CROSS-REFERENCE TO RELATED APPLICATION

This application is related to, and derives priority from, United States Provisional Patent Application Serial Number 61/969,918, filed 25 March 2014 and titled "Method for Bulk Optical Polymer Composite Fabrication and Applications," the content of which is incorporated herein fully by reference.

STATEMENT OF GOVERNMENT INTEREST

Not applicable.

BACKGROUND

1. Field

The embodiments relate generally to optical composites, related methods for fabricating the optical composites and related applications of the optical composites. More particularly, the embodiments relate to enhanced performance optical composites, related methods for fabricating the enhanced performance optical composites and related applications of the enhanced performance optical composites.

2. Description of the Related Art

The current state of the optical composite art (i.e., where an optical composite generally comprises a generally homogeneous mixture of an inorganic particulate base material and an organic polymer base material that are together used for forming an optical component) is often limited by a relatively low content (i.e., less than about 20 volume percent) of the inorganic particulate base material in comparison with the organic polymer base material. As a result of the relatively low loading of the inorganic particulate base material in comparison with the organic polymer base material, conventional optical composites often exhibit inhibited compositional scalability, comparatively high cost and comparatively poor stopping power for gamma rays and high energy (i.e., greater than about 5 kev) X-rays (i.e., when the optical composite is used as an optical component within a gamma ray or high energy X-ray radiation detector). Since applications of optical composites and optical components that derive from the optical composites are likely to increase in the near term future, and also increase further into the forseeable longer term future, desirable are optical composites with enhanced performance, methods for fabricating the optical composites with enhanced performance and additional applications of the optical composites with enhanced performance.

SUMMARY

Embodiments include an optical composite and a method for fabricating the optical composite. The optical composite in accordance with the embodiments exhibits enhanced performance insofar as the optical composite possesses enhanced optical transparency with an increased inorganic particulate base material loading. A method for fabricating an optical composite in accordance with the embodiments provides the enhanced optical transparency with the increased inorganic particulate base material loading by: (1) first fabricating a substantially uniform (i.e., less than about 5 volume percent variation between any two locations of 100 nm space) porous compact of the inorganic particulate base material that may provide a substantially uniform volume concentration of the inorganic particulate base material of at least about 30 volume percent; and (2) next infiltrating (and, if needed, also curing) the porous compact with an appropriate optical filler material which is generally a monomer, monomer mixture, pre-polymer or polymer (i.e., selected within the context of desirable optical properties of a resulting optical component) to provide an optical composite in accordance with the embodiments.

As is understood by a person skilled in the art, a "compact" in accordance with the embodiments is intended as a solid dimensional shape that has some residual porosity and where the pores within the solid dimensional shape are defined by a plurality of particles within the solid dimensional shape. A "compact" is thus also defined as a solid form composed of powders which are made cohesive, with void space connecting around, through mechanical pressing.

In particular with respect to an optical composite that includes a scintillation inorganic particulate material, the embodiments also contemplate a selection of a porous compact inorganic particulate material particle size sufficiently small with respect to a wavelength of light emitted by the scintillation inorganic particulate material such that any difference in index of refraction of the scintillation inorganic particulate material and the optical filler material does not appreciably compromise an optical transparency (i.e., less than about 2 percent decrease in transmittance) of the optical composite which comprises the porous compact and the infiltrated optical filler material, where the optical filler material itself has a low (i.e., less than about 0.5 percent) absorption within a transparent wavelength window of interest. Generally, an optical composite in accordance with the embodiments has an optical transmittance from about 0.60 to about 0.99, more preferably from about 0.80 to about 0.99 and most preferably from about 0.90 to about 0.99.

A particular optical component in accordance with the embodiments includes a compact comprising at least about 30 volume percent of a substantially uniform porous inorganic particulate material defining a plurality of pores. The particular optical component in accordance with the embodiments also includes an optical filler material located in the plurality of pores.

