U.S. patent application number 13/274871 was filed with the patent office on 2013-04-18 for spacecraft and spacesuit shield.
This patent application is currently assigned to CELLA ENERGY LIMITED. The applicant listed for this patent is Stephen BENNINGTON, Tom HEADEN, Arthur LOVELL, Atahl NATHANSON, David ROYSE, Stephen VOLLER. Invention is credited to Stephen BENNINGTON, Tom HEADEN, Arthur LOVELL, Atahl NATHANSON, David ROYSE, Stephen VOLLER.
Application Number | 20130095307 13/274871 |
Document ID | / |
Family ID | 48086182 |
Filed Date | 2013-04-18 |
United States Patent
Application |
20130095307 |
Kind Code |
A1 |
BENNINGTON; Stephen ; et
al. |
April 18, 2013 |
SPACECRAFT AND SPACESUIT SHIELD
Abstract
A spacecraft or spacesuit that provides shielding to reduce
exposure to ionizing radiation such as high energy electrons and
protons. Further, methods are provided for reducing exposure
through spacesuits and manufacturing spacecraft and spacesuit
shields.
Inventors: |
BENNINGTON; Stephen;
(Abingdon, GB) ; LOVELL; Arthur; (Oxford, GB)
; HEADEN; Tom; (Wantage, GB) ; ROYSE; David;
(Reading, GB) ; NATHANSON; Atahl; (Oxford, GB)
; VOLLER; Stephen; (Hampshire, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BENNINGTON; Stephen
LOVELL; Arthur
HEADEN; Tom
ROYSE; David
NATHANSON; Atahl
VOLLER; Stephen |
Abingdon
Oxford
Wantage
Reading
Oxford
Hampshire |
|
GB
GB
GB
GB
GB
GB |
|
|
Assignee: |
CELLA ENERGY LIMITED
Didcot
GB
|
Family ID: |
48086182 |
Appl. No.: |
13/274871 |
Filed: |
October 17, 2011 |
Current U.S.
Class: |
428/213 ;
156/313; 252/478; 264/212; 264/465; 264/639; 428/221; 428/421;
428/473.5; 428/474.4; 428/515 |
Current CPC
Class: |
G21F 3/025 20130101;
D01D 5/0007 20130101; B29C 48/08 20190201; Y10T 428/3154 20150401;
D01F 8/04 20130101; G21F 3/00 20130101; Y10T 428/2495 20150115;
B29C 48/05 20190201; B29C 67/04 20130101; Y10T 428/31721 20150401;
B29C 43/02 20130101; Y10T 428/249921 20150401; D01F 1/10 20130101;
G21F 1/10 20130101; B29C 39/02 20130101; Y10T 428/31725 20150401;
G21F 1/103 20130101; Y10T 428/31909 20150401 |
Class at
Publication: |
428/213 ;
252/478; 428/221; 428/515; 428/474.4; 428/473.5; 428/421; 264/212;
156/313; 264/639; 264/465 |
International
Class: |
B32B 7/02 20060101
B32B007/02; B32B 3/00 20060101 B32B003/00; B32B 27/06 20060101
B32B027/06; B29C 47/06 20060101 B29C047/06; B29D 7/00 20060101
B29D007/00; B29C 70/28 20060101 B29C070/28; B28B 5/00 20060101
B28B005/00; G21F 1/10 20060101 G21F001/10; B32B 27/34 20060101
B32B027/34 |
Claims
1. A spacecraft or spacesuit having a radiation shield, the shield
comprising a hydrogen-containing material and a polymer binder.
2. The spacecraft or spacesuit of claim 1, wherein the
hydrogen-containing material has a hydrogen content greater than
14% by weight.
3. The spacecraft or spacesuit of claim 2, wherein the
hydrogen-containing material has a hydrogen content of 17% or more
by weight.
4. The spacecraft or spacesuit of claim 1, wherein the
hydrogen-containing material has a higher hydrogen content by
weight than the polymer binder.
5. The spacecraft or spacesuit of claim 1, wherein the
hydrogen-containing material is an inorganic compound.
6. The spacecraft or spacesuit of claim 1, wherein the
hydrogen-containing material is a hydride.
7. The spacecraft or spacesuit of claim 1, wherein the
hydrogen-containing material comprises borane and/or a
borohydride.
8. The spacecraft or spacesuit of claim 1, wherein the shield
comprises a neutron absorber element.
9. The spacecraft or spacesuit of claim 8, wherein the neutron
absorber element has a neutron absorption cross-section of at least
50 barns
10. The spacecraft or spacesuit of claim 8, wherein the neutron
absorber element has a neutron absorption cross-section of at least
500 barns.
11. The spacecraft or spacesuit of claim 8, wherein the
hydrogen-containing material comprises the neutron absorber
element.
12. The spacecraft or spacesuit of claim 8, wherein the neutron
absorber element is one or more of lithium and boron.
13. The spacecraft or spacesuit of claim 12, wherein the neutron
absorber is lithium isotopically enriched with lithium-6
14. The spacecraft or spacesuit of claim 12, wherein the neutron
absorber is boron isotopically enriched with boron-10.
15. The spacecraft or spacesuit of claim 1, wherein the polymer
binder forms a matrix through which the hydrogen-containing
material is distributed.
16. The spacecraft or spacesuit of claim 1, wherein the polymer
binder encapsulates the hydrogen-containing material.
17. The spacecraft or spacesuit of claim 1, wherein the shield is
formed as a bulk solid.
18. The spacecraft or spacesuit of claim 1, wherein the shield is
formed of layers or films.
19. The spacecraft or spacesuit of claim 1, wherein the shield is
formed of fibres.
20. The spacecraft or spacesuit of claim 1, wherein the binder is a
thermoplastic or thermosetting polymer.
21. The spacecraft or spacesuit of claim 20, wherein the binder is
a thermoplastic polymer and is one or more of poly
(methylmethacrylate), polyester, polyethylene, polybutylene,
polyisobutylene, poly vinylidene fluoride, poly vinyl acetate,
polybutadiene, polystyrene, polytetrafluoroethylene, polysulphone,
or polypropylene.
22. The spacecraft or spacesuit of claim 20, wherein the binder is
a thermosetting polymer and is one or more of polyepoxide,
polyimide, polyamide, polyaramide and melamine formaldehyde.
