This invention relates to a composite shielding structure for space applications.

BACKGROUND OF THE INVENTION

Satellites, space structures, and space vehicles deployed beyond low earth orbits (LEO) are subjected to a radiation environment that includes protons and electrons trapped in the Van Allen Belt, galactic cosmic rays (GCR), and solar energetic particles (SEP) as well as space charging. New materials are required to shield against this harmful space environment and are the key to the successful completion of the intended mission by these spacecraft. These materials should be light weight and should have a life time beyond the intended life of these spacecraft.

US Patent 7855157 (Kaul) issued December 21 2010 to NASA discloses a cosmic and solar radiation shielding structure that also has structural attributes is comprised of three layers. The first layer is 30-42 percent by volume of ultra-high molecular weight (UHMW) polyethylene fibers, 18-30 percent by volume of graphite fibers, and a remaining percent by volume of an epoxy resin matrix. The second layer is approximately 68 percent by volume of UHMW polyethylene fibers and a remaining percent by volume of a polyethylene matrix. The third layer is a ceramic material.

US Patent 7923709 (Chen) issued April 12 2011 to NASA discloses a system for shielding personnel and/or equipment from radiation particles. In one embodiment, a first substrate is connected to a first array or perpendicularly oriented metal-like fingers, and a second, electrically conducting substrate has an array of carbon nanostructure (CNS) fingers, coated with an electro-active polymer extending toward, but spaced apart from, the first substrate fingers. An electric current and electric charge discharge and dissipation system, connected to the second substrate, receives a current and/or voltage pulse initially generated when the first substrate receives incident radiation, In another embodiment, an array of CNSs is immersed in a first layer of hydrogen-rich polymers and in a second layer of metallike material. In another embodiment, a one- or two-dimensional assembly of fibers containing CNSs embedded in a metal-like matrix serves as a radiation-protective fabric or body covering.

The disclosures of the above references are incorporated herein by reference.

SUMMARY OF THE INVENTION

It is one object of the invention to provide a composite radiation shielding structure.

According to the invention, a composite shielding structure is provided for use in shielding against radiation comprising:

a minimum of five layers

wherein the top and bottom layers are a metal,

wherein a central layer or layers is one type of composite and the other two layers are another type of composite; and wherein the layers are arranged in a symmetrical and balanced lay-up configuration.

Preferably, at least one of the metal layers is Al or its alloys or is Ta or its alloys.

Preferably, one type of composite used is PEFC (Ultrahigh Molecular

Weight Polyethylene Fiber Reinforced Polyethylene Composite) consisting of polyethylene (PE) matrix reinforced with ultra-high molecular weight polyethylene fibers (PEF).

Preferably, one type of composite used is GFC (Graphite Fiber Reinforced epoxy or Polyethylene Composite), consisting of PE or epoxy matrix reinforced with graphite or carbon fibers.

Preferably, the total number of layers of the first type of composite is selected relative to the number of layers of the second type of composite to meet the desired mechanical properties.

Preferably, the structure comprises a solid panel.

Preferably, the structure is bonded to a honeycomb core to form a sandwich structure for space applications.

Preferably, the thickness and the number of the layers, and hence the volume fraction of the Al, PEFC and GFC, is tailored to meet the desired levels of transmitted TID (Total Ionizing Radiation Dose), induced electric charge, and mechanical properties.

Preferably, the thickness and the number of the Al, PEFC and GFC layers, and hence the volume fraction of the layers, as well as the volume fraction and the orientation of the PE fibers and the graphite fibers in the composites depend on the type of orbit and mission type, and are tailored to meet the desired levels of transmitted TID, induced electric charge, and mechanical properties.

Preferably, the volume fractions of Al, PEFC, and GFC in the shielding structure are in the range of 3-6%, 25-65%, 72-29% respectively.

Preferably, the volume fraction of Al, PEFC, and GFC in the shielding structure required to provide 50 krad transmitted TID in a specific orbit are 4.6%, 29.7%, and 65.7%, respectively.

Preferably, the volume fraction of ultra-high molecular weight PE fibers (PEF) within the PEFC and the volume fraction of Graphite fibers within the GFC are in the range of 50-65% and more preferably of the order of 55%.

