NOVELTY - The composite structure (30) comprises a matrix (32), a connected percolating thermally conducting network structure embedded within the matrix, and phase change material particles dispersed within the matrix. The connected percolating network structure comprises interconnected thermally conducting pathways, and is a metallic network having a volume fraction of 0.1-20% within the composite structure. The interconnected thermally conducting pathways comprise nanoparticles (34) that form interconnected structures. The nanoparticles have a dimension of 5-500 nm.    USE - The composite structure is useful for switched power systems,and as thermal interface material in electronic and photonic device packages and as thermal management materials for thermal energy storage in buildings and solar power plants.    ADVANTAGE - The composite structure has high thermal conductivity (greater than 1 W/mK) and dielectric constant, and increased heat capacity, and can provide improved performance and reliability to the device packages.    DETAILED DESCRIPTION - The composite structure (30) comprises a matrix (32), a connected percolating thermally conducting network structure embedded within the matrix, and phase change material particles dispersed within the matrix. The connected percolating network structure comprises interconnected thermally conducting pathways, and is a metallic network having a volume fraction of 0.1-20% within the composite structure. The interconnected thermally conducting pathways comprise nanoparticles (34) that form interconnected structures. The nanoparticles have a dimension of 5-500 nm. The matrix comprises a polymer material, and a phase change material. The phase change material particles comprise phase change salts, lamellar metallo-alkylphosphonates, indium nanoparticles, and/or low melt element or alloy. The phase change material comprises an organic phase change material, an inorganic phase change material, and/or a eutectic of two phase change materials. An INDEPENDENT CLAIM is included for a method of forming a compound.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross-sectional view of a composite structure.    First substrate (10)    Second substrate (20)    Composite structure (30)    Matrix (32)    Metallic nanoparticles. (34)