23. The spacecraft or spacesuit of claim 1, wherein the polymer is
polyethylene oxide and/or polyvinyl pyrrolidone.
24. The spacecraft or spacesuit of claim 1, wherein the polymer is
a co-polymer.
25. The spacecraft or spacesuit of claim 1, wherein the hydrogen
containing material is at least one of ammonia borane, ammonium
borohydride, methylammonium borohydride, lithium borohydride, an
ammoniate of lithium borohydride, a methyl amine borane, ammonia
triborane, ammonium octahydrotriborate, and beryllium hydride.
26. The spacecraft or spacesuit of claim 1, wherein the
hydrogen-containing material and binder form layers having a
thickness less than 500 .mu.m.
27. The spacecraft or spacesuit of claim 1, wherein the shield is a
mat of fibres.
28. A material comprising a cast or sintered mixture of a polymer
and a hydrogen containing material, wherein the polymer forms a
matrix through which the hydrogen-containing material is
distributed.
29. (canceled)
30. The material of claim 28, wherein the polymer is one or more of
polyethylene, polypropylene, polyisobutylene, polybutadiene, poly
(methylmethacrylate), polysulphone, polystyrene, poly (vinyl
pyrrolidone), poly vinylidene fluoride, poly tetrafluoroethylene,
poly ethylene oxide, poly vinyl acetate, polyester, poly
(styrene-co-ethylene-ran-butadiene-styrene), polyepoxide,
polyimide, polyamide, polyaramide and melamine formaldehyde, or is
a copolymer thereor.
31. Use of the material of claim 28 as a radiation shield of a
spacecraft, spacesuit, or lunar or planetary habitation module.
32. A method of manufacturing a radiation shield for a spacecraft
or spacesuit, comprising: mixing a hydrogen-containing material
with a polymer or polymer precursor; shaping the mixture; and
solidifying the mixture such that the polymer forms a matrix
through which the hydrogen-containing material is distributed; and
incorporating the solid in a radiation shield of a spacecraft or
spacesuit.
33. The method of claim 32, wherein the step of mixing comprises
mixing the hydrogen-containing material and polymer or polymer
precursor in a liquid to form a solution and/or suspension, and the
step of shaping comprises electrospinning or electrospraying the
solution and/or suspension to form fibres or beads.
34. The method of claim 33, further comprising heating the
mixture.
35. The method of claim 32, wherein the hydrogen containing
material has a decomposition temperature above the melting point of
the polymer, the method further comprising melting the polymer and
the step of shaping comprises electrospinning or electrospraying
the mixture to form fibres or beads.
36. The method of claim 32, wherein the step of mixing comprises
mixing the hydrogen-containing material and polymer or polymer
precursor in a liquid to form a solution and/or suspension, and the
step of shaping comprises casting the solution and/or suspension to
a film.
37. The method of claim 36, further comprising sandwiching the film
between two sheets of a gas-impermeable polymer.
38. The method of claim 32, wherein the hydrogen-containing
material has a decomposition temperature above the melting point of
the polymer, the method further comprising melting the polymer and
the step of shaping comprises casting the mixture in moulds or by
rolling to form a sheet.
39. The method of claim 32, wherein the step of mixing comprises
mixing powdered hydrogen-containing material with powdered polymer,
and the step of shaping comprises pressing or extruding the
mixture, and further comprising sintering the mixture.
40. The method of claim 32, wherein the polymer is a thermoplastic
or thermosetting polymer.
41. The method of claim 32, further comprising the step of adding a
surfactant or dispersant prior to, or during, the step of
mixing.
42. The method of claim 32, further comprising adding a
polymerisation catalyst prior to, or during, the step of
mixing.
43. The method of claim 38, wherein the mould is airtight.
44. The method of claim 38, wherein the mixture in the mould is
cooled.
45. The method of claim 44, wherein the mixture is cooled to
maintain its temperature at 50.degree. C. or less.
46. The method of claim 44, wherein the mixture is cooled to
maintain its temperature at 40.degree. C. or less.
47. The method of claim 38, further comprising lining the mould
with releasing agent.
48. The method of claim 32, wherein the hydrogen-containing
material has a hydrogen content greater than 14% by weight.
49. The method of claim 32, wherein the hydrogen-containing
material has a hydrogen content of 17% or more by weight.
50. The method of claim 32, wherein the hydrogen-containing
material has a higher hydrogen content by weight than the polymer
binder.
51. The method of claim 32, wherein the hydrogen-containing
material is an inorganic compound.
52. The method of claim 32, wherein the hydrogen-containing
material is a hydride.
53. The method of claim 32, wherein the hydrogen-containing
material comprises borane and/or a borohydride.
54. The method of claim 32, wherein the hydrogen-containing
material comprises a neutron absorber element.
55. The method of claim 54, wherein the neutron absorber element is
one or more of lithium and boron.
56. The method of any of claim 32, wherein the neutron absorber is
lithium-6 and/or boron-10.
57. The method of claim 32, wherein polymer is a thermoplastic or
thermosetting polymer.
58. The method of claim 32, wherein the polymer is one or more of
polyethylene, polypropylene, polyisobutylene, polybutadiene, poly
(methylmethacrylate), polysulphone, polystyrene, poly (vinyl
pyrrolidone), poly vinylidene fluoride, poly tetrafluoroethylene,
poly ethylene oxide, poly vinyl acetate, polyester, poly
(styrene-co-ethylene-ran-butadiene-styrene), polyepoxide,
polyimide, polyamide, polyaramide and melamine formaldehyde.
59. The method of claim 32, wherein the polymer is a copolymer.
60. The method of claim 32, wherein the hydrogen-containing
material is selected from ammonia borane, ammonium borohydride,
methylammonium borohydride, lithium borohydride, an ammoniate of
lithium borohydride, a methyl amine borane, ammonia triborane,
ammonium octahydrotriborate, and beryllium hydride.
61. A method of manufacturing a radiation shield for a spacecraft
or spacesuit, comprising: mixing a polymer or polymer precursor in
a first solvent to form a shell mixture; mixing a
hydrogen-containing material in a second solvent to form a core
mixture; co-axial electrospinning the shell mixture through an
outer aperture of a coaxial nozzle and the core mixture through a
central aperture of a nozzle to form a fibre having a core formed
of the hydrogen-containing material surrounded by a shell formed of
the polymer from the shell mixture or formed from the polymer
precursors therein; and incorporating the fibre in a radiation
shield for a spacecraft or spacesuit.
