Research Article www.acsami.org
Heat Dissipation Interfaces Based on Vertically Aligned Diamond/ Graphite Nanoplatelets N. F. Santos,*,† T. Holz,† T. Santos,‡ A. J. S. Fernandes,† T. L. Vasconcelos,§ C. P. Gouvea,§ B. S. Archanjo,§ C. A. Achete,§ R. F. Silva,∥ and F. M. Costa† †
i3N and Physics Department of University of Aveiro, 3810-193 Aveiro, Portugal CICECO and Physics Department of University of Aveiro, 3810-193 Aveiro, Portugal § Materials Metrology Division, INMETRO, 25250-020, Duque de Caxias - RJ, Brazil ∥ CICECO and Dept. of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal ‡
ABSTRACT: Crystalline carbon-based materials are intrinsically chemically inert and good heat conductors, allowing their applications in a great variety of devices. A technological step forward in heat dissipators production can be given by tailoring the carbon phase microstructure, tuning the CVD synthesis conditions. In this work, a rapid bottom-up synthesis of vertically aligned hybrid material comprising diamond thin platelets covered by a crystalline graphite layer was developed. A single run was designed in order to produce a high aspect ratio nanostructured carbon material favoring the thermal dissipation under convection-governed conditions. The produced material was characterized by multiwavelength Raman spectroscopy and electron microscopy (scanning and transmission), and the macroscopic heat flux was evaluated. The results obtained confirm the enhancement of heat dissipation rate in the developed hybrid structures, when compared to smooth nanocrystalline diamond films. KEYWORDS: diamond, graphite, nanoplatelets, CVD, surface area, convective cooling
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INTRODUCTION Nanocrystalline diamond (NCD) combines a unique set of propertieshigh hardness, high thermal conductivity, optical transparency, negative electron affinity, biocompatibility, and chemical inertness−that makes it an interesting solution for a wide plethora of applications such as in wear-resistant components, heat dissipators, infrared lenses, electron field emitters, and biosensors.1,2 High-quality NCD coatings can be synthesized by the chemical vapor deposition (CVD) technique based on a continuous renucleation mechanism of nanometric equiaxial crystals, resulting in very smooth surfaces.3,4 However, changing the deposition conditions in order to obtain peculiar configurations can modify this type of morphology, envisaging specific applications. This was the purpose of a few authors that have been focused on the development of NCD coatings with vertically aligned 2D nanodiamond crystals named as nanoplatelets or nanosheets.5−14 The CVD growth of vertically aligned 2D nanodiamond has been promoted by different approaches. In a pioneer work in 2004, Chen et al. prepared a microcrystalline diamond substrate by hot-filament CVD on which a 100 nm catalyst nickel film was deposited.6 The diamond nanoplatelets (DNPs) synthesis was then carried out in a microwave plasma CVD reactor. Such nanoplatelets, with a uniform thickness of a few tenths of nanometers and a flat well-faceted morphology, were covered © XXXX American Chemical Society
with an ultrathin graphite layer (1−2 nm). Later, the same group varied the type of catalyst film (Fe, Au−Ge, ...), the microwave power, and the relative positioning of the substrates to the microwave plasma ball.5,10 A mechanism was first proposed for this unique surface morphology: graphitization of the NCD crystals on their surface by hydrogen plasma etching at high temperature favoring the lateral growth of diamond platelets.6 Later, this model was improved based on the observation that the platelets present {111} facets.5 Under the high temperature and intensive plasma environment, the adsorbed hydrocarbon radicals by these facets are easily abstracted by atomic hydrogen, hampering the growth rate normally to the {111} facets and perpetuating the lateral growth. The group of Raina and coauthors proposed another approach to grow vertically aligned 2D nanodiamond, which they called nanodiamond film with a “ridge” surface.13,14 Microwave plasma CVD (MPCVD) was again used to grow the NCD film, this time without a catalyst film. The interlaced “ridges”, with nanocrystalline grains on their side walls, result in a surface morphology with a high degree of surface roughness. Received: August 17, 2015 Accepted: October 23, 2015
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DOI: 10.1021/acsami.5b07633 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
