U.S. patent application number 10/887683 was filed with the patent office on 2007-10-25 for self-healing and adaptive materials and systems.
Invention is credited to Anagi Balachandra, Parviz Soroushian.
Application Number | 20070246353 10/887683 |
Document ID | / |
Family ID | 38618453 |
Filed Date | 2007-10-25 |
United States Patent
Application |
20070246353 |
Kind Code |
A1 |
Soroushian; Parviz ; et
al. |
October 25, 2007 |
Self-healing and adaptive materials and systems
Abstract
Solid electrolyte and at least one of piezoelectric and
thermoelectric materials are incorporated into material systems to
provide them with self-healing and adaptive qualities. The
piezoelectric and thermoelectric constituents convert the
mechanical and thermal energy, respectively, concentrated in
critical areas into electrical energy which, in turn, guides and
drives electrolytic transport of mass within solid electrolyte
towards and its electrodeposition at critical areas to render
self-healing and adaptive effects. Material systems incorporating
the solid electrolyte but not the piezoelectric and thermoelectric
constituents are also amenable to healing and adaptive effects
through external application of electric potential for electrolytic
transport of mass towards and its electrodeposition at critical
areas.
Inventors: |
Soroushian; Parviz;
(Lansing, MI) ; Balachandra; Anagi; (Lansing,
MI) |
Correspondence
Address: |
Parviz Soroushian
1232 Mizzen Drive
Okemos
MI
48864
US
|
Family ID: |
38618453 |
Appl. No.: |
10/887683 |
Filed: |
July 12, 2004 |
Current U.S.
Class: |
204/279 |
Current CPC
Class: |
C25D 7/00 20130101 |
Class at
Publication: |
204/279 |
International
Class: |
C25B 9/00 20060101
C25B009/00 |
Goverment Interests
[0001] This invention was made with U.S. government support under
Contract W911W6-04-C-0024 by U.S. Army. The U.S. government has
certain rights in the invention.
Claims
1. Self-healing and adaptive materials and systems incorporating
solid electrolytes with dissolved salts, and at least one of
piezoelectric and thermoelectric materials, wherein gradient stress
or temperature distributions indicating development of critical
areas with elevated stress or temperature levels induce, via at
least one of piezoelectric and thermoelectric effects, gradient
electric potentials which transport substance towards and deposit
it at said critical areas by electrolytic processes within solid
electrolyte, rendering self-healing and adaptive effects.
2. The self-healing and adaptive materials and systems of claim 1,
wherein at least one of structural, protective and functional
constituents are incorporated to meet service requirements.
3. The self-healing and adaptive materials and systems of claim 1,
wherein the solid electrolytes are at least one of inorganic,
organic and composite ion-conducting materials.
4. The self-healing and adaptive materials and systems of claim 1,
wherein the salts dissolved in solid electrolytes are metal salts,
with self-healing and adaptive effects involving electrolytic
transport of metal cations and electrodeposition of metals at
critical areas.
5. The self-healing and adaptive materials and systems of claim 1,
wherein the salts dissolved in solid electrolytes are metal salts,
and metal fillers are also incorporated into the solid electrolyte,
with electrostripping of metal fillers providing additional metal
cations to be transported and electrodeposited to render
self-healing and adaptive effects.
6. The self-healing and adaptive materials and systems of claim 1,
wherein the piezoelectric constituents are at least one of
inorganic, organic and composite piezoelectric materials.
7. The self-healing and adaptive materials and systems of claim 1,
wherein the thermoelectric constituents are at least one of
metallic, inorganic, organic and composite thermoelectric
materials.
8. Materials and systems that are amenable to externally stimulated
healing and adaptive effects, incorporating solid electrolytes with
dissolved salts and optionally at least one of structural,
protective and functional constituents, wherein external
application of electric potential to the system transports
substance towards and deposits it at said critical areas by
electrolytic processes within solid electrolyte, to heal damaged or
defective areas, or to adapt the material to new service
requirements.
9. The materials and systems of claim 8, wherein at least one of
structural, protective and functional constituents are incorporated
to meet service requirements.
10. The materials and systems of claim 8, wherein the solid
electrolytes are at least one of inorganic, organic and composite
ion-conducting materials.