62. The method of claim 61, wherein the first and second solvents
are immiscible.
63. The method of claim 61, further comprising adding polymer to
the core mixture.
64. The method of claim 61, wherein the step of mixing the
hydrogen-containing material and second solvent forms a colloid or
suspension.
65. The method of claim 61, further comprising heating the mixture
of polymer or polymer precursor and first solvent.
66. The method of claim 61, wherein the hydrogen-containing
material is an inorganic compound.
67. The method of claim 61, wherein the hydrogen-containing
material is a hydride.
68. The method of claim 61, wherein the hydrogen-containing
material has a higher hydrogen content by weight than the polymer
binder.
69. The method of claim 61, wherein the hydrogen-containing
material comprises borane and/or a borohydride.
70. The method of claim 61, wherein the hydrogen-containing
material comprises one or more of lithium and boron.
71. The method of claim 61, wherein the hydrogen-containing
material comprises lithium-6 and/or boron-10.
72. The method of claim 61, wherein the polymer is a thermoplastic
or thermosetting polymer.
73. The method of claim 61, wherein the polymer is one or more of
polyethylene, polypropylene, polyisobutylene, polybutadiene, poly
(methylmethacrylate), polysulphone, polystyrene, poly (vinyl
pyrrolidone), poly vinylidene fluoride, poly tetrafluoroethylene,
poly ethylene oxide, poly vinyl acetate, polyester, poly
(styrene-co-ethylene-ran-butadiene-styrene), polyepoxide,
polyimide, polyamide, polyaramide and melamine formaldehyde.
74. The method of claim 61, wherein the polymer is a copolymer.
75. The method of claim 61, wherein the hydrogen containing
material is selected from ammonia borane, ammonium borohydride,
methylammonium borohydride, lithium borohydride, an ammoniate of
lithium borohydride, a methyl amine borane, ammonia triborane,
ammonium octahydrotriborate, and beryllium hydride.
Description
TECHNICAL FIELD
[0001] The present invention relates to space shielding and more
particularly to shielding to reduce exposure to ionizing radiation
in spacecraft, such as high energy electrons and protons. The
present invention also relates to reducing exposure through
spacesuits and to methods of manufacturing spacecraft and spacesuit
shields.
BACKGROUND ART
[0002] Terrestrial radiation comes largely from nuclear decay and
the remnants of cosmic and solar radiation that has interacted with
the atmosphere. This radiation is made up of light charged
particles with energies of around 1 to 2 MeV.
[0003] Radiation in space is made up of particles having much
higher energies. Space radiation can be divided into three types.
Firstly, solar particles are ejected from the sun in solar flares.
The magnitude of these varies according to the sun's 11-year solar
magnetic cycle. The particles are electrons and protons with
energies typically in the range of hundreds of MeV to low GeV. The
protons cause the majority of the damage. Extreme events are known
as solar proton events (SPE). Galactic cosmic rays (GCR) are a
second type of space radiation. These consist of energetic
particles from events in deep space. The particles are protons,
helium nuclei, and small numbers of heavier nuclei such as carbon
and iron. These particles often have very high energy, for example
in the range 1 to 20 GeV. A third type of radiation is that trapped
by the earth's magnetic field. This trapped radiation is formed by
some of the flux of protons and electrons coming from the sun. They
are concentrated into two radiation bands called the Van Allen
belts. The outer band consists mostly of electrons with energies in
the range 0.1-10 MeV and the inner consists mostly of protons with
energies up to 600 MeV.
[0004] In close earth orbit satellites benefit from the proximity
of the earth and its magnetic field, but such satellites are
outside the earth's atmosphere so they still receive significantly
more radiation than on the surface of the earth. On earth space
radiation produces an average dose of 0.4 mSv/year, whereas on the
International Space Station this rises to 150 mSv/year. Deep space
missions would subject humans to even greater doses, perhaps as
high as 900 mSv/year.
[0005] Space radiation also has an impact on materials and
electronics. Heavy ions, neutrons and protons can displace atoms in
a semiconductor, introducing noise and error sources. The
characteristics of capacitor dielectrics, metal resistor films,
other passive electronic components and even wiring and cabling can
be degraded by radiation over time.
[0006] It is also possible for high energy charged particles to
alter the bits stored in computer memory. These are called single
event upsets and can cause anything from a short-term denial of
service to the loss of the satellite.
[0007] Most of the energy lost by an incoming particle in matter is
through the interaction with electrons. So for space applications
materials with the highest number of electrons per unit mass are
best, which usually means materials with a high hydrogen
content.
[0008] Very high energy irradiation can cause nuclear fragmentation
in heavy elements such as aluminium, which is a common construction
material for spacecraft. The materials that are the most efficient
at shielding space radiation are those with the most electrons per
unit mass. For these reasons, hydrogen, and high
hydrogen-containing materials, such as polyethylene, are preferred.
Lead on the other hand is less efficient at absorbing energy per
unit mass, and is more suited to terrestrial situations where
volume, not mass, is more important.
[0009] Polyethylene, which is used on the International Space
Station (ISS) as a radiation shield, contains only 14 wt % of
hydrogen. Thus it is desirable to produce a material that has a
higher hydrogen content per unit weight than polyethylene.
SUMMARY OF THE INVENTION
[0010] The present invention provides a spacecraft or spacesuit
having a radiation shield, the shield comprising a
hydrogen-containing material, compound or complex, and a polymer
binder. The hydrogen containing compound or complex may also
contain a neutron-absorbing element, such as boron or lithium. The
radiation shield absorbs and dissipates the energy of high energy
particles such as protons, electrons, helium nuclei and others
described above to reduce their ability to cause damage, for
example by ionization. The hydrogen-containing material is
optionally also a neutron absorber. The types of spacecraft may be
satellites or space stations. The invention is not however limited
to these, but also includes any other types of spacecraft in which
a reduction in exposure to the types of space radiation described
above is desired.
[0011] The shield may comprise a first component which comprises a
hydrogen-containing material or combination of materials, and a
second component which is a polymer binder to hold the first
component together thereby increasing its structural strength
beyond that of the first component alone. The shield may be
considered a composite.