A particular method for fabricating the particular optical component in accordance with the embodiments includes infiltrating into a compact comprising at least about 30 volume percent of a substantially uniform porous inorganic particulate material defining a plurality of pores an optical filler material into the plurality of pores.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the embodiments are understood within the context of the Detailed Description of the Non-Limiting Embodiments, as set forth below. The Detailed Description of the Non-Limiting Embodiments is understood within the context of the accompanying drawings, which form a material part of this disclosure, wherein:

FIG. 1A, FIG. IB, FIG. 1C and FIG. ID show a series of schematic diagrams illustrating the results of progressive stages in fabricating an optical composite in accordance with the embodiments. FIG. 2A shows a sample optical component fabricated from a europium doped calcium fluoride based optical composite in turn fabricated in accordance with the embodiments.

FIG. 2B shows a scanning electron microscopy image of the europium doped calcium fluoride based optical composite fabricated in accordance with the embodiments.

DETAILED DESCRIPTION OF THE NON-LIMITING EMBODIMENTS The embodiments provide an optical composite and a method for fabricating the optical composite. The optical composite in accordance with the embodiments comprises: (1) a compacted porous inorganic particulate material mass at a volume percent at least about 30 volume percent and defining a plurality of pores; and (2) an optical filler material (i.e., which is generally but not necessarily an organic polymer material) located in the plurality of pores.

General Considerations

FIG. 1A to FIG. ID shows a series of schematic diagrams illustrating the results of progressive stages in fabricating an optical composite in accordance with the embodiments.

FIG. 1A first shows a quantity of inorganic particles 12a located as a powder upon a first platen 10a. Within the embodiments, the quantity of inorganic particles 12a may comprise any of several inorganic materials. Most specifically, the inorganic materials in accordance with the embodiments comprise an inorganic particle scintillation material such as but not limited to a doped metal halide scintillation material. Such a doped metal halide scintillation material may be selected from the group including but not limited to europium doped calcium fluoride or calcium iodide or strontium iodide or barium iodide or barium bromo-iodide or barium chloride or cerium doped cesium lithium yttrium chloride, lanthanum bromide doped halide scintillation materials. As noted below, however, embodiments are not limited to optical composites that comprise scintillation materials.

Although the embodiment that follows illustrates the embodiments most specifically within the context of a particular europium doped calcium fluoride optical composite the embodiments are also not intended to be so limited. To that end, in a more general sense embodiments may provide optical composites using inorganic materials including but not limited to metal oxide inorganic materials, metal nitride inorganic materials and metal oxynitride inorganic materials which need not comprise scintillation materials. Specific examples of inorganic metal oxide materials include but are not limited to gadolinium oxide, cerium oxide, yttrium oxide, yttrium aluminum garnet, yttrium vanadate, silicon oxide materials, titanium oxide materials and aluminum oxide materials. Additional metal nitride and metal oxynitride materials correlate with the metal oxide materials with respect to metal components. Both stoichiometric and non- stoichiometric compositions of all materials are considered within the context of the embodiments. Most particularly the quantity of inorganic particles 12a as illustrated in FIG. la comprises, as indicated above, at least one scintillation material, and as an example discussed further below a europium doped calcium fluoride scintillation material having a europium dopant concentration from about 0.1 to about 5 atomic percent with respect to a stoichiometric calcium fluoride base material.

Notable in accordance with further discussion below, and in particular with respect to an inorganic particulate scintillation material, a size of the particles from which is comprised the quantity of inorganic particles 12a is selected such that optical scattering from an optical component that is formed from the quantity of inorganic particles 12a is limited. From a practical perspective, this provides a particle size range for the inorganic material particles from about 5 to about 50 nanometers, more preferably from about 5 to about 30 nanometers and most preferably from about 5 to about 10 nanometers within the context of most scintillation materials that provide emitted scintillation light in a range from about 350 to about 650 nanometers.