11. The materials and systems of claim 8, wherein the salt
dissolved in solid electrolyte is a metal salt, with the healing
and adaptive effects involving electrolytic transport of metal
cation and electrodeposition of metal at critical areas.
12. The materials and systems of claim 8, wherein the salt
dissolved in solid electrolyte is a metal salt, and metal fillers
are also incorporated into the solid electrolyte, with
electrostripping of metal fillers providing additional metal
cations to be transported and electrodeposited to render
self-healing and adaptive effects.
Description
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention is generally related to self-healing and
adaptive materials. Particularly, the invention is directed to
materials which can alter their internal mass distribution in
response to stress and temperature gradients in order to optimally
utilize the available structural substance in critical areas
subjected to stress and temperature rise.
[0004] 2. Description of the Relevant Art
[0005] Altering service environments as well as damaging effects
change the stress and/or temperature distribution within
structures. Biological systems such as bone are capable of adapting
to changes in stress distribution through transport of substance
towards and its deposition at highly stressed areas. This
adaptive/self-healing capability enables biological structural
systems make optimal use of available materials as new
circumstances evolve. Various efforts have been made to develop
synthetic materials which mimic the self-healing/adaptive qualities
of biological systems.
[0006] U.S. Pat. No. 6,518,330 discloses a self-healing material
with the polymeric healing agent stored in microspheres which are
dispersed within the material systems. Damage (cracking) of the
material would cause breakage of the microspheres and release of
the healing agent, which fills the crack and rebonds the crack
faces. U.S. Pat. No. 5,790,304 discloses self-healing coatings
incorporating sacrificial constituents which react with oxygen at
defects (e.g., cracks and voids) to produce compounds which
condense on such defects and thereby restore the integrity of
coating. U.S. Pat. No. 5,965,266 discloses a self-healing
high-temperature materials incorporating constituents capable of
reacting with oxygen to produce compounds to plug cracks and
mitigate access of oxygen to the core of the material. U.S. Pat.
No. 4,599,256 discloses a high-temperature material incorporating
multiple constituents which, when exposed to the elevated service
temperature at cracks, react with each other to produce compounds
which seal the cracks. U.S. Pat. No. 5,738,664 discloses a material
incorporating a viscous flowable constituent which can flow into
defects to restore the integrity of the material.
[0007] The above inventions rely on damaging effects (e.g., cracks)
to either release the healing agent or to promote chemical
reactions (e.g., upon exposure to oxygen or elevated temperatures)
which render self-healing and adaptive effects. Unlike the
invention described herein, they do not rely on electrolytic mass
transport to strengthen highly stressed areas, and they do not
convert the destructive mechanical energy concentrated in critical
areas to electrical potential and energy which guide and drive the
self-healing/adaptive effects.
SUMMARY OF THE INVENTION
[0008] It is an object of this invention to provide solid material
systems within which substance can be transported for an optimum
mass distribution to be realized.
[0009] It is another object of this invention to convert the
destructive mechanical and/or thermal energy concentrated within
critical areas of the material into the electrical energy needed to
drive the mass transport phenomenon.
[0010] It is another object of this invention to convert the stress
and/or temperature gradients within the material into the electric
potential which guides transport of mass towards critical
areas.
[0011] It is another object of this invention to integrate the
energy conversion and mass transport capabilities into a material
system which is inherently capable of transporting substance
towards critical areas to render self-healing and adaptive
effects.
[0012] Applicant has discovered that electrolytic transport and
electrodeposition of mass within solid electrolytes can strengthen
and densify areas within which electrodeposition has taken place.
Applicant has also discovered that the piezoelectric effect can
generate sufficient electric potential and energy, by conversion of
mechanical energy, to drive and guide electrolytic mass transport
within solid electrolyte.
[0013] According to the invention, there is provided composite
materials incorporating solid electrolyte and at least one of
piezoelectric and thermoelectric constituents, which can strengthen
and densify highly stressed areas through electrolytic mass
transport and electrodeposition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows a fiber reinforced composite under stress,
where rupture of one fiber has caused local stress rise in an
adjacent fiber.
[0015] FIG. 2 shows a carbon fiber which has received a hybrid
coating comprising a piezoelectric layer and a solid electrolyte
layer with dissolved metal salt.
[0016] FIG. 3 shows the cross-section of the carbon fiber which has
received a hybrid coating comprising a piezoelectric layer and a
solid electrolyte layer (with dissolved metal salt).