[0012] The polymer binder may consist of single or multiple polymer
types. The polymer binder may form a matrix through which the
hydrogen-containing material is distributed to provide structural
strength and rigidity.
[0013] The polymer binder may also confer other advantageous
properties to the shield such as impact protection.
[0014] The polymer binder may encapsulate the hydrogen containing
material. This is preferable if the material is sensitive to oxygen
or moisture.
[0015] The shield may be formed as a bulk solid, as layers or
films, or as fibres. A combination of forms may be used in the
shield.
[0016] The binder may be a thermoplastic or thermosetting
polymer.
[0017] If the binder is a thermoplastic polymer, it may be
polyethylene, polypropylene, polyisobutylene, polybutadiene, poly
(methylmethacrylate), polysulphone, polystyrene, poly (vinyl
pyrrolidone), poly vinylidene fluoride, poly tetrafluoroethylene,
poly ethylene oxide, poly vinyl acetate or polyester. It may be a
co-polymer comprising two or more polymers. It may be poly
(styrene-co-ethylene-ran-butadiene-styrene). If the binder is a
thermosetting polymer, it may be polyepoxide, polyimide, polyamide,
polyaramide or melamine formaldehyde.
[0018] The hydrogen-containing material is preferably an inorganic
material. It may be a hydride.
[0019] The hydrogen containing material may be at least one of
ammonia borane, ammoniumborohydride, methylammonium borohydride,
lithium borohydride, an ammoniate of lithium borohydride, a methyl
amine borane, ammonia triborane, ammonium octahydrotriborane, and
beryllium hydride.
[0020] The shield may comprise a fibre mat.
[0021] The present invention also provides a method of
manufacturing a radiation shield for a spacecraft or spacesuit,
comprising: mixing a polymer or polymer precursor binder with a
hydrogen-containing compound or complex; shaping the mixture; and
allowing the mixture to solidify; and incorporating the solid in a
radiation shield for a spacecraft. Optionally, the
hydrogen-containing compound may comprise a neutron absorber such
as boron or lithium.
[0022] The polymer may be a thermoplastic or a thermosetting
polymer.
[0023] The hydrogen containing material preferably has a hydrogen
content greater than 14%, 15%, or 16% by weight. More preferably
the hydrogen content is 17% or more by weight. The maximum hydrogen
content for an inorganic complex may be around 25%, which is that
for ammonium borohydride.
[0024] The hydrogen containing material has a higher hydrogen
content by weight than the polymer binder. The hydrogen-containing
material is preferably a solid at temperatures from -40 degrees C.
to 150 degrees C. The step of solidifying may occur through
polymerisation. For example, a polymer monomer and hydride may be
mixed together and then a polymerisation initiator, accelerator or
catalyst is added before the mixture is poured into a mould.
Polymerisation produces heat and so cooling may be required.
Polymerisation can also create radicals so it may be necessary to
keep the mixture in an oxygen-free environment.
[0025] The production of bulk material may also be achieved using
epoxy. For example, the two halves of the epoxy (resin and
hardener) are mixed with the hydrogen-containing material just
prior to pouring into a mould.
[0026] The method may further comprise the step of adding a
surfactant or dispersant prior to, or during, the step of mixing. A
surfactant helps materials or solvents to wet each other by
reducing the surface tension between the two. A dispersant is a
special kind of surfactant that helps colloid systems to be better
dispersed by preventing settling or clumping. The dispersant may be
a non-ionic surfactant. More particularly, the dispersant may be a
poloxamer (also known by the trade name Pluronics.RTM.), sorbitan
monopalmitate, ethylenediamine
tetrakis(ethoxylate-block-propoxylate) tetrol average and/or
poly(ethylene glycol)-block-poly(propylene
glycol)-block-poly(ethylene glycol).
[0027] The method may further comprise adding a polymerisation
catalyst prior to, or during, the step of mixing.
[0028] The step of shaping may comprise pressing the mixture into a
mould. The mould may be airtight to prevent absorption of moisture
as the mixture sets. The mixture in the mould is preferably cooled.
The mixture may be cooled to maintain its temperature below the
decomposition temperature of the hydrogen containing material. The
mould may be lined with releasing agent. Some hydrogen containing
materials will require little or no cooling, such as lithium
borohydride which is stable to 200.degree. C.
[0029] The method may be carried out in an environment of reduced
oxygen and/or water vapour.
[0030] The step of mixing may comprise mixing the polymer or
polymer precursor with a boron- and hydrogen-containing compound or
complex in a shared solvent or solvent blend to form a composite
solution. A shared solvent is a solvent in which the polymer and
hydrogen-containing material are soluble.
[0031] Melt casting may also be performed by pouring a liquid
polymer into a mould with the hydrogen-containing material already
mixed therein. Bulk solid material or sheets may be produced in
this way.
[0032] The polymer precursor may be methyl methacrylate monomer,
and the boron- and hydrogen-containing compound or complex may be
ammonia borane.
[0033] The method may further comprise adding a catalyst prior to,
or during, the step of mixing, wherein the catalyst is methyl ethyl
ketone peroxide.
[0034] The polymer precursor may be epoxide, and the hydrogen
containing compound or complex may be lithium borohydride.
[0035] The step of mixing may further comprise adding polyamine as
a hardener and the resulting formation of polyepoxide as
binder.
[0036] The step of shaping may comprise extruding a layer of
solution onto a drum or belt and drying the layer to form a film or
sheet.
[0037] The method may further comprise sandwiching the layer or
film between sheets of oxygen- and/or moisture-impermeable polymer
prior to the step of incorporating. Typically, the sheets may be
high density polyethylene or polyisobutylene. In some embodiments
the bulk or slab materials may also have oxygen- and/or
moisture-impermeable polymer sheets bonded thereto.
[0038] The polymer may be polyethylene oxide or poly(vinyl
pyrrolidone), the boron- and hydrogen-containing compound or
complex may be ammonia borane, and the step of mixing may comprise
mixing the polyethylene oxide or poly(vinyl pyrrolidone) and
ammonia borane in water.
[0039] A further method of making the shield material is to use
sintering of a mixture of powdered polymer and hydrogen-containing
material. Sintering may take place at elevated pressures or
temperatures.
