Understanding the negative thermal expansion in planar graphite–metal composites

Abstract

The addition of graphitic fibers and flakes as fillers is commonly used to control the thermal expansion of metals. Sintered metal matrix composites with a planar distribution of graphite flakes show a low or negative thermal expansion coefficient perpendicular to the orientation plane of the graphite (z-CTE). Since the metal matrix has a positive isotropic expansion and graphite has a high z-CTE, this effect cannot be explained by a simple model of stapled metalgraphite layers. Instead, a mechanical interaction between graphite and matrix must be considered. With neutron scattering measurements we show that there is little or no strain of the graphite flakes caused by the matrix, which can be explained by the high modulus of graphite. Instead, we suggest that a macroscopic crumpling of the flakes is responsible for the low z-CTE of the composite. The crumpled flakes are thicker at low temperature and get stretched and flattened by the expanding matrix at high temperature, explaining the reduction in the thermal expansion across the orientation plane.

From 1D to 2D arrangements of graphite flakes in an aluminium matrix composite: Impact on thermal properties

“…However, it was reported in the aligned Gf/Al composite that the overall CTE along the base plane of graphite merely decreases from 25 × 10 −6 /K to 22 × 10 −6 /K when the volume fraction of Gf increases up to 50% [9]. Comparatively, the CTE along the direction perpendicular to the plane decreases drastically even reaching negative values [18,19]. Therefore, such above-mentioned high anisotropies of the Gf gave rise to major obstacles in fabrication once both TC and CTE should be taken into account for potential applications.…”

Composite metamaterial square grids with sign-flipping expansion coefficients leading to a type of Islamic design

“…Their investigation provides distinctive sensing results and therefore avails a novel method to the microfluidic sensor applications by using the sample holder. Although negative material properties-such as negative Poisson's ratio [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], negative thermal expansion [23][24][25][26][27][28][29][30][31], negative compressibility [32][33][34][35][36][37] and negative moisture expansion [38][39][40]-have been investigated for a few decades due to their capability of behaving in ways that are not achievable by conventional materials, it remains that some applications may require both positive and negative properties depending on the circumstances. For this reason, a series of metamaterials in which their effective material properties can switch between positive and negative signs have been introduced.…”

“…Recall that the length of a bimaterial strip is typically 2 or 3 orders higher than its thickness, thereby implying r ∼ l ≫ h ∼ f ∼ R , such that Eqs. (31) and (32) simplify to a common expression…”