[0017] FIG. 4 shows a carbon fiber with piezoelectric and solid
electrolyte coating layers where local stress rise within fiber has
prompted piezo-induced electric potential difference along the
fiber surface which, in turn, drives electrolytic phenomena within
the solid electrolyte layer which transport mass towards and
electrodeposit it at the highly stressed area.
[0018] FIG. 5 shows a layered composite incorporating
piezoelectric, solid electrolyte, conductive and structural layers,
experiencing a local stress rise under concentrated force, with
piezo-driven electrolytic mass transport and deposition
strengthening the highly stressed area where the concentrated force
is applied.
[0019] FIG. 6 shows a layered composite incorporating
piezoelectric, solid electrolyte, conductive and structural layers,
experiencing a local stress rise due to the presence of a
manufacturing defect, with piezo-driven electrolytic mass transport
and deposition strengthening the highly stressed area around the
manufacturing defect.
[0020] FIG. 7 shows a cylindrical structural element, made of a
layered composite incorporating piezoelectric, solid electrolyte
conductive and structural layers, subjected to a gradient stress
system, with piezo-driven electrolytic mass transport and
deposition strengthening regions within the structural element
which are subjected to higher stress levels.
[0021] FIG. 8 shows a layered composite incorporating solid
electrolyte layer, conductive and structural layers, subjected to a
structural damage, where electrolytic mass transport and deposition
via external power supply is used to strengthen the damaged
area.
[0022] FIG. 9 shows a thermal protection coating on a substrate,
with thermoelectric and solid electrolyte layers introduced as
coating constituents, where a damage to thermal protection coating
causes local temperature rise, with electrolytic mass transport and
deposition driven by thermoelectric effect bracing the damaged
area.
[0023] FIG. 10 shows the solid electrolyte specimen sandwiched
between two aluminum electrodes which are connected to a DC power
supply.
[0024] FIG. 11 shows the cathode electrode where electrodeposition
of copper has taken place for the case with solid electrolyte
incorporating dissolved copper salt but no copper filler.
[0025] FIG. 12 shows the cathode electrode where electrodeposition
of copper has taken place for the case with solid electrolyte
incorporating both dissolved copper salt and copper filler.
[0026] FIG. 13 shows the electrolysis cell comprising a solid
electrolyte sheet sandwiched between two stainless steel
electrodes.
[0027] FIG. 14 shows a piezo-driven electrolysis test set-up where
a piezoelectric sheet is subjected to stress in order to generate
the electric potential and charge needed to drive electrolysis
phenomena within a solid electrolyte.
[0028] FIG. 15 shows: (a) solid electrolyte sheet (with dissolved
metal salt) prior to piezo-driven electrolysis; (b) the cathode
face of the solid electrolyte sheet after piezo-driven
electrolysis, where electrodeposition has taken place; and (c) the
anode face of the solid electrolyte sheet after piezo-driven
electrolysis.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0029] Damaging effects, changes in service environment and
manufacturing defects modify the stress and/or temperature
distributions which develop within materials, with local stress
and/or temperature rise occurring in critical areas which govern
eventual failure. Material systems can optimally utilize their
available material resources through partial transport of these
resources towards critical areas which experience local stress
and/or temperature rise, where the rise in material concentration
can render strengthening and densification effects to mitigate the
initiation or propagation of damage.
[0030] Solid electrolytes are solids which can dissolve metal
salts. Electrolytic phenomena can occur within solid electrolytes,
and can be used to transport structural substance towards and
deposit it at particular locations in order to strengthen such
locations. The structural substance is present in solid electrolyte
in the form of dissolved salt; additional structural substance can
be introduced in the form of metals which are in contact with the
solid electrolyte.
[0031] The electrolysis phenomena within solid electrolyte can be
guided and driven by the piezoelectric effect. Piezoelectric and
thermoelectric materials generate electric potential and charge
under stress and temperature gradient, respectively. If
piezoelectric and thermoelectric materials are in proper contact
with a solid electrolyte, the electric potential resulting from
stress and/or temperature gradients can guide and the corresponding
electric charge can drive electrolytic phenomena within the solid
electrolyte to transport structural substance towards and deposit
it at critical areas experiencing stress and/or temperature rise.