[0040] The step of mixing may comprise mixing the polymer or
polymer precursor with a boron- and hydrogen-containing compound or
complex in a shared solvent or solvent blend to form a composite
solution, and the step of shaping may comprise electrospinning the
composite solution to form fibres. Single phase electrospinning of
this kind can also be performed using a polymer melt. The polymer
melt may be mixed with a powdered hydrogen-containing compound or
complex, having a decomposition temperature higher than the polymer
melt temperature.
[0041] The polymer may be polyethylene oxide or poly (vinyl
pyrrolidone), the hydrogen-containing compound or complex may be
ammonia borane, and the shared solvent may be water.
[0042] The present invention further comprises a method of
manufacturing a radiation shield for a spacecraft, comprising:
mixing a binder comprising a polymer or combination of polymers
and/or polymer precursors in a first solvent, or combination of
solvents, to form a shell solution, suspension or mixture; mixing a
boron- and hydrogen-containing compound or complex in a second
solvent or combination of solvents to form a core mixture;
co-axially electrospinning the shell mixture through an outer, or
annular, aperture of a coaxial nozzle, and the core mixture through
a central, or core, aperture of a coaxial nozzle to form a fibre
having a core formed solely or mostly of the boron- and
hydrogen-containing compound or complex, surrounded by a shell
formed of the polymer or combination of polymers mixed or formed
from the polymer precursors; and incorporating the fibre in a
radiation shield for a spacecraft.
[0043] Coaxial electrospinning can also be performed using a
polymer melt as shell material, if a suitable high boiling-point
fluid is used to dissolve or form a suspension with the
hydrogen-containing material as a core mixture. Coaxial
electrospinning may be performed at temperatures greater than 30
degrees C., with a shell mixture comprising a polymer solution or
polymer melt and a core solution with suspended or dissolved
hydrogen-containing material. This may include material with a
decomposition temperature higher than the temperature of
spinning.
[0044] The first (shell) and second (core) solvents are preferably
immiscible.
[0045] The method may further comprise adding polymer to the core
mixture.
[0046] The hydrogen-containing material may not be soluble, or may
be only partly soluble, in the second solvent such that the core
mixture is a colloid system or slurry.
[0047] The hydrogen-containing material may be ammonia borane and
the binder may be polystyrene, polypropylene, poly vinylidene
fluoride, polyisobutylene or polybutylene.
[0048] The shell mixture may be a solution, and the solvent may be
toluene and/or xylene and/or N,N-dimethylformamide and/or
N-dimethylacetamide and/or dimethyl sulphoxide. The core mixture
may be a slurry of ammonia borane in water with poly (ethylene
oxide).
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] Embodiments of the present invention, along with aspects of
the prior art, will now be described with reference to the
accompanying drawings, of which:
[0050] FIGS. 1a to 1c are photos of the structure of materials made
according to the present invention;
[0051] FIG. 2 is a flow chart showing steps in manufacturing a
spacecraft with a shield according to the present invention;
[0052] FIG. 3 is a chart summarising exemplary manufacturing
techniques and material examples;
[0053] FIG. 4 is a flow chart showing steps in manufacturing a
material for a spacecraft shield according to a first detailed
embodiment using casting of thermoplastics;
[0054] FIG. 5 is a flow chart showing steps in manufacturing a
material for a spacecraft shield according to a second detailed
embodiment using casting of thermosetting plastics;
[0055] FIG. 6 is a flow chart showing steps in manufacturing a
material for a spacecraft shield according to a third specific
embodiment using solution casting;
[0056] FIG. 7 is a flow chart showing steps in manufacturing a
material for a spacecraft shield according to a fourth specific
embodiment using single phase electrospinning;
[0057] FIG. 8 is a flow chart showing steps in manufacturing a
material for a spacecraft shield according to a fifth specific
embodiment using coaxial electrospinning; and
[0058] FIG. 9 is a schematic diagram of a spacecraft with
shielding.
DETAILED DESCRIPTION
[0059] As mentioned above polyethylene is used in the International
Space Station (ISS) as a radiation shield because this is a stable
non-toxic material with high hydrogen content. The amount of
hydrogen by weight for polyethylene is 14%.
[0060] Some boron and hydrogen containing compounds have a hydrogen
percentage by weight which is greater than for polyethylene. For
example, ammonia borane has greater than 19% hydrogen by weight.
Boron is also an excellent neutron absorber and as mentioned above,
although space radiation does not contain neutrons, the nuclear
fragmentation which occurs due to bombardment with high energy
radiation does produce neutrons. Therefore, hydrogen- and
boron-containing compounds such as ammonia borane provide better
radiation shielding than polyethylene with the added advantage of
also absorbing neutrons. Other solid materials with a high hydrogen
content include those based on lithium, and beryllium (such as
BeH.sub.2). Lithium is also a good neutron absorber. The
neutron-absorbing isotope of boron is boron-10 and the
neutron-absorbing isotope of lithium is lithium-6. Using
hydrogen-containing compounds or complexes enriched with these
lighter isotopes confers both improved neutron absorption and a
higher hydrogen weight content in the materials. Lithium has a
neutron absorption coefficient of 70.5 barns which increases to 940
for lithium-6. Boron has a neutron absorption coefficient of 767
barns which increases to 3835 for boron-10.
[0061] Ammonia borane is a waxy solid with little structural
strength. A number of other boron compounds with high hydrogen
content are available but many do not have the required structural
strength or stability alone for use in a radiation shield. Table 1
lists various hydrogen and boron compounds along with wt % of
hydrogen and their stability.
TABLE-US-00001 TABLE 1 Hydrogen Compound Formula Content Stability
Ammonia NH.sub.3BH.sub.3 19.6 wt % Loses H slowly borane above
50.degree. C. Melts & Degrades at 105.degree. C. Ammonium
NH.sub.4BH.sub.4 24.5 wt % Loses H above borohydride room
temperature Methylammonium CH.sub.3NH.sub.3(BH.sub.4) 21.4 wt %
Loses H rapidly borohydride above 40.degree. C. Lithium
Li(BH.sub.4) 18.4 wt % Stable to 300.degree. C. borohydride Reacts
with moisture and oxygen Ammoniates of Li(NH.sub.3).sub.nBH.sub.4
18.1 wt % (n = 1) Loses H lithium at 200.degree. C. borohydride (n
= 1-3) 17.9 wt % (n = 2) Melts at 57.degree. C. 16.5 wt % (n = 3)
Methyl amine (CH.sub.3).sub.nNH.sub.3-nBH.sub.3 18.1 wt % (n = 1)
borane (n = 1-2) 17.9 wt % (n = 2) Ammonia NH.sub.3B.sub.3H.sub.7
17.7 wt % triborane
[0062] Some of the above materials are not sufficiently stable and
must be stabilized before they will be useful as a shield material.