Preferably, the orientation of the fibers in each layer is in a [0/90] configuration.

Alternatively, the orientation of the fibers in each layer is in a [+4S/-45] configuration.

Alternatively, the orientation of the fibers in some layers is in a [0/90] configuration and the orientation of the fibers in other , layers is in a [+45A 5] configuration.

Preferably, the orientation of the fibers and the volume fraction of al, PEFC, and CFC are chosen to result in tensile modulus for the shielding panel in the range of 40-70 GPa, tensile strength in the range of 400-600 Pa, compressive modulus in the range of 40-70, compressive strength in the range of 400-600 MPa, shear modulus in the range of 20-30 GPa, and shear strength in the range of 75 - 150 MPa.

Preferably, the metal layer is of thickness in the range of 50-200 micro meters.

Preferably, the metal layer is obtained by physical vapor deposition.

Alternatively, the layers are formed by compression molding using metal layers with required thickness; Alternatively, the layers are formed by compression molding using metal layers with thickness more than the desired values, following which the metal layers are machined to the required thickness.

Preferably, the shield structure is used for shielding satellites in Highly Elliptical Orbits like a Molniya orbit.

This structure could also be used for terrestrial application with minor modification. Such applications include personnel and equipment protection in medical imaging, transporting medical isotopes, and nuclear installations.

Therefore, one objective of the present invention is to provide a structure that can shield the electronics and payloads of a satellite from harmful effects of the space radiation environment, which includes trapped protons and electrons, GCR, and SEP, by absorbing the incident radiation and reducing the magnitude of transmitted radiation to an acceptable level. The magnitude of radiation is normally measured as TID (Total Ionizing Radiation Dose), using appropriate detectors, which is known to person skilled in this area.

Acceptable levels of TID are normally decided by the satellite designers depending on the orbit and mission duration; for example 100 Krad for orbit 1 in 10 years or 75 Krad for orbit 2 in 7.5 years. This objective is achieved by a hybrid composite consisting of layers of a metal (Al, Ta) or its alloy, GFC and PEFC with specified thickness and arranged in a specific pattern. The number of layers of GFC and PEFC is determined by the specified thickness divided by the thickness of a single layer,

Another objective of the present invention is to provide a radiation shielding structure with additional attributes, namely acceptable level of electric charging (required to protect against ESD [Electrostatic Discharge]), and mechanical properties.

Acceptable levels are normally decided by the satellite designers and depend on the orbit and mission duration. For example, a mission duration of 10 years in orbit 1 may require a maximum induced electrical charge be 0.002 MV/m, and minimum tensile modulus and strength of 58 GPa and 407 MPa, respectively. This objective of acceptable level of electric charging is achieved by a hybrid composite consisting of layers of a metal (Al, Ta) or its alloy, GFC and PEFC with specified thickness and arranged in a specific pattern. This, along with a specified amount and orientation of graphite fibers in the GFC layer, and of PE fibers in PEFC layer, is used to achieve the acceptable level of mechanical properties.

Another objective of the present invention is to provide a radiation shielding structure that can provide shielding to achieve acceptable transmitted TID without exceeding the limit for the TID absorbed by the component material of the structure. For example, the PE proposed to be used in the invention has been found by prior research to have a threshold value of 2*10 ⁷ rad for the absorbed TID and radiation beyond this has been found to significantly degrade the properties of PE. This objective is achieved by a hybrid composite consisting of layers of a metal (Al, Ta) or its alloy, GFC and PEFC with specified thickness and arranged in a specific pattern.

