-solid
lattice distortive structural phase transformation. The performance
of shape memory alloy based actuators strongly depends on the
amount of recoverable deformation. In turn, recoverable deformation
itself is a function of the lattice distortions which take place
during martensitic phase transformation in the particular SMA. For
an individual grain (single crystal) of SMA, the amount of possible
recoverable strain after uniaxial loading, depends on the
particular crystallographic orientation of the deformation tensor
relative to the crystallographic axes of the high temperature
(austenite) phase and the sign of applied load (tension or
compression).
[0057] For a given deformation mode, the recoverable strain is
strongly orientation dependent, and for the various
crystallographic directions it differs by approximately a factor of
two.
[0058] The recoverable deformation of these polycrystalline SMA
alloys, due to the lattice distortion during diffusionless
solid-solid phase transition, is substantially lower than is
theoretically possible for a given material. The main reason for
this is that for a conglomerate of randomly oriented grains (as is
normally the case for polycrystalline materials), the average
deformation will always be less than the maximum available value
for a given grain. The diffusionless nature of phase transitions in
SMA results in strict lattice correspondence between the high
temperature (austenite) and low temperature (martensite) lattices.
As the symmetry of the martensite lattice is lower than that of
austenite, maximum deformation in each grain can only be attained
in one particular crystallographic direction. This means that for
randomly oriented grains (as normally is the case for
polycrystalline materials), the average deformation will be at
least a factor of two less than the maximum.
[0059] The restrictions imposed on a polycrystalline body by the
deformation mechanism is another reason for diminished recoverable
deformation in polycrystals as compared with a single crystal. To
maintain integrity of the polycrystal, deformation of each
particular grain has to be less than that corresponding to the
theoretical limit for lattice distortion.
[0060] Therefore, for polycrystalline material, resultant recovery
is the vector sum of particular grain deformations over the whole
range of grain orientations, and is significantly smaller than the
maximum value for an individual single crystalline grain.
[0061] By comparison, recoverable deformation close to the
theoretical value (lattice distortion) can be achieved in single
crystalline SMA. In addition to the substantially increased
recoverable deformation, absence of grain boundaries results in
increased strength and longer fatigue life. Specifically, as a
single crystal, the strength of the grain for CuAlNi SMA can be as
high as 800 MPa with the practical limit for recoverable
deformation up to 9 percent and potentially even higher for special
deformation modes. An additional advantage of a single crystal SMA
is that not only the thermally induced phase transformation may
contribute to the recoverable deformation, as in the case for
polycrystals, but also stress-induced martensite-to-martensite
phase transitions. Depending on the material, this additional
contribution may be up to 15 percent. Therefore the total
theoretical recovery can be as high as 24 percent.
[0062] The graphs of FIGS. 6(a) and 6(b) show stress-strain curves
for a CuAlNi single crystal SMA and a polycrystal TiNi SMA. Curve
20 shows the single crystal SMA in its austenitic phase while curve
22 shows the martensitic phase. Curve 24 shows the polycrystal SMA
in its austenitic phase while curve 26 shows the martensitic
phase.
[0063] Hyperelastic single crystal SMA is an enabling technology
for mechanical energy storage. It has the following advantages over
polycrystal SMA for mechanical devices: [0064] 1. Large strain
recovery. In FIG. 6 the region 28 of curve 22 for the austenitic
phase of the single `hyperelastic` SMA shows the magnitude of its
strain recovery in comparison to the comparable region 30 of curve
26 for an austenitic polycrystal SMA. There is a three-fold gain in
performance over the conventional SMA materials made from bulk
materials, such as TiNi. Depending on how the sample is used, the
great strain recovery can either be used in the high temperature
state as a spring, for example, or deformed when in martensite
phase and then heated to recovery as an actuator. [0065] 2. True
constant force deflection. Unlike polycrystalline materials which
reach their strain/stress plateau strength in a gradual fashion and
maintain an upward slope when deformed further, hyperelastic SMA
materials have a very sharp and clear plateau strain/stress that
provides a truly flat spring rate. This is shown in FIG. 6 by the
region 32 of curve 20. The stress level at which the plateau occurs
depends on the temperature difference between the transformation
temperature and the loading temperature. Additionally, single
crystal SMAs exhibiting hyperelasticity benefit from a second
stress plateau which can increase the total recoverable strain to
24 percent. [0066] 3. Very narrow loading-unloading hysteresis. As
a result there is substantially the same constant force spring rate
during both loading (increasing stress) and unloading (decreasing
stress). This is shown in FIG. 6 by the narrow vertical spacing 34
between the upper portion of curve 20 which represents loading and
the lower portion representing unloading. This characteristic is
key in applications where the flexure undergoes repeated cycling.
In comparison, there is a relatively wide spacing between the
corresponding loading and unloading portions of curve 24. [0067] 4.
Recovery which is 100 percent repeatable and complete. One of the
drawbacks of polycrystalline SMA materials has always been the
`settling` that occurs as the material is cycled back and forth.