Some of the materials are also sensitive to air and moisture and
will require protection or encapsulation if exposed to atmosphere
in processing or during use. The hydrogen content shown may be
improved further by using materials enriched with boron-10 and/or
lithium-6.
[0063] The present invention provides composite materials for use
as a radiation shield in spacecraft and methods of manufacturing
the material.
[0064] FIG. 2 is a flow chart showing a method of manufacturing a
spacecraft having a shield. A hydrogen-containing compound or
complex is used as an absorber of energy from high energy
electrons, protons etc for the shield. This is mixed with a
polymer, or a combination of polymers or polymer precursors. The
polymers or polymer precursors may be a liquid or in a solution.
This mixing step is shown at step 110. Thermosetting plastics and
thermoplastics may be used as the polymer binder. Thermoplastics
are formed when a catalyst is added to the monomer. Thermosetting
plastics form through a mixture of a resin and a hardening agent.
In both cases the chemical reaction will generate heat. The mixture
of hydrogen-containing compound and binder should be actively
cooled to below the hydrogen-containing compound decomposition
temperature to prevent degradation and release of hydrogen.
[0065] After mixing the mixture will begin to set and any solvents
used will begin to evaporate. The material should be shaped
immediately after mixing, for example by pressing into a mould. The
step of shaping is shown at 120 in FIG. 2. Material pressed into a
mould, or cast, will produce slabs of shield material in which the
hydrogen-containing compound is distributed throughout the binder
and is held in a matrix of binder. The percentage by weight of the
binder and hydrogen-containing compound is such that the majority
of the slab is hydrogen-containing compound. To permit easy removal
of the slab from the mould, the mould may be lined with a releasing
agent. As well as slabs, other shapes of mould may be used to
fabricate other shapes of shield material. After shaping the
material, for example by pressing into a mould, the cooling of the
mixture should continue, as indicated at step 130 in FIG. 2.
[0066] The binder provides structural rigidity while also
protecting the hydrogen-containing compound from oxygen and
moisture. This protection allows it to be handled during assembly
on earth as well as allowing it to be used inside satellites or
space stations. The rigidity provided by the binder also permits
use as an independent structure outside, or spaced from, a
spacecraft.
[0067] Once the mixture has cooled and set, the shield material can
be incorporated in a spacecraft as shown at step 140 in FIG. 2.
[0068] In an alternative embodiment, the process of mixing
hydrogen-containing compound and binder together and shaping the
mixture may be performed simultaneously.
[0069] Examples of suitable polymers for the binder include
poly(methyl methacrylate), polyethylene, polypropylene,
polystyrene, poly vinylidene fluoride, polybutylene, polybutadiene,
polyisobutylene polyester, and co-polymers comprising two or more
of these. An example of a copolymer may be SEBS. These polymers are
thermoplastics. Other examples of suitable polymers include
polyepoxide, polyimide, polyamide, polyaramide and melamine
formaldehyde. These are thermosetting plastics. Another alternative
is polyethylene oxide or poly(vinyl pyrrolidone). Preferably
molecular weights of greater than 1,000,000 (1M) and preferably 2M,
4M or 8M polyethylene oxide are used.
[0070] Depending on the compatibility of the hydride and monomer it
may be necessary to use surfactants to reduce the surface tension
between the absorber and polymer monomer in solution to keep the
absorber in suspension until casting is complete. For example, the
absorber is likely to be a strongly polar molecule whereas the
polymer may only have weak polarity. This may mean there is a
difference in the types of solvent in which the absorber and
polymer will be soluble. The surfactant improves the solubility of
the absorber and/or polymer in the chosen solvent or solvent blend.
The surfactant may be a non-ionic surfactant. Typically, the
surfactant may be sorbitan monopalmitate, ethylenediamine
tetrakis(ethoxylate-block-propoxylate) tetrol average and/or
poly(ethylene glycol)-block-polypropylene
glycol)-block-poly(ethylene glycol).
[0071] Alternative embodiments provide different shaping techniques
to the above casting technique. A first alternative is to use
solution casting. In this technique the hydrogen-containing
compound and binder are dissolved in a common solvent or
combination of solvents. The use of a single solvent in which the
binder and hydrogen containing material are both soluble limits the
choice of binders and solvents that can be used. After mixing the
solution a thin layer of solution is extruded onto a drum or belt.
As the solvent evaporates the hydrogen containing material adheres
to the binder to form a solid solution or mixture. After
evaporation a film of less than 500 .mu.m thickness is produced,
such as a film 10s or 100s of microns thick. Some of the hydrogen
containing material mentioned above are sensitive to air or
moisture. In such cases thin sheets of gas- and
moisture-impermeable polymer, for example high-density polyethylene
or polyisobutylene, are bonded to either side of the shield
material. For shielding applications a large number of layers of
the film will need to be stacked together to create the required
thickness or areal density of absorber. Such a stacking process is
a widely performed industrial process and is not expensive.
[0072] A second alternative is to use electrospinning to produce
fibres. The produced fibres have micron- or sub-micron scale
dimensions. Electrospinning is the process of extruding a solution
or melt through a nozzle where a large electrostatic field causes a
jet to issue from a Taylor cone. Solvents in the jet evaporate, or
the melt solidifies, such that as the jet is pulled by the
electrostatic field a fine fibre is produced.
[0073] Electrospinning can be performed in two ways; single phase
electrospinning and co-axial electrospinning (also known as
co-phase spinning or co-electrospinning). In single phase
electrospinning the hydrogen containing material and binder are
mixed in a common solvent in which both hydrogen containing
material and binder are soluble, in a similar way to the solution
cast method. The solution, or mixture, is fed to a nozzle with a
single aperture to electrospin the fibre. Single phase
electrospinning is not suitable for air- and moisture-sensitive
hydrogen-containing absorber materials without further processing
and encapsulation in a gas-impermeable layer. This is because the
surface area of the fibres is large in comparison to bulk material
so a large amount of absorber will be subjected to air or
moisture.