In accordance with the above objectives, the present invention is a composite shielding structure with multi-functional attributes. It consists of layers of metal and two types of composite arranged in a symmetrical and balanced lay-up configuration. The top and bottom layers are a metal, which is typically Al (Aluminum) and its alloys or Ta (Tantalum) and its alloys. The central layer (in a five layer arrangement) or layers (in an arrangement with six or more layers) is one type of composite and the other two layers are another type of composite. The two type of composites used are PEFC, consisting of polyethylene (PE) matrix reinforced ultra-high molecular weight polyethylene fibers (PEF), and GFC, consisting of PE or epoxy matrix reinforced with graphite or carbon fibers. In structures with more than five layers, the total number of layers of GFC will be less than, equal to or more than the number of layers of PEFC depending on the design. The number of layers of GFC and PEFC is determined by the specified thickness divided by the thickness of a single layer. The thickness and hence volume fraction of a composite, the specific lay-up pattern of the layers of the two composites, and the fiber volume fraction and orientation within the composites are optimized to yield the desired attributes. The multi-functional attributes include desired levels of radiation shielding, electric charging, and mechanical properties.

This structure can be used as a solid panel or can be bonded to honeycomb cores to form sandwich structures for space applications.

The thickness of the three constituents and hence the volume fraction of the three constituents as well as the volume fraction and the orientation of the PE fibers and the graphite fibers in the composite depend on the type of orbit and mission type, and can be tailored to meet the desired levels of transmitted TID, induced electric charge, and mechanical properties. For example, the volume fraction of Al, PEFC and GFC in the shielding structure required to provide 50 krad transmitted TID in a specific orbit is 4.6%, 29.7%, and 65.7%, respectively. These values can be in the range of 3-6%, 25-65%, 72-29% respectively. The volume fraction of ultra-high molecular weight PE fibers (PEF) within the PEFC and the volume fraction of Graphite fibers within the GFC are typically 55% and can be in the range of 50-65%.

The orientation of these fibers in each layer are typically in a [0/90] configuration or [+45Λ 5] configuration. The orientation of fibers in various layers of a composite are typically [0/90] only or [+45/-4S] only or [0/90] in some layers and [+45/-4S] in others. These have been chosen to result in tensile modulus for the shielding panel in the range of 40-70 GPa, tensile strength in the range of 400-600 MPa, Compressive modulus in the range of 40-70 GPa, compressive strength in the range of 400-600 MPa, shear modulus in the range of 20-30 GPa, and shear strength in the range of 75 - 150 MPa.

The graphite fibers are those that are commercially available such as Hexcel Corporation's IM7 fibers. Carbon fibers are those that are commercially available such as Hexcel Corporation's AS4 fibers. The Ultra High Molecular weight PE fibers (PEF) are those that are commercially available such as Honeywell Corporation's Spectra fibers or DSM Corporation's Dyneema fibers. The PE matrix can be any commercially available PE classified as LDPE (Low density Polyethylene), HDPE (High Density Polyethylene) or UHMWPE (Ultra High Molecular Weight Polyethylene).

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention will now be described in conjunction with the accompanying drawings in which:

Fig. 1 is a schematic view of a six-layered composite shielding structure according to the invention.

Figs. 2 and 3 each show a schematic view of the six-layered composite shielding structure of Figure 1 according to the invention where the composite 1 is GFC and composite 2 is PEFC and particularly show two possible lay-up configurations of the resulting composite.

Fig. 1A is a schematic view of a five-layered composite shielding structure according to the invention.

Figs. 2A and 3A each show a schematic view of the five-layered composite shielding structure of Figure 1A according to the invention where the composite 1 is GFC and composite 2 is PEFC and particularly show two possible lay-up configurations of the resulting composite.

Figs. 4 and 5 each show a schematic view of the composite shielding structure according to the invention where more than five layers are illustrated using an example of a structure with 10 layers.

Figs. 6 and 7 each show a schematic view of the composite shielding structure according to the invention where the fibers in GFC and PEFC are woven to yield a typically [0/90] or [+4S/-45] configuration.

Figs. 8 and 9 each show a schematic view of the composite shielding structure according to the invention where the fibers in GFC and PEFC can also be non-woven to yield a typically [0/90] or [+45/-45] configuration.

In the drawings like characters of reference indicate corresponding parts in the different figures.

DETAILED DESCRIPTION

The invention shown in Figure 1 is a composite shielding structure with multifunctional attributes. Figure 1 shows six layers made-up of three types of material namely, a metal, GFC (Graphite Fiber Reinforced epoxy or Polyethylene Composite), and PEFC (Ultrahigh Molecular Weight Polyethylene Fiber Reinforced Polyethylene Composite). In Figure 1A is shown a modified arrangement where there is a single center layer 3 of composite 2. Thus in the center there may be a single layer or more layers in the balanced construction shown.

