Isotropic thermal expansion in anisotropic thermal management composites filled with carbon fibres and graphite

Light materials with high thermal conductivity and low thermal expansion have a wide application potential for the thermal management of high-performance electronics, in particular in mobile and aerospace applications. We present here metal matrix composites with a mixture of graphite flakes and pitch-based carbon fibres as filler. The production by spark plasma sintering orients the filler particles on to a plane perpendicular to the pressing axis. The obtained materials have lower density than aluminium combined with a thermal conductivity significantly outperforming the used metal matrix. Depending on the ratio of the filler components, a low thermal expansion along the pressing direction (high graphite flakes content) or across the pressing direction (high carbon fibre content) is achieved. For a 1:3 ratio of carbon fibres to graphite, we measured an isotropic reduction of the thermal expansion of the matrix by up to 55%. We present a detailed characterisation of composites with two aluminium alloys as matrix and an overview of the properties for six different metal matrices including magnesium and copper. With the goal of a technical application, we show that the described properties are intrinsic to the material compositions and are achieved with a wide spectrum of production methods

Einfluss der Oberflächenmodifikation auf das thermophysikalische Verhalten von Metall/Diamant-Verbundwerkstoffen

Die Leistungs-, Opto- und Mikroelektronikindustrie tendiert stetig hin zu höheren Leistungen bei gleichzeitiger Verringerung der Größe des elektronischen Geräts. Lokale Leistungsdichten werden so enorm angehoben und nähern sich denen eines Kernreaktors. Die Thermal Management Materialien, die heutzutage verwendet werden, sind nicht in der Lage, solche Energiemengen abzuführen. Es ist also offensichtlich, dass die Zeit für neue Materialien auf diesem Bereich des Thermal Mangements gekommen ist. Eines der Mitglieder dieser so genannten Thermal Management Materialien der 3. Generation, nämlich Diamant-Metall-Matrix-Verbundwerkstoffe (MMC), wurde in dieser Arbeit eingehend untersucht, um ein tieferes Verständnis dieser Materialklasse zu erlangen. Wie bei allen Kompositmaterialien diktiert die Schnittstelle zwischen dem Füllstoff (Diamant) und der Matrix (Metall) die Eigenschaften des MMC. Der Schlüssel zur Erzielung hoher Wärmeleitfähigkeiten ist daher die Optimierung der Metall-Diamant-Grenzfläche. Dies wurde in dieser Arbeit hauptsächlich dadurch erreicht, dass die Diamantoberflächenmodifikation durch verschiedene Diamantvorbehandlungen verändert wurde. Andere Ansätze, die in dieser Arbeit verwendet werden, umfassen das Legieren der Metallmatrix und Variationen der Herstellungsbedingungen. Die Diamantoberflächenmodifikation wurde durch nasschemische Behandlungen in H2SO4 und Königswasser sowie durch thermische Behandlungen mit verschiedenen Gasatmosphären durchgeführt. Die Änderung der Modifikation wurde durch Kontaktwinkel- und XPS Messungen von sowohl reinen Modellsystemen als auch Diamantpartikeln, wie sie für die Herstellung von MMCs verwendet wurden, untersucht. Bei einer geeigneten Behandlung der Diamanten konnte die Menge an O-terminierenden Gruppen, von denen angenommen wird, dass sie die Grenzfläche verstärken, fast um das zehnfache erhöht werden. Die so vorbehandelten Diamantpartikel wurden dann durch gasdruckunterstützte Flüssigmetallinfiltration in einen MMC verarbeitet und anschließend hinsichtlich ihrer thermischen Eigenschaften charakterisiert. Es konnte gezeigt werden, dass nicht nur die Diamantoberflächenmodifikation einen Einfluss auf die thermischen Eigenschaften hat, sondern auch die Rauhigkeit der Diamantoberfläche, die Menge an Legierungselement sowie die Herstellungsbedingungen signifikante Einflüsse auf diese Eigenschaften zeigen.The power-, opto- and micro-electronic industries are all trending towards higher performances while decreasing the size of the electronic device, therefore driving local power densities up to values that are approaching the ones of a nuclear reactor. Thermal management materials used today are not capable of dissipating such amounts of energy. Hence it is evident, that the time for new thermal management materials to emerge has come. One of the members of these so called 3rd generation thermal management materials, namely diamondmetal matrix composites (MMC), was investigated in depth in this work in order to get a deeper understanding of this material class. As in all composite materials the interface between the filler (diamond) and the matrix (metal) is dictating the properties of the MMC. Therefore the key to getting high thermal conductivities is the optimization of the metal-diamond interface. This was done in this work mainly by changing the diamond surface modification by different diamond pre-treatments. Other approaches used in this work comprise alloying of the metal matrix and variations of the processing conditions. The diamond surface modification was done by wet-chemical treatments in H2SO4 and aqua regia, as well as by thermal treatments with various gas atmospheres. The change in modification was monitored by contact angle and XPS measurements of both clean model systems and diamond particles as used for the preparation of MMCs. Upon a proper treatment of the diamonds the amount of O-terminating groups, which are believed to strengthen the interface, could be almost increased tenfold. The prepared diamond particles were then incorporated into a MMC by gas pressure assisted liquid metal infiltration and subsequently characterized concerning their thermal properties. It could be shown, that not only the diamond surface modification shows an effect on the thermal properties but also the roughness of the diamond surface, the amount of alloying element as well as the processing conditions