[0074] Co-axial electrospinning uses a nozzle having a central
aperture surrounded by an annular aperture to produce a fibre
having a central core surrounded by an outer shell. The binder is
dissolved in, or mixed with, a first solvent or combination of
solvents to form a binder solution or mixture. The absorber is
dissolved in, or mixed with a second solvent or combination of
solvents to form an absorber solution or mixture. The two solvents,
or combinations of solvents, are preferably immiscible. The binder
mixture is supplied to the outer aperture of the nozzle, namely the
annular aperture. The absorber mixture is supplied to the inner
aperture of the nozzle, namely the central core aperture.
Electrospinning is then the same as for the single phase method.
The absorber mixture may also include a small quantity of polymer
to maintain stability during electrospinning and prevent collapse
of the fibre as the solvents evaporate.
[0075] The fibres produced from single phase or co-axial
electrospinning techniques can be packed into complex shapes to fit
in the spacecraft. The fibres may be produced as a non-woven mat.
The fibres are flexible, which allows them to be easily fitted into
small and complex spaces, or can be made into moving or flexible
parts such as in spacesuits or inflatable structures. Nevertheless,
because the fibres pack with gaps between them, there will be some
unused space between the fibres which will result in a reduction in
absorber density compared to slab materials. A packing fraction for
fibres is 0.8.
[0076] Another alternative is sintering in which fine powders of
polymer and hydrogen-containing material are mixed together. The
powder mixture is then sintered under pressure or heat to form the
material into the required shape. Any heat applied during the
sintering process should not result in a temperature being exceeded
above which the hydrogen-containing material decomposes.
[0077] A summary of techniques with examples of compounds that may
be used is shown in FIG. 3. The examples are not considered
limiting and other techniques for those materials may be used. In
the figure the following abbreviations are used:
[0078] PEO=polyethylene oxide
[0079] PP=polypropylene
[0080] PE=polyethylene
[0081] PMMA=poly methyl methacrylate
[0082] AB=ammonia borane
[0083] LB=lithium borohydride
[0084] The techniques listed in FIG. 3 can be summarised as:
[0085] i) sintering of a mixture of polymer and hydrogen containing
material in powdered form;
[0086] ii.a) casting by solution or melt;
[0087] ii.b) casting by polyemerisation, resin, or epoxy; and
[0088] iii) single phase and coaxial electrospinning.
[0089] We now describe examples of specific embodiments of the
techniques.
First Example
[0090] As a first specific embodiment, we cast a shield slab having
ammonia borane as the absorber in a poly(methyl methacrylate)
binder. The steps of this method are shown in FIG. 4. At step 210
ammonia borane was mixed with the monomer, methyl methacrylate
liquid using a shear mixer. A surfactant or more preferably a
dispersant was added to the mixture to keep the ammonia borane in
suspension for a sufficiently long time to allow the mixture to be
handled and to solidify. This is shown at step 220. Since
poly(methyl methacrylate) is a thermoplastic polymer a catalyst is
used to initiate the polymerisation of the monomer. In this case
methyl ethyl ketone peroxide (MEKP) was used as catalyst. At step
230 this is mixed in with the monomer and ammonia borane to produce
a viscous slurry. The slurry is pressed into an airtight mould at
step 240. Ammonia borane starts to lose hydrogen at 50.degree. C.
so the temperature of the contents of the mould are actively
maintained below this, such as at 40.degree. C. or less, as shown
at step 250. This example produces shield slabs of polymethyl
methacrylate and ammonia borane (PMMA/AB in FIG. 3). The ratio of
ammonia borane absorber to polymer binder is at least 80:20 wt %
and even as high as 90:10 wt %.
[0091] A similar method may be used for resins or epoxies where the
polymer or precursor is a liquid and is mixed with the
hydrogen-containing material. An initiator can then be added to
start hardening or polymerisation.
Second Example
[0092] FIG. 5 shows the steps of a second specific embodiment which
uses lithium borohydride in epoxide, Epoxide was mixed with lithium
borohydride and polyamine in an inert atmosphere at step 310.
Epoxide is a thermosetting epoxy resin and the polyamine acts as
hardener. The mould is lined with releasing agent at step 320. The
mixture is pressed into the mould (step 330) and allowed to
set.
Third Example
[0093] A third specific embodiment used the solution cast
technique. The method steps are shown in FIG. 6. The materials used
were polyethylene oxide (PEO) as binder and ammonia borane as
absorber in water. 2,000,000 (2M) molecular weight polyethylene
oxide at a concentration of around 3 wt % was used, and mixed (step
410) with ammonia borane at a concentration of around 8-10 wt % in
water. The mixture was stirred and at the same time heated to
30-40.degree. C. for several hours. The solution was then extruded
on to a drum to form a film (step 420) and allowed to dry (step
430). The resultant film has a ratio of ammonia borane to
polyethylene oxide of 70:30 wt %. This technique is shown in the
summary of FIG. 3 as PEO/AB from solution casting. This composition
has a preferred combination of structural strength and high
hydrogen content, although higher ammonia borane contents by weight
are possible, for example 80:20 wt % and 85:15 wt %. Ammonia borane
is hygroscopic and absorbs moisture from the air. In the
environment of space this will not be a problem. However, the
spacecraft will be assembled, at least partly, on earth where
moisture can be absorbed. The structure of the film may be impaired
by the absorption of water, so the film may be encapsulated in
polyethylene sheets bonded on either side of the film (step
440).
Fourth Example
[0094] FIG. 7 lists the method steps of a fourth specific
embodiment. This embodiment used polyethylene oxide as binder and
ammonia borane as absorber. The technique used to produce the
shield material was single phase electrospinning. The materials
used were similar to the solution cast technique. 2M molecular
weight polyethylene oxide at a concentration of approximately 3 wt
% was taken and mixed with ammonia borane at a concentration of
8-10 wt % in water, as shown at step 510. The mixture was stirred
for several hours at room temperature. The resulting solution was
electrospun at step 520 using conventional single phase spinning.