In the case of particles with sub-micron size, the Rayleigh-Gans-Debye approximation gives the real inline transmittance (RIT):

RIT = (l - R _s ) exp( —— ) where R _s describes the total reflection losses from the sample interfaces, d is the particle size, t is the sample thickness, λ is the wavelength and Δη is the refraction mismatch.

FIG. IB shows the results of further processing of the optical composite whose schematic diagram is illustrated in FIG. 1A. FIG. IB shows the results of processing the quantity of inorganic particles 12a that is illustrated in FIG. 1A through use of the first platen 10a counter-opposed in conjunction with a second platen 10b to provide a compacted inorganic particulate mass 12b that is generally, in the art and within this disclosure, referred to as a "compact." In general within the context of the embodiments, the quantity of inorganic particles 12a as illustrated in FIG. 1A is processed to form the compacted inorganic particulate mass 12b as illustrated in FIG. IB via compaction while using a compressive method that yields the "compact" having an inorganic particle material volume fraction at least about 30 volume percent, more preferably at least about 40 volume percent and still more preferably at least about 50 volume percent, and up to about 80 volume percent. Within the embodiments the foregoing volume percent conditions may be met for the compact using an isostatic pressing processes and in particular a cold isostatic pressing process, at a cold isostatic pressure from about 4000 to about 35000 pounds per square inch. Such a cold isostatic pressing process may use a temperature from about 0 to about 100 degrees centigrade. The use of an isostatic pressing process, and in particular a cold isostatic pressing process, does not limit the embodiments.

FIG. 1C shows the results of further processing of the optical composite whose schematic diagram is illustrated in FIG. IB.

FIG. 1C shows the results of infiltrating an organic polymer material 14 into the compact to provide an optical composite. Within the embodiments, the organic polymer material that may be infiltrated into the pores of the compact to form the optical composite may include, but is not necessarily limited to a monomer material, a plurality of different monomer materials, a pre- polymer or a fully polymerized material that is otherwise thermoplastic. Specific examples include, but are not necessarily limited to, acrylic monomer and polymer materials, silicone monomer and polymer materials and epoxy monomer and polymer materials. Preferably the embodiments use as an organic polymer material a methyl-methacrylate polymer material that is infiltrated into the compact at a negative pressure from about 550 to about 750 torr. FIG. ID shows the results of further processing of the optical composite whose schematic diagram is illustrated in FIG. 1C.

FIG. ID shows an optical composite 12b/14 in accordance with the embodiments which is intended as a uniform compacted inorganic particulate mass 12b defining a plurality of pores having infiltrated therein the organic polymer material 14.

As is understood by a person skilled in the art, the embodiments provide but are not limited to a general method for fabricating an optical composite that may be (but is not necessarily limited to) a photonic application. The particularly disclosed method in accordance with the embodiments enables the optical composite to have a comparatively large inorganic particulate material volume fraction, thereby improving on the physical properties exhibited by the optical composite of the embodiments. As is understood by a person skilled in the art, an optical composite in accordance with the embodiments may be used in an optical component selected from the group including but not limited to radiation detectors (i.e., in particular scintillation radiation detectors), low power solid-state lasers, display components, lighting components, wavelength shifter components, solar cell module components and light filtering components.

In accordance with the embodiments an optical composite comprises a compacted inorganic particulate mass, with volume fractions up to about 80 volume percent, infiltrated by an optical fill material which is generally, but not necessarily, an organic polymer material. High optical transmittances of the optical composite are made possible by the minimization of light scattering in the optical composite through the control of particle size (i.e., in a range from about 5 to about 50 nanometers as noted above, porosity volume fraction (i.e., in a range from about 20 to about 70 as noted above and the judicious choice of the relative indices for the inorganic and organic phases (i.e., in a difference range no greater than about 0.12 consistent with as noted below). If the inorganic particle size is large (micrometer range), the refractive indices must be well matched to within 0.001 or better, while if inorganic nano-particle powders are used when fabricating an optical composite in accordance with the embodiments, the refractive indices can have a relatively larger difference of greater than about 0.01. Theoretically, In fact, as the particle size increases, it becomes exponentially difficult to enhance transmission through refractive index matching.