The combination of solid electrolyte with at least one of
piezoelectric and thermoelectric materials can thus be used to
develop material systems which can partially adapt their internal
mass distribution to internal stress and/or temperature systems
which are altered by at least one of damaging effects, changing
service environments, and manufacturing defects.
[0032] Material systems incorporating solid electrolyte,
piezoelectric, thermoelectric and other constituents can assume
diverse configurations. One configuration introduces the solid
electrolyte and at least one of piezoelectric and thermoelectric
constituents as a multilayer coating system on reinforcing fibers
in composites. One other configuration comprises solid electrolyte
matrices reinforced with at least one of piezoelectric and
thermoelectric fibers; yet another configuration is in the form of
laminated composites comprising solid electrolyte, at least one of
piezoelectric and thermoelectric, and other layers. Alternatively,
the piezoelectric and thermoelectric constituents could be absent,
with external power supply used to adjust distribution of
structural substance within the solid electrolyte constituent of
the material systems.
[0033] FIGS. 1 through 4 present the example configuration where
the piezoelectric and solid electrolyte constituents are introduced
as a hybrid coating on reinforcing fibers in a composite system.
FIG. 1 shows a fiber reinforced composite comprising reinforcing
fibers and the matrix subjected to external stress; rupture of one
fiber is shown to cause local stress rise in an adjacent fiber.
FIG. 2 shows a length segment of a carbon fiber that has received a
hybrid coating comprising a piezoelectric layer and a solid
electrolyte layer with dissolved metal salt. FIG. 3 shows the cross
section of the carbon fiber which has received the piezoelectric
and solid electrolyte coating layers. FIG. 4 shows the same fiber
as in FIGS. 2 and 3 subjected to local stress rise along its
length. The stress gradient in piezoelectric material produces
electric potential on the surface of the piezoelectric layer which
is in contact with the solid electrolyte. This electric potential
drives electrolytic transport of metal cations within the solid
electrolyte and their electrodeposition at the highly stressed
location along the fiber length. This electrodeposition strengthens
the fiber at the highly stressed location where fiber rupture could
otherwise occur. This process of mass transport towards and its
deposition at the highly stressed location would, in the example of
FIG. 1, strengthen the damaged zone of the composite material where
fiber rupture has occurred, and could thus mitigate the propagation
of an otherwise catastrophic failure process.
[0034] The examples of FIGS. 1 through 4 introduced the
self-healing features of the invention. FIG. 5 depicts the adaptive
features of the invention in an example where the piezoelectric and
solid electrolyte constituents are introduced as layers within a
laminated structural material. Application of a concentrated force
in this example, with the laminated composite placed on a flat
surface, causes a local stress rise which drives electrolytic mass
transport and electrodeposition phenomena to strengthen the highly
stressed region under the concentrated force. FIG. 6 presents the
laminated composite of FIG. 5 subjected to tensile stress, where a
local stress rise is caused by a manufacturing defect, and the
electrolytic mass transport and electrodeposition phenomena
strengthen the critical area around the defect. FIG. 7 presents a
cylindrical element made of a laminated composite similar to that
presented in FIG. 5, with an eccentric load generating an
unsymmetric stress distribution; electrolytic mass transport and
electrodeposition phenomena in this case tend to normalize the
stress distribution and approach an optimum use of structural
materials. Finally, FIG. 8 shows an application where the
piezoelectric constituent is not present, and external application
of electric potential drives the electrolytic mass transport and
electrodeposition phenomena within solid electrolyte to strengthen
a damaged location of the material system.
[0035] The key applications of the technology introduced above
focus on structural applications where conversion of mechanical to
electrical energy drives electrolytic phenomena which strengthen
highly stressed areas of the structure. Piezoelectricity is the
specific effect which converts mechanical energy to electrical
energy in structural applications. Another implementation of the
technology is in thermal protection systems where conversion of
thermal energy to electrical energy, via the thermoelectric effect,
drives electrolysis phenomena within solid electrolyte to transport
substance towards and deposit it at locations experiencing elevated
temperature in order to enhance local thermal protection qualities.
FIG. 9 shows an application where damage to thermal protection
coating causes a local rise in temperature of the substrate with
thermoelectric and solid electrolyte coating layers. The
thermoelectric effects generates electric potential which guides
electrolytic transport and deposition of mass at damaged area in
order to restore the integrity of the damaged protective
coating.