The fibres usually dry during the electrospinning process, but may
be dried further afterwards. Drying is listed at step 530. The
produced fibres have a ratio of ammonia borane to polyethylene
oxide of 70:30 wt % and this material is listed as PEO/AB single
phase fibres in summary FIG. 3. This composition has a preferred
combination of structural strength and high hydrogen content,
although higher ammonia borane contents by weight are possible, for
example 80:20 wt % and 85:15 wt %. The produced fibres may be
optionally moulded, collected or woven together to form a mat at
step 540.
Fifth Example
[0095] A fifth specific embodiment produced core-shell fibres with
ammonia borane encapsulated by polypropylene. The preparation
technique was coaxial electrospinning. The steps of the method are
shown schematically in FIG. 8. The core solution or mixture
comprised a 10-15% solution of ammonia borane in
N,N-dimethylformamide and is prepared at step 610. A small amount
of polymer, polyethylene oxide, was added to the core solution to
maintain stability during spinning. The shell solution or mixture
comprised polypropylene of molecular weight 250,000 at
concentrations of 10-15 wt % in cyclohexane or xylene mixed with
10-20 wt % acetone or N,N-dimethylformamide. This is prepared at
step 620. At step 630, the core and shell solutions or mixture were
coaxially electrospun to produce fibres having at least 40 wt %,
and preferably at least 50 wt % ammonia borane content. As
mentioned above for single phase electrospinning, the fibres dry
during electrospinning or can be dried further afterwards, as shown
at step 640. The practical limit for core:shell weight ratio
appears to be 50:50 because of shell viscosity and flow rate
requirements. This ratio of AB to PS produces only a 14 wt %
material. To improve on this a polyolefin (PE, PP, possibly
polyisobutylene, polybutylene and co-polymers thereof) shell is
required. PP dissolves at elevated temperatures but can be spun at
below <30.degree. C. so 50:50 PP:AB fibres are possible without
AB decomposition producing .about.17 wt % H.
[0096] Optionally the fibres can be collected or woven to form mats
of shield material. The five techniques described above are
examples and numerous variations in the materials for the binder
and hydrogen-containing absorber may be made without departing from
the scope of the invention. Furthermore, the moulding and shaping
techniques described may be interchanged to use other materials
described.
[0097] In a final embodiment polyethylene as a current preferred
choice for shield materials could be used as the binder according
to the present invention. However, polyethylene (PE) is insoluble
in most common solvents which makes its use in solution casting or
electrospinning difficult. Polyethylene is normally manufactured
using gaseous precursors and this also prevents use by the bulk
casting technique. However, it is also possible to sinter the
hydride and polyethylene into composite materials using high
pressure techniques. For example, powdered lithium borohydride and
polyethylene can be mixed together and subjected to high pressures
in a press or through extrusion to make solid or flexible sheets,
or shaped bulk materials.
[0098] This example embodiment may optionally include melting the
polyethylene and carrying out melt casting as the decomposition
temperature of lithium borohydride is above the polyethylene
melting temperature. Polyethylene can be dissolved and electrospun
in solvents such as cyclohexane or xylene at elevated temperatures,
typically above 100 degrees C. Therefore a material may be made in
a version of the fifth embodiment stated above, where single-phase
or co-axial electrospinning at temperatures of 100 degrees C. or
higher is used, and where a solution or suspension of the
hydrogen-containing material with decomposition temperature higher
than 100 C is combined with a polyethylene solution either as a
single phase or as a core-shell composition. Other polymers with a
similar hydrogen content that could be processed in a version of
this embodiment include polybutylene, polyisobutylene and
polypropylene.
[0099] Table 2 below summarises some of the materials described
above.
TABLE-US-00002 TABLE 2 AB:polymer Hydrogen Density Packing H
density ratio wt % g/cm3 density g/cm3 PMMA + AB 90/10 18% 0.81 1.0
15.0 by casting PEO + AB by 80/20 17% 0.86 1.0 15.0 spin casting
PEO-based 80/20 17% 0.86 0.8 12.0 fibres PP-based 70/30 18% 0.83
1.0 14.8 fibres
[0100] FIG. 1 shows some shield materials manufactured according to
the above techniques. FIG. 1a shows a fibre pellet. FIG. 1b shows
PEO-AB spun fibres. FIG. 1c shows sheets of material.
[0101] After the shield material has been prepared the material is
incorporated into a spacecraft for example in the manner shown in
FIG. 8. The spacecraft 800 includes a region which can be occupied
by humans 820 and also in which sensitive electronics 830 are
housed. It is therefore required that this region be shielded from
space radiation. A shield 810 is formed in the hull of the
spacecraft to reduce the exposure of humans 820 to the radiation
and also prevent degradation of the electronics 830 by the
radiation. Part or all of the electronics 830 may also be provided
with an extra layer of shielding to provide further radiation
protection. Furthermore, the spacecraft may also have additional
components that require shielding from radiation. For example, the
spacecraft may include a "storm shelter" 840 inside the spacecraft.
This would provide human occupants with a region of increased
radiation protection such as may be useful in times of peak solar
activity. The storm shelter would be exposed to air within the
spacecraft and if a neutron absorbing element is present in the
shield, the storm shelter would also protect against secondary
radiation (nuclear fragmentation) from the hull. Additional
components may be fitted to the outside of the spacecraft.
Alternatively, because of the orientation of the spacecraft and the
contents it carries it may only be necessary to shield one side of
the spacecraft, namely that facing away from the earth and/or
towards the sun. In such cases a shield 850 mounted externally from
the spacecraft may be used. If the shield is manufactured from
fibres or sheet-based materials described above, the shield may be
folded up during launch and deployed after the spacecraft has
reached the correct orbit. Furthermore, the flexibility of
fibre-based materials makes them particularly useful for spacesuits
and inflatable structures. Shield materials that are flexible may
also be used in spacesuit 860.
[0102] The shield may be incorporated into or comprise a structural
member or impact shield, for example a micrometeorite shield.
[0103] As well as using the bulk, film, fibre or mat-based shield
materials individually it is also possible to combine the three
types of materials. For example, in a given area a mixture of
materials may be used to produce optimum shield packing in enclosed
or complex spaces by using a mixture of bulk and film or fibre
shield materials. The materials described herein for use in
spacecraft and spacesuits may also be used in other space objects
in which radiation shielding is required, for example lunar or
planetary habitation modules.
* * * * *