Polymer compositions exhibiting a PTC property and methods of fabrication
Various embodiments of the invention provide polymeric compositions including PTC composites that exhibit highly non-linear PTC effects together with extremely rapid, repeatable switching within a predetermined temperature range. In one embodiment, the polymer composite includes a polymer base material with dispersed conductive particles that have very low densities and very low thermal conductivity properties. The conductive particle component can comprise hollow glass microspheres to provide low mass and low thermal conductivity properties together with a nanoscale conductive cladding of silver or gold. The conductively clad microspheres have a core portion with a bulk density of less than about 2.0 g/cm 3 and a mean thermal conductivity of less than about 5.0 W/m-° K.
1. A conductive polymer composition exhibiting a positive temperature coefficient (PTC) property, the composition comprising:
a matrix comprising at least one polymer component; and
a plurality of conductively clad microspheres dispersed within the at least one polymer component, each conductively clad microsphere comprising an electrically conductive cladding over a core portion, wherein the plurality of core portions has a bulk density of less than about 2.0 g/cm 3 and wherein the plurality of conductively clad microspheres comprises at least about 20% by volume of the composition.
2. The conductive polymer composition of claim 1 , wherein the plurality of core portions has a bulk density of less than about 1.0 g/cm 3 .
3. The conductive polymer composition of claim 1 , wherein the plurality of core portions has a bulk density of less than about 0.5 g/cm 3 .
4. The conductive polymer composition of claim 1 , wherein the plurality of conductively clad microsoheres comprises at least about 40% by volume of the composition.
5. The conductive polymer composition of claim 1 , wherein the microspheres are hollow.
6. The conductive polymer composition of claim 1 , wherein the plurality of microspheres comprise at least one of microspheres, filaments, tubules, grains, flakes, particles or powder.
7. The conductive polymer composition of claim 6 , wherein at least a portion of the plurality of microspheres are hollow, porous or solid.
8. The conductive polymer composition of claim 1 , wherein at least a portion of the plurality of microspheres comprise at least one of a glass, a ceramic or a polymer.
9. The conductive polymer composition of claim 1 , wherein a majority of the plurality of microspheres have a mean dimension across a principal axis ranging from about 0.5 micron to about 200 microns.
10. The conductive polymer composition of claim 1 , wherein a majority of the plurality of microspheres have a mean dimension across a principal axis ranging from about 1 micron to about 100 microns.
11. The conductive polymer composition of claim 1 , wherein the polymer component comprises at least one of a polyethylene, polyamide, polystyrene, polyacrylonitrile, polyethylene oxide, polyacetal, thermoplastic modified cellulose, polysulfone, thermoplastic polyester, poly(ethyl acrylate), poly(methyl methacrylate), nylon, polyvinytidene fluoride or ethylene tetrafluoroethylene.
12. The conductive polymer composition of claim 1 , wherein a conductive cladding of the plurality of elements comprises at least one of silver, gold, platinum, nickel, tin, copper, palladium, magnesium, aluminum, iron, molybdenum, tungsten, tantalum, zirconium, zinc, cobalt, chromium or carbon.
13. A conductive polymer composition exhibiting a positive temperature coefficient (PTC) property, the composition comprising:
a matrix comprising at least one polymer component; and
a plurality of conductively clad glass microspheres dispersed within the at least one polymer component, each condictively clad glass microsphere comprising an electrically conductive cladding over a core portion, wherein a plurality of core portions has a mean thermal conductivity of less than about 5.0 W/m.-° K.
14. The conductive polymer composition of claim 13 , wherein a thermal conductivity of each core portion is less than about 5.0 W/m-° K.
15. The conductive polymer composition of claim 13 , wherein the plurality of core portions has a mean thermal conductivity of less than about 2.0 W/m-° K.
16. The conductive polymer composition of claim 13 , wherein the plurality of core portions has a mean thermal conductivity of less than about 0.5 W/m-° K.
17. The conductive polymer composition of claim 13 , wherein the microspheres are hollow.
18. A conductive polymer composition exhibiting a PTC property, the composition comprising:
a matrix comprising at least one polymer component comprising at least one of polyethylene, polyamide, polystyrene, polyacrylonitrile, polyethylene oxide, polyacetal, thermoplastic modified cellulose, polysulfone, thermoplastic polyester, poly(ethyl acrylate), poly(methyl methacrylate), nylon, polyvinylidene fluoride or ethylene tetrafluoroethylene; and
a plurality of conductively clad elements dispersed within the at least one polymer component, the plurality of elements comprising at least about 20% by volume of the composition, each element comprising an electrically conductive cladding portion over a core portion, wherein the plurality of core portions has a bulk density of less than about 2.0 g/cm 3 and the cladding portion comprises at least one of silver, gold, platinum, nickel, tin, copper, palladium, magnesium, aluminum, iron, molybdenum, tungsten, tantalum, zirconium, zinc, cobalt, chromium or carbon.
19. The polymeric composition of claim 18 , wherein each conductively clad element comprises an electrically conductive cladding over a core portion and each core portion has a mean resistivity of greater than about 1.0 megohm-cm.
20. The polymeric composition of claim 18 , where in the plurality of conductivelv clad elements has a mean resistivity of less than about 10 1 ohm-cm.
21. The polymeric composition of claim 19 , wherein at least a portion of the plurality of core portions comprises at least one of a glass or a ceramic.
22. The polymeric composition of claim 19 , wherein at least a portion of the plurality of core portions is hollow.