The organic phase can also be sensitized to emit light by energy transfer from the inorganic particulate scintillation emitter or directly from the environment, where the energy gap of the organic phase matches with energy of photons from inorganic phase or environment. Moreover, in accordance with the embodiments a transparency range of an optical composite can be tuned with temperature, where the extent of refractive mismatch between inorganic phase and organic phase varies with temperature or wavelength due to refractive index dispersion (Christiansen filter).

The fabrication process of an optical composite in accordance with the embodiments involves the preparation of a near net-shape (i.e., final shape) ceramic powder inorganic particulate material compact (using preferably dry consolidation methods such as cold isostatic pressing), and the infiltration of this porous medium under vacuum by a monomer or a molten polymer. The monomer is subsequently polymerized using methods and materials as are conventional in the art, including but not limited to adequate amounts of polymerization initiators, heat or UV curing. In that respect, a method in accordance with the embodiments differs from conventional particle dispersions and polymerization methods for which the inorganic volume fractions are limited to about 20 volume percent and in which high loading contents lead to particle agglomeration, enlarging the effective particle size, and result in increased light scattering. This novel and simple approach in accordance with the embodiments is also a cost effective alternative to single crystal and optical ceramic materials, which require high temperatures and long processing times. As well, in accordance with the embodiments the absence of a sintering process also precludes any shape or dimensional change during fabrication.

Advantages and benefits of the embodiments over currently available technology include but are not limited to greater fabrication efficiency, lower cost, simplicity, low temperature process, size scalable and rapid production process. Applications of the embodiments include but are not limited to photonic devices (displays, Christiansen filters, wavelength shifters), scintillation detectors and low power (i.e., from about 0.001 to about 0.01 watt) lasers.

The embodiments provide a novel approach to fabricate low-cost, near net-shape scintillation detectors with increased neutron detection capability. Because of the high neutron scattering and low capture cross section of 1H, neutrons will be trapped in the composite by elastic scattering and pass the energy to an adjacent inorganic phase with high efficiency. Those particular embodiments rely on carefully engineered composites of inorganic nano-scintillation materials embedded in dissimilar matrices that enable high-volume inorganic particle material loading while maintaining full optical transparency. Those composites can easily be integrated into large area and directional scintillation detectors in order to improve on their signal-to-noise characteristics.

Recent developments in scintillation materials research have highlighted the potential of nano- scintillation composites for the detection of special nuclear materials (SNM). SNM are radioactive materials of strategic significance containing isotopes such as plutonium, uranium- 233, or uranium enriched in the isotopes uranium-233 or uranium-235. Because radioluminescent nanoparticles dispersed in polymers or glasses exhibit reduced optical scattering, it is theoretically possible to embed them in a transparent matrix and form optically clear composites that enable efficient scintillation output, despite a relatively large refractive index contrast. This approach opens the door to a wide range of high performance scintillations that may be difficult to grow in large single-crystals, mechanically fragile or environmentally sensitive (such as LaBr3 or SrI2), and to producing large, rugged and inexpensive detectors with near net-shapes. The ability to engineer composite scintillation materials in volume, by alternating layers of varied compositions for example, also enables the design of large imaging screens and directional neutron detectors with improved background discrimination and time-of- flight capabilities. However, the poor dispensability of powders, which leads to particle clumping and light scattering, has hampered continued progress in this direction since powder volume fractions below 20 percent have only been achieved. Disclosed herein is a fabrication approach different from prior attempts based on dispersed granular scintillation materials, where this embodied approach enables volume loading of up to 80 volume percent of an inorganic particle material. The present work in the preparation of high-quality inorganic nanoparticle nano-powder compacts for the fabrication of laser and nuclear detector transparent ceramics leads one to consider the fabrication of optical composites by infiltration of inorganic green-bodies with liquid monomers followed by curing or by molten inorganic phases at high temperature. An example of such an optical composite or optical component is illustrated in FIG. 2A. A scanning electron microscope image is illustrated in FIG. 2B.