This is shown in FIG. 6 for curve 24 by the spacing 36 of the curve
end representing the beginning of the loading and the curve end
representing the end of the unloading. The settling problem has
required that the material be either `trained` as part of the
manufacturing process, or designed into the application such that
the permanent deformation which occurs over the first several
cycles does not adversely affect the function of the device. By
comparison, hyperelastic SMA materials do not develop such
permanent deformations and therefore significantly simplify the
design process into various applications. This is shown in FIG. 6
where the beginning of curve 20 representing unloading coincides
with the end of the curve representing loading. [0068] 5. Very low
yield strength when martensitic. This property is shown by the
horizontal portion 38 of curve 22, which is relatively much lower
than the corresponding portion of curve 26. The property is key for
designing an SMA actuator which is two way (i.e., it cycles back
and forth between two states). This is typically done by
incorporating a biasing element, which overcomes the SMA when cold
or martensitic, and establishes position one until the SMA is
heated and overcomes the biasing element for driving the mechanism
to position two. The problem with this type of device when using
polycrystalline SMA is that the biasing element robs a significant
amount of work output from the SMA. By comparison, an equivalent
hyperelastic SMA element has a much lower yield strength when
martensitic, enabling a much softer biasing element, and therefore
generating a much greater net work output. [0069] 6. Ultra-low
transition temperature. Hyperelastic SMA materials made from CuAlNi
can be manufactured with transition temperatures near absolute zero
(minus 270 Celsius). This compares to SMA materials made from TiNi
which have a practical transition temperature limit of -100
Celsius. [0070] 7. Intrinsic hyperelastic property. TiNi SMA can be
conditioned, through a combination of alloying, heat treatment and
cold working, to have hyperelastic properties. Single crystal
CuAlNi SMA has intrinsic hyperelastic properties: a crystal of
CuAlNi is hyperelastic immediately after being formed (pulled and
quenched) with no further processing required. Method of
Fabricating Single Crystal SMA
[0071] Since single crystals cannot be processed by conventional
hot or cold mechanical formation without breaking single
crystallinity, a special procedure is required for shaping single
crystals in the process of growth as the crystal is pulled from
melt, resulting in finished shape.
[0072] Single crystal SMA is fabricated in a special
crystal-pulling apparatus. A seed of the desired alloy is lowered
into a crucible containing a melted ingot of the alloy composition,
and gradually drawn up. Surface tension pulls the melted metal
along with the seed. The rising column cools as it leaves the
surface of the melt. The rate of drawing is controlled to
correspond with the rate of cooling so that a solid crystal is
formed at a region that becomes a crystallization front. This front
remains stationary while the crystal, liquid below and solid above,
travels through it. The top surface of the melt can contain a die
(of the desired cross-sectional shape) that forms the shape of the
crystal as it grows. This procedure generally is known as the
Stepanov method of making single crystals. The principal elements
are shown in FIG. 7.
[0073] After the ingot is melted the crucible with the metal is
lifted up until the hydrostatic pressure pushes the molten metal
almost to the upper edge of the shape-forming die. The metal should
be overheated above the melting point for about 10-15 C. After the
temperature becomes stable the seed is lowered into the die opening
until it reaches the surface of the molten metal. This operation
requires a port for visual observation. The seed material melts and
the column of molten metal confined by the capillary forces rises
above the upper edge of the shape-forming die. The combination of
the proper pressure (defined by the vertical position of the
crucible), metal temperature (defined by the power applied to the
heater) and capillary forces will maintain the corresponding shape
of the molten material and at the same time prevent it from
spilling. Now the pulling mechanism is turned on and the seed
starts moving up, away from the shape-forming die. The column of
molten material becomes longer until its length gets stabilized
with the crystallization front at the top few millimeters above the
edge of the shape-forming die. Continuous adjustment of pressure
and temperature is needed to stabilize the shape of the molten
column of the metal for a given pulling rate.
[0074] The shape and size of the single crystal produced according
to method described above are defined by the geometry of the
shape-forming die, the pressure of liquid at the edge of the
shape-forming die, and the position of the crystallization front
relative to the shape-forming die. There is no specific physical
limitation on the length of the pulled single crystal. However,
technically it is limited by the stroke of the pulling
mechanism.
[0075] From the known Cu--Al phase diagram, rapid cooling
(quenching) of the drawn crystal is necessary for production of
single crystal beta phase that has the desired hyperelastic
properties. Starting with beta phase at 850-1000 Celsius, if the
alloy is cooled slowly the beta phase precipitates as beta+gamma,
and at lower temperatures, as alpha+gamma-2. Single crystal beta
phase, which requires that Al remains in solution at room
temperature, is formed by rapid cooling in salt water from 850
Celsius. At elevated temperatures, above 300 Celsius, some
decomposition gradually occurs; in fact, beta phase is not entirely
stable at room temperatures but the time constant for decay is many
years. The known phase diagram for the ternary CuAlNi alloy has
similar characteristics.
* * * * *