{"id":929,"date":"2026-06-09T15:51:05","date_gmt":"2026-06-09T13:51:05","guid":{"rendered":"https:\/\/segdielectriques.com\/en\/non-classe\/filler-morphology-aspect-ratio-thermal-percolation\/"},"modified":"2026-06-09T15:51:05","modified_gmt":"2026-06-09T13:51:05","slug":"filler-morphology-aspect-ratio-thermal-percolation","status":"publish","type":"post","link":"https:\/\/segdielectriques.com\/en\/article\/filler-morphology-aspect-ratio-thermal-percolation\/","title":{"rendered":"Filler Morphology: Aspect Ratio, Size and Thermal Percolation"},"content":{"rendered":"<p>The thermal conductivity of a composite does not depend solely on the nature of the fillers. The shape of the particles, their size, distribution and concentration play an equally decisive role. A breakdown of the key morphological mechanisms.<\/p>\n<h2>1. Aspect ratio<\/h2>\n<p>The aspect ratio is defined as the ratio between the length and the thickness of a filler (or between the length and diameter for spherical particles). The further it deviates from 1, the more the particles promote contact between them and therefore heat transfer.<\/p>\n<figure class=\"wp-block-table\">\n<table>\n<thead>\n<tr>\n<th>Shape<\/th>\n<th>Aspect ratio<\/th>\n<th>Inter-particle contact<\/th>\n<th>Impact on \u03bb<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Sphere<\/td>\n<td>AR = 1<\/td>\n<td>Low<\/td>\n<td>Unfavourable<\/td>\n<\/tr>\n<tr>\n<td>Prolate (elongated oval)<\/td>\n<td>AR &gt; 1<\/td>\n<td>Moderate<\/td>\n<td>Moderate<\/td>\n<\/tr>\n<tr>\n<td>Oblate (flattened disc)<\/td>\n<td>AR &lt; 1<\/td>\n<td>Good<\/td>\n<td>Good<\/td>\n<\/tr>\n<tr>\n<td>Platelet (blade)<\/td>\n<td>AR \u226a 1<\/td>\n<td>Very high<\/td>\n<td>Excellent<\/td>\n<\/tr>\n<tr>\n<td>Cylinder (fibre)<\/td>\n<td>AR \u2192 \u221e<\/td>\n<td>Very high<\/td>\n<td>Excellent<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n<p>For hexagonal BN platelets, conductivity in the layer plane can reach ~400 W\/m\u00b7K, making it the most effective shape for directing heat flux in a preferred direction.<\/p>\n<h2>2. Particle size<\/h2>\n<p>Filler size comes into play in two ways:<\/p>\n<ul>\n<li><strong>Specific surface area and filler\/matrix interface:<\/strong> small particles create many small interfaces that increase overall thermal resistance. Large fillers, with a lower specific surface area, are therefore preferable for effective conductivity.<\/li>\n<li><strong>Size combination (jar principle):<\/strong> mixing particles of different diameters optimises packing and maximises the density of the conductive network, similar to the principle of filling a jar with pebbles and sand.<\/li>\n<\/ul>\n<h2>3. Thermal percolation phenomenon<\/h2>\n<p>The percolation threshold corresponds to the critical volume fraction at which the particles form a continuous network in the matrix, creating preferred paths for thermal conduction.<\/p>\n<figure class=\"wp-block-table\">\n<table>\n<thead>\n<tr>\n<th>Below threshold<\/th>\n<th>At percolation threshold<\/th>\n<th>Above threshold<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Isolated particles in matrix. Low conductivity improvement. Rate &lt; 10 vol.%.<\/td>\n<td>First continuous conductive network formed. Significant improvement. Progressive transition for \u03bb.<\/td>\n<td>Dense, interconnected network. Strong improvement in \u03bb. Rate &gt; 50 vol.% required.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/figure>\n<h2>4. Volume fraction (filler content)<\/h2>\n<p>The volume fraction represents the percentage of fillers over the total volume of the composite material. Its influence on effective conductivity is non-linear:<\/p>\n<ul>\n<li>Rate &lt; 10 vol.%: low improvement, filler network is non-existent<\/li>\n<li>Rate 10-50 vol.%: progressive improvement, first network forming<\/li>\n<li>Rate &gt; 50 vol.%: strong impact on effective \u03bb, but risk of high viscosity<\/li>\n<\/ul>\n<p><strong>Warning:<\/strong> Increasing the filler content also increases the number of filler\/matrix interfaces, which tends to reduce local heat transfer. Particle size distribution and morphology must therefore be optimised jointly with the volume fraction.<\/p>\n<h2>5. Interactions between fillers: synergistic effects<\/h2>\n<p>Some studies mention the combination of fillers of different natures and shapes to obtain synergistic effects. For example, Zhifang Gao and Lei Zhao showed that a mixture of AlN and Al\u2082O\u2083 nanoparticles achieves 0.57 W\/m\u00b7K with low volume fractions. Dual-filler synergies can provide an additional gain of 20 to 40% on effective conductivity.<\/p>\n<p><em>Do you want to optimise the morphology of fillers in your thermal composite? Contact our technical team at <a href=\"mailto:sales@segdielectriques.com\">sales@segdielectriques.com<\/a> for tailored support.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Particle shape, size, percolation threshold and volume fraction all shape composite thermal conductivity. Understand the key morphological mechanisms at play.<\/p>\n","protected":false},"author":1,"featured_media":928,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"pdf_url":"","secteur":"","cta_label":"","footnotes":""},"categories":[16],"tags":[],"class_list":["post-929","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-article"],"_links":{"self":[{"href":"https:\/\/segdielectriques.com\/en\/wp-json\/wp\/v2\/posts\/929","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/segdielectriques.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/segdielectriques.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/segdielectriques.com\/en\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/segdielectriques.com\/en\/wp-json\/wp\/v2\/comments?post=929"}],"version-history":[{"count":0,"href":"https:\/\/segdielectriques.com\/en\/wp-json\/wp\/v2\/posts\/929\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/segdielectriques.com\/en\/wp-json\/wp\/v2\/media\/928"}],"wp:attachment":[{"href":"https:\/\/segdielectriques.com\/en\/wp-json\/wp\/v2\/media?parent=929"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/segdielectriques.com\/en\/wp-json\/wp\/v2\/categories?post=929"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/segdielectriques.com\/en\/wp-json\/wp\/v2\/tags?post=929"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}