Negative coefficient thermal expansion engineered particles for composite fabrication

Methods for the fabrication of negative coefficient thermal expansion engineered elements, and particularly, wherein such elements provide for fillers possessing a low or even potentially zero coefficient thermal expansion and which are employable as fillers for polymers possessing high coefficients of thermal expansion. Further, disclosed are novel structures, which are obtained by the inventive methods.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to diverse methods for the fabrication of negative coefficient thermal expansion engineered elements, and particularly, wherein such elements provide for fillers possessing negative coefficient of thermal expansion and which are employable as fillers for polymers possessing high coefficients of thermal expansion to result in filled polymers with net low, zero, or negative coefficient of thermal expansion. The invention further relates to novel structures, which are obtained by the inventive methods.

In essence, polymers, which are adapted to be filled with negative coefficient of thermal expansion elements or particles that possess a low, zero or negative coefficient of thermal expansion (CTE), are in demand for intended applications in, for instance, the electronics and aeronautics industries.

In particular, the methods of fabricating these negative CTE elements or particles, and that serve as volume increasers for the polymer components may be produced by means of the novel methods so as to provide unique volumetrically expanded structures.

Accordingly, it is an object of the present invention to provide novel methods for the fabrication of negative coefficient of expansion engineered elements or particles, preferably utilized for fillers in polymer structures.

Another object of the invention resides in the provision of structures incorporating negative coefficient of expansion elements or particles that are fabricated pursuant to the inventive methods.

DETAILED DESCRIPTION OF THE INVENTION

Pursuant to the fabrication of a first embodiment of the present invention, there is employed the following method, as represented inFIGS. 1A through 1Dof the drawings.

Referring in particular toFIG. 1Aof the drawings, there are provided in superposition two separate material layers10,12in sheet form, which possess different coefficients of thermal expansion (CTEs), i.e. such as Al, Cu or the like. These two layers, which form a bilayer sheet14may be stacked with a reversely or oppositely oriented and symmetrically arrayed bilayer16consisting of identical materials10,12in flat superposition, as shown inFIG. 1B. A first projection laser L1is adapted to weld the superimposed bilayers14,16together in a repeating annular pattern along line18, so as to produce essentially disk-shaped structured configurations, as inFIG. 1C. The annular welding by projection laser L1need not be continuous, but may be in the nature of spot welds; for example, three spots spaced at 120-degree intervals about the perimeter of each disk-shaped structure. This will enable the remaining areas to maintain freedom of movement. These disk-shaped configurations, or essentially ring structures formed on the bilayers14,16are not touching neighboring ring structures across the repeating pattern on the sheet and leave a small linear distance on the sheet between each annulus18, possibly such as a half-radius. A further process step, as inFIG. 1C, resides in employing a second laser L2, to cut these annular structures18out from the sheets14,16by focusing the laser in an annular pattern along a circular cutting line20, which is larger then the annular weld pattern at18, and whereby the cutting laser conditions differ from those of the welding laser conditions. The cutting laser intensities may hereby be higher or the pulse frequencies and/or widths may differ. Moreover, a different type of laser may also be advantageous or possibly by employing a different laser wavelength.

These foregoing method steps, as described in connection withFIGS. 1A through 1C, are preferably performed at elevated temperatures, so that when the final disk-shaped particles18are cooled, the differential coefficient thermal expansions within each of the bilayers force the disk-shape structure to open up, as inFIG. 1Dof the drawings, and thus increase the volume of the overall structure.

As illustrated in the embodiment of the method as described inFIGS. 2A through 2Eof the drawings, a single type of material is initially employed for each of two sheet-like layers24, as shown inFIG. 2A, for example, such as aluminum or the like. The two layers10are laser welded together, as shown inFIG. 2B, by a suitable projection laser L1into an annular or disk-shaped structure26along an annular weld line28. Each of the layers10are then converted by processing into a bilayer30,32with different coefficients of thermal expansion, as represented inFIG. 2C. Thus, the structure26is oxidized in a controlled-atmosphere furnace so that there is grown a precise thickness of aluminum oxide30on each outer surface. The aluminum oxide (Al2O3) has a lower CTE than the Al thickness portion. The resultant annular sheet is then laser cut by a laser L2, as shown inFIG. 2D, so as to release or separate disk-shaped or annular particles or elements from the remaining sheet. The method steps are all implemented at elevated temperatures so that a zero stress condition is obtained at higher temperature, where the bilayers30,32are flat. Upon cooling, these disk-shaped particles or elements are then opened or spread apart internally, increasing their volume, as shown inFIG. 2Eof the drawings.

In accordance with modified methods employed in forming structures of the type under consideration, the following approaches are possible:

Another example starting with a single layer, similar to that ofFIGS. 2A through 2E, would be the coating of a sacrificial core polymer disk with aluminum, followed by controlled oxidation to A1203 through a fraction of the A1thickness. After the polymer is decomposed a hollow disk shaped structure remains which is in a low stress state at the temperature at which the oxidation was performed. Upon cooling, these particles would open up increasing their volume.

Pursuant to a further modification, the following concept may be used:

Another embodiment is to cause a sheet of material to be adhered to a second sheet of the same material, where the adhesive would be applied in an annular pattern, and wherein the adhesive processes a higher CTE than the sheet material. Laser cutting could then be used to excise the disk shaped device around the outside of the adhesive ring. The processing, including the adhesion step, should be carried out at elevated temperatures so that upon final cooling the shrinkage of the high CTE adhesive would be greater than that of the sheet material and cause a curvature to develop in the sheet and possibly even occurring in buckling. A variant of this approach would be to use a bilayer sheet material with a low CTE layer on the outside to force the development of positive curvature upon cooling (convex from the outside) and to negate the possibility of particle collapse where one sheet would have positive curvature and the other negative (where the two layers would nest as with two spoons).

Another embodiment, as disclosed inFIG. 3, can be obtained utilizing the inventive method. Hereby, it is possible to have the directions of the bilayer reversed near the edge so that it would curve in the opposite direction and thus help reduce edge stress during opening.

FIG. 3shows a crossection of the right hand side of the device. The full structure would additionally show the mirror image attached seamlessly to the left side of the figure. Three sections are shown. An “A width” a “B width” and a “C width”. The A section is typical of the negative CTE devices previously described to the extent that the outer layers40consist of low CTE material, the inner layers42consists of a higher CTE material, and there is a gap44consisting of no material. The B section reverses this general sequence and has a high CTE layer46on the outsides, and a lower CTE material48on the inside. With a gap50separating them. The C section has the same sequence as the B section (high CTE on the outside52and low CTE on the inside54) except that a layer56that welds the inner low CTE layers together now replaces the gap.

The relative lengths of the A, B, and C sections, as well as the relative thicknesses of the high and low CTE layers and the weld layer, are variable depending on the material properties of the constituent materials used. Such as the elastic modulus, the CTE, etc., as well as the optimized performance for a use as a filler in a particular polymer.

The effect of reversing the layer sequence near the device edges is that upon imposing a delta T, this creates an edge curvature which deviates from the plane faster than the reverse sequence and results in a larger overall volumetric change. It is to be understood that many varieties of such a structure could be easily conceived, each with different advantages and tradeoffs. One such would merge layers46with42by co-depositing them in a single process step followed by a following step to deposit layer40.