INVENTION AND COMPARISON EXAMPLES
Example 1
[0036] Solid electrolytes were prepared with dissolved metal salt,
without and with fine copper filler. Electrolysis phenomena
occurring in the context of a solid electrolyte, causing
electrodeposition of metal at cathode, were verified
experimentally.
Materials
[0037] Poly(acrylonitrile) (PAN, M.sub.w=86,200), ethylene
carbonate (EC, 98%), propylene carbonate (PC, 99%), copper (II)
trifluoromethanesulfonate (CuTf, 98%), copper powder (3 micron,
dendritic, 99.7%), and acetonitrile (99.93%+, HPLC grade) were
purchased from Aldrich, and were used without any further
purification. The use of copper slat in this investigation implies
that copper is the metal to be ionically transported and
electrodeposited to render self-healing effects. A variety of other
metals (nickel, etc.) can replace copper in the process.
Preparation of Solid Electrolyte without Copper Filler
[0038] PAN (1.06 g or 20 mole %), EC (3.6 g or 41 mole %) and CuTf
(1.8 g or 5 mole %) were weighed into a ceramic crucible and mixed
well before adding PC (3.4 g or 34 mole %). PC was then added, and
the blend was stirred until thorough dissolution and a mixture of
uniform light blue color was obtained. The mixture was then heated
to 120.degree. C. and maintained at this temperature for 45 minutes
(using a temperature-programmed oven with heating rate of
20.degree. C./min, and total heating duration of 51 minutes). The
mixture was allowed to cool down to room temperature, and was then
vacuum dried for 24 hours, and further dried at 60.degree. C. under
vacuum for 2 hours. The end product was light green in color, and
it was pressed to yield the test specimen.
Preparation of Solid Electrolyte with Copper Filler
[0039] The copper salt dissolved in solid electrolyte can act as
the source of metallic ion to be transported and deposited for
self-healing effects. In addition, one can add copper fillers to
raise the quantity of metal available to render self-healing
effects. In order to prepare the PAN-based solid electrolyte
incorporating copper filler, first PAN, EC and CuTf were weighed in
a ceramic crucible, and mixed well before adding PC. PC was then
added, and the mix was magnetically stirred until thorough
dissolution (a uniform mixture) was achieved after about 1 hour.
Different amounts of copper particles were then added to the mix
and magnetically stirred until a mixture with uniform light
brown/blue color was obtained; the intensity of brown color
depended on the dosage of copper filler. The mixture incorporated
1.0 g of water for 10% filler content. The remaining steps in
synthesis and pressing of solid electrolyte specimens with copper
filler were similar to those taken for the specimen without
filler.
Experimental Procedure
[0040] The solid electrolyte was tightly sandwiched between two
aluminum electrodes, as shown in FIG. 10, and a constant voltage
was applied for a period of three days. After three days, the
aluminum electrodes at anode and cathode were inspected
visually.
Test Results and Discussion
[0041] Since the solid electrolyte has some copper salt dissolved
in it, even with no copper filler added to the solid electrolyte,
indications of electrodeposition of copper was observed to occur on
the aluminum sheet at cathode, as shown in FIG. 11, with no such
deposition observed at anode.
[0042] Copper fillers were added to the PAN-based solid electrolyte
to complement the dissolved metal salt as the source of metal for
electrolysis processes which render self-healing effects. In the
case of solid electrolyte with metallic filler, dispersed copper
fillers as well as the dissolve copper salt were the sources of
copper for the electrolysis process. FIG. 12 shows the aluminum
sheet surface at cathode after application of constant voltage.
Electrodeposition of copper on aluminum sheet at cathode is
apparent in FIG. 12, with no such deposition observed at anode.
Example 2
[0043] Materials: The materials used for preparation of PVDF-HFP
solid electrolyte included poly(vinylidine
fluoride-co-hexafluropropylene) (PVDF-HFP) (pellets, crystalline
copolymer, 15% HFP, average M.sub.w.about.400,000), ethylene
carbonate (EC, 98%), propylene carbonate (PC, 99%), copper (II)
trifluoromethanesulfonate (CuTf, 98%), and tetrahydrofuran (THF,
99.9+HPLC grade, inhibitor free). The electrodes were made of 50
micron thick stainless steel shims. The copper salt was used in
this verification study as an example; other metal salts could
replace the copper salt to yield self-healing and adaptive effects
by deposition of metals with higher performance-to-weight rations
than copper.