Einfluss der Oberflächenmodifikation von Diamanten auf die Wärmeleitfähigkeit in Metall-Diamantverbundwerkstoffen

Al- and Ag-alloys were used as matrices in diamond composites. The diamonds were surface terminated by different means in order to influence the interfacial thermal conductance positively. The composites were fabricated by gas preessure assisted liquid metal infiltration, subsequently the ambient thermal conductivity was determined. Accomplishing measurements by AFM and wetting behaviour were performed on diamond monocrystals. It was possible to show, that the ambient thermal conductivity strongly depends on the alloying element concentration, the diamond particle size, heat treatment operation and the type of surface modifiaction of the diamonds

Influence of the Diamond Surface Termination on the Thermal Conductivity of Al/Diamond- and Ag/Diamond MMCs

MMCs consisting of diamonds and highly conductive metal matrices have been produced via gas pressure assisted liquid metal infiltration and their thermal properties have been investigated. Special attention was paid towards the diamond surface termination and its influence on the diamond-metal-interface and the resulting heat transport across this interface. Altering the diamond terminating surface layer can lead to a rather drastic increase in the thermal conductivity, rendering MMCs with pretreated diamonds double the thermal conductivity of the ones with as-received diamonds. The evolution of those terminating layers with different pretreatment conditions and their influence on the thermal conductivity of the resulting MMCs is rather complex and an ever-growing field of interest for diamond heat sink materials.The observed thermal properties of the MMCs produced in this study will be linked with the established diamond surface termination and will demonstrate the potential that lies within the method of diamond surface modification.</jats:p

Thermal Conductivity Behaviour of Al/Diamond and Ag/Diamond Composites in the Temperature Range 4 K &amp;lt; T &amp;lt; 293 K

Two different systems, the non-reactive Ag–diamond and the reactive Al–diamond system, were assessed by their thermal conductivity behaviour, both were fabricated by gas pressure assisted infiltration of densely packed diamond bulks with aluminium or silver and different Si-concentration and diamonds of varying particle sizes. The effect of Si-concentration on the interface thermal conductance h between Al, Ag and diamonds was investigated in dependence of temperature by measuring thermal conductivity of composites with different sized diamond particles in the temperature range from 4 K up to ambient. Composite thermal conductivities κc(T) can be as high as 860 W m-1 K-1 at roughly 100 K for Al/diamond and 1100 W m-1 K-1 for Ag–Si/diamond at approx. 150 K. Although the Si concentration in the matrix plays an eminent role for κc(T), i.e. the lower the Si concentration, the higher κc(T), interface thermal conductance is almost unaffected in the reactive Al-diamond system. Furthermore, they are close to values determined on clean model systems, i.e. sputtered and evaporated metal layers on diamond monocrystals. For Ag–diamond composites, the matrix composition of Ag–1Si seems to reflect an optimal composition, as the highest thermal conductivity κc(T) and an extraordinary higher interface conductance was achieved compared to Ag–3Si/diamond composites.</jats:p

Effect of Processing Conditions on Bonding Strength at Al(Si)/Diamond Interfaces

Understanding thermos-physical properties of MMCs includes considering interfacial processes and interactions between the constituents in MMCs. In this context, interfacial bonding is of vital interest for a deeper understanding of composites. Neutron diffraction experiments on Al/diamond composites were performed and reconciled with their thermo-physical properties and quantification of interfacial carbides formation. To create different interfacial conditions both, the contact time during processing the MMCs by liquid metal infiltration and the nominal composition of the matrix were changed, thus creating different amounts of interfacial Al4C3 carbides. Neutron diffraction showed the increase in contact time and the addition of Si to Al both increase the bonding strength, although going with a significant decrease of the composite`s thermal conductivity.</jats:p