Implications

The embodiments also contemplate establishing the science behind the fabrication of these composites and their optimization for enhancing neutron detection capability by using nanopowder compacts of known scintillation and ΙΗ-rich filling phases. Specifically, one might carry out systematic analyses of the scintillation performance as a function of i) particle size, ii) volume fraction, iii) composition of the scintillating and non-scintillating phases, and choose nanoparticles synthesis routes that minimize scintillation quenching induced by surface-defects. One may model the light scattering properties and light collection efficiency of the composites in order to optimize the selection of the filling phase and one may study the temperature dependency of the composite mechanical strength, light yield and decay. One may propose to carry out these studies using several scintillation material systems including CaF ₂ and BaF ₂ , as well as 6Li-containing scintillation phases such as LiCaAlF ₆ :Ce, LiBaF ₃ :Ce, Cs ₂ LiYCl ₆ :Ce (CLYC). One may also explore optical composite systems in which both phases are inorganic and share a similar refractive index such as but not limited to CsLYAG.

Within the context of the foregoing, one might sequentially:

1. Model light scattering for CaF ₂ , BaF ₂ , LiCaAlF ₆ , LiBaF ₃ , CLYC and polymer systems;

synthesize certain size nanoparticles of these materials; eliminate surface-defect induced luminescence quenching; establish green body forming methods of cold press, slip and gel casting; fabricate transparent and mechanically stable composite with varied volume fraction. 2. Optimize light collection by metal nanoparticle coating; characterize scintillation properties and non-proportionality of different composites; learn the complex temperature dependence of intrinsic emission and possible deep trap recombination; integrate arrays of composites with varied thickness and volume fraction to tune the angular resolution for intrinsically directional detection.

3. Fabricate optical composite with variable volume fraction; determine the

combined scintillation and energy transfer mechanism; identify the proper particle size, volume fraction with best gamma detection energy resolution and light yield; couple composites with different volume fraction to utilize non-proportionality as it is related to chemical composition, dopant type and refractive index.

Experimental

A CaF ₂ :Eu (Eu 0.5 at%) scintillation optical composite was fabricated generally in accordance with the methodology described above.

The fabrication sequence used 1.5 grams of 10 nanometer sized CaF ₂ :Eu (0.5 atomic percent) scintillation material that was isostatically pressed at a pressure of about 29000 pounds per square inch at a temperature of about 20 degrees centigrade to provide a compacted CaF ₂ :Eu particulate mass of thickness about 2 millimeters and diameter about 25 millimeters.

The compacted CaF ₂ :Eu particulate mass was then infiltrated with a methyl-meth-acrylate polymer material and then heat treated at 55 degree centigrade post-processed to provide a relatively flat circular component. An image of the optical component is illustrated in FIG. 2A, which showed an optical clarity of the optical component. FIG. 2B shows a scanning electron microscopy image of the optical component, illustrating the compacted inorganic particulate material domains and the organic polymer material domains. The composite presents a transmittance of 80% at wavelength of 850 nm, and a transmittance of 65% at wavelength of 600 nm. All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference in their entireties to the extent allowed, and as if each reference was individually and specifically indicated to be incorporated by reference and was set forth in its entirety herein.

The use of the terms "a" and "an" and "the" and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms "comprising," "having," "including," and "containing" are to be construed as open-ended terms (i.e., meaning "including, but not limited to,") unless otherwise noted. The term "connected" is to be construed as partly or wholly contained within, attached to, or joined together, even if there is something intervening.

The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it was individually recited herein.

All methods described herein may be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate embodiments of the invention and does not impose a limitation on the scope of the invention unless otherwise claimed.

No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. There is no intention to limit the invention to the specific form or forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention, as defined in the appended claims. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.