[0044] Two different solid electrolytes were prepared by varying
the proportions of copper salt, EC and PC while keeping the
PVDF-HFP percentage constant. In order to prepare the solid polymer
electrolyte with 3% copper ion concentration, PVDF-HFP was
dissolved in THF (30% by weight, 3 g) at 60.degree. C.
Subsequently, CuTf (1.8084 g), EC (3.5224 g) and PC (1.7865 g) were
added to the mix (70% by weight at CuTf:EC:PC ratios of
1.0:8.0:3.5), and dissolved until a uniform solution was obtained.
The solution was cast on a Petri dish, and left at room temperature
until all the THF was evaporated. A free standing polymer sheet of
blue/green color was obtained, which was cut into pieces for use in
electrochemical experiments. Since the most common coordination
number of copper is four, each copper ion will bind with four
fluorine atoms (FIG. 12a). This defines the maximum copper
ion-to-polymer molar ratio of 2, which guides our efforts to
increase the concentration of copper ions in PVDF-HFP.
[0045] In order to prepare the solid polymer electrolyte with 6%
copper ion concentration, PVDF-HFP was dissolved in THF (30% by
weight, 3 g) at 60.degree. C. Subsequently, CuTf (3.6168 g), EC
(1.7612 g) and PC (0.89325 g) were added to the mix (70% by weight
at CuTf plus EC plus PC), and dissolved until a uniform solution
was obtained. The solution was cast on a Petri dish, and left at
room temperature until all the THF was evaporated. A free standing
polymer sheet of blue/green color was obtained, which was cut into
pieces for use in electrochemical experiments. Since the most
common coordination number of copper is four, each copper ion will
bind with four fluorine atoms. This defines the maximum copper
ion-to-polymer molar ratio of 2, which guides efforts to increase
the concentration of copper ions in PVDF-HFP.
Experimental Procedures
[0046] In order to validate piezo-induced electrolysis within solid
electrolyte, PVDF-HFP specimens with dissolve copper salt was
sandwiched between two stainless steel electrodes, as shown in FIG.
13. Piezoelectric (PZT fiber reinforced composite) sheets were then
subjected to repeated stress application, as shown schematically in
FIG. 14, and the piezo-induced voltage was applied between the
electrodes. Current was measured at pico amp precision (between the
piezo-setup and electrodes). The basic elements of the test set-up
are depicted in FIG. 13. The current flowing through the solid
electrolyte was found to be 20 .mu.A; a load frequency of 3 Hz was
used in this experiment which lasted 18 hours. After this period,
the solid electrolyte surfaces at anode and cathode were inspected
visually, and were subjected to hardness tests (ASTM D 2240) in
order to assess any changes in mechanical attributes associated
with electrolytic mass transport and deposition.
Experimental Results
[0047] The experimental results provided clear evidence of metal
deposition at cathode interface under piezo-driven electrolysis in
solid electrolyte. FIG. 15a shows the solid electrolyte with
dissolved metal salt prior to piezo-driven electrolysis.
Observation of the cathode and anode interfaces of the solid
electrolyte after the test, shown in FIGS. 15b and 15c,
respectively, provided clear evidenced for piezo-driven
electrolysis at cathode. After piezo-driven electrolysis, the solid
electrolyte adhered to the electrode at cathode. The hardness
values at anode and cathode after piezo-driven electrolytic mass
transport and deposition were 33.3 and 48.1 Shore A (ASTM D 2240),
respectively, compared with a hardness value of 34.0 Shore A (ASTM
D 2240) for the solid electrolyte prior to piezo-driven
electrolysis. The results indicate more than 40% gain in hardness
(representing mechanical attributes) at cathode where
electrodeposition has taken place, confirming the gain in
mechanical properties at cathode associated with piezo-driven
electrolysis within solid electrolyte. On the other hand, anode
experiences only about 2% loss of hardness, indicating that the
local gains in mechanical performance at cathode are achieved
through piezo-driven electrolysis without any major loss of
mechanical performance elsewhere.
* * * * *