down was conducted, lowering both microwave power and total pressure until a shut-off was possible without compromising the samples by thermal shock. This final step is quite important and plays a dual role, permitting to avoid premature delamination due to thermal stresses at the interface, and promoting a plasma cleaning operation due to its harsh chemical environment, namely, by the action of atomic hydrogen. The standard morphological characterization by SEM was carried out in a Hitachi SU70 field emission instrument, while the transmission microscopy data including the electron diffraction analysis were performed using a probe-corrected Titan 80−300 kV (FEI Company) working at 80 kV. The cross-sectional sample for TEM analysis was prepared using a dual beam microscope (Helios Nanolab 650; FEI Company). A micro-Raman analysis was conducted in the backscattering configuration on a Jobin Yvon HR800 instrument (Horiba), using a 1800 l/mm grating for the visible lines (632.8 nm red, 532 nm green, and 442 nm blue) and a 2400 l/mm one for the near-UV line (325 nm). In all cases, the spot size was smaller than 2 μm, while the Rayleigh rejection was made by edge filters allowing Raman acquisition at least from 200 cm−1, the backscattered Raman radiation being detected by a Peltier cooled (223 K) CCD sensor. The experimental apparatus used to appraise the heat dissipation ability of the samples is sketched in Figure 1. In this setup, two
Vertically aligned 2D nanodiamond has two additional advantages in respect to planar NCD: a much higher specific surface area and the conspicuous edge shape. The former has been proposed for electrochemical applications such as electrochemical bioanalytical surfaces8 or enzyme-free amperometric biosensors for glucose,9 while the latter has been characterized for field emission purposes once the ridge edge geometry results in low turn-on electrical fields.7 A potentially innovative application for this nanomaterial is in thermal dissipation interfaces. Diamond is intrinsically an excellent heat conductor due to its high Debye temperature. When heat is transferred to diamond, it is readily diffused out mainly due the emission of phonons, the electronic component playing a little role that can be neglected.15 In addition, diamond is stable up to 700 and 1700 °C in oxidizing and inert atmospheres, respectively,15 keeping a low thermal expansion coefficient.16 All together, these properties explain the widespread usage of diamond in the semiconductor industry not only to prevent from circuitry overheating but also during the manufacturing process itself.17 Nevertheless, the relatively smooth surfaces of NCD films obtained by the CVD process are not the most adequate to efficiently remove heat under natural or forced convection. Patterning of diamond surfaces into suitable dissipating geometries using standard lithography processes is possible but lack in time and cost effectiveness. Therefore, a bottom-up approach to rapidly produce efficient and cheap heat dissipating surfaces with appropriate micromorphology is highly desirable. Considering all the above, in the present work, MPCVD grown films of nearly vertically aligned diamond thin platelets, encapsulated by a graphitic layer, were produced by a simple and fast single-run process in only 20 min, envisaging thermal dissipation purposes. The dissipation performance of the nanoplatelet-based films was evaluated under natural convection conditions, using smooth NCD films as reference for comparison purposes.
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Figure 1. Sketch of the experimental apparatus used for heat dissipation measurements. samples (Si/NCD as reference and Si/NCD/DNPs as test sample) are heated simultaneously on the same Peltier cell (RS, 4 × 4 cm2) at room conditions. The thermal coupling between the cell and samples was accomplished using the same amount of a heat sink material. A plastic cuvette with open ends enclosing the samples ensured that the air columns above each sample did not mix. Two similar thermocouples A and B, placed at a 4 cm height from the samples’ surface, were used to measure the air columns average temperature, and a third thermocouple C was used to measure the temperature at the Peltier cell surface. The thermocouples used were all commercial grade K-type (RS) with a 0.2 mm diameter tip (ungrounded and exposed). The data from thermocouples A and B were retrieved and logged by the same instrument (RS 1316 Dual Datalogger thermometer). The Peltier cell was then placed inside a vertically positioned acrylic cylinder with opened tops to suppress circulating air currents striking the apparatus. After each increment of the electrical current in the Peltier by steps of 0.2 A, the system was allowed to stabilize for 30 min, before 7 measurements spaced by 10 s took place. This allows one to build statistics and to account for the effects of convection at the thermocouple tip, especially at a higher temperature range (>90 °C), where measurement fluctuations of a few tenths of degrees Celsius were observed.
EXPERIMENTAL SECTION
Silicon substrates were prepared from a ⟨100⟩ oriented p-type single crystal wafer, which was diced into squares of 1 × 1 cm2. Afterward, a pretreatment step was performed to enhance diamond nucleation. For that purpose, a mechanical abrasion was conducted in a soft cloth impregnated with diamond powder (