The thickness of the three constituents, specific lay-up sequence of layers of GFC and PEFC, and fiber volume fraction and orientation within the GFC and PEFC are chosen to meet desired levels of Transmitted TID without exceeding the limit for the absorbed TID for the constituent materials, electric charging, and mechanical properties. This structure can be used as a solid panel or can be bonded to honeycomb cores to form sandwich structures for space applications.

The metal layer on the top surface reduces surface charging and the graphite fibers in GFC reduce internal charging. Together, they result in acceptable levels of overall electric charging.

The metal layer on the top surface also reduces the radiation transmitted to the underlying GFC and PEFC so that desired levels of transmitted TID is obtained without exceeding the limit for the absorbed TID for the GFC and PEFC.

The thickness of the metal, GFC, and PEFC layers as well as lay-up sequence of GFC and PEFC layers can be varied to meet the desired transmitted TID without exceeding the absorbed dose limit for the GFC and PEFC.

The volume fraction and orientation of fibers in GFC and PEFC are chosen to yield a composite shield with desired modulus and strength.

The symmetrical and balanced lay-up sequence prevents warping of the structure during manufacturing. The composite shielding structure is manufactured by stacking various constituents of desired thickness in desired lay-up sequence and laminating them using compression molding. The Al layer will be obtained in suitable thickness from a supplier.

Each GFC composite layer is obtained by sandwiching layers of graphite or carbon fiber fabric in desired orientation between layers of PE film. Alternatively, this composite layer can be obtained by sandwiching layers of parallel graphite or carbon fibers in desired orientation between layers of PE films. Alternatively, a GFC layer, consisting of carbon or graphite fabric pre-impregnated with epoxy matrix, can be obtained from material supplier such as Hexcel corporation. Alternatively, a GFC layer, consisting of parallel carbon or graphite fibers pre-impregnated with epoxy matrix, can be obtained from material supplier such as Hexcel corporation.

Each PEFC composite layer is obtained by sandwiching layers of ultra- high molecular weight PE fiber fabric in desired orientation between layers of PE film. Alternatively, this composite layer can be obtained by sandwiching layers of parallel ultra-high molecular weight PE fibers in desired orientation between layers of PE films. The spaces between the fibers are filled with PE matrix during compression molding.

In another variation of the above process, the metallic layer of thickness in the range of 50-200 micro meters can also be obtained by physical vapor deposition. The shielding structure without the top and bottom metallic layers is manufactured first by the aforementioned compression molding process. Subsequently, the composite laminate surfaces are coated with the metallic layer using physical vapor deposition.

In another variation of the above process, the composite shielding structure is first manufactured using the aforementioned compression molding using metal layers with thickness more than the desired values. Subsequently, the metal layers on the top and bottom surfaces are machined to required thickness.

If composite 1 is GFC and composite 2 is PEFC, two possible lay-up configurations of the resulting composite are provided in Fig.2 and Fig. 3

Possible lay-up configuration for the composite shielding structure with more than five or six layers is illustrated in Fig.4 and Fig. 5 using an example of a structure with 10 layers. The arrangement in each of these figures can be modified by interchanging composite 1 with composite 2, and vice versa (similar to Fig. 2 and 3).

The fibers in GFC and PEFC are woven to yield a typically [0/90] or

[+4S/-45] configuration as shown in Fig. 6 and 7 respectively.

The fibers in GFC and PEFC can also be non-woven to yield a typically [0/90] or [+4S/-45] configuration as shown in Fig. 8 and 9 respectively.

In structures with more than five layers, the total number of layers of GFC would be equal to the number of layers of PEFC. The number of layers for a given volume fraction of a composite, the specific lay-up pattern of the layers of the two composites, and the fiber volume fraction and orientation within the composites are optimized to yield the desired attributes. The multi-functional attributes include desired levels of radiation shielding, electric charging, and mechanical properties. This structure can be used as a solid panel or can be bonded to honeycomb cores to form sandwich structures for space applications.