New epoxy composites with enhanced thermal conductivity keeping electrical insulation
- Autores: Isaac Isarn Garcia
- Directores de la Tesis: Francesc Ferrando Piera (dir. tes.), Àngels Serra i Albet (codir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: John Hutchinson (presid.), Albert Fabregat Sanjuan (secret.), Marco Sangermano (voc.)
- Programa de doctorado: Programa de Doctorado en Nanociencia, Materiales e Ingeniería Química por la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
The tendency in electronics to produce smaller and lighter devices with higher power output causes the need to improve some properties that existent materials do not meet. Reducing the size of these devices while increasing their work speed results in an increase in the frequency of electrons circulating in a specific area. As it is known, the circulation of electrons emits heat by Joule effect, which must be removed to maintain the operating temperature. This is directly related to efficiency, useful lifetime and prevention of premature equipment failures.
In this doctoral thesis we delve into the field of high thermal conductive and electrically insulating epoxy thermosets. The next generation of packaging materials are expected to possess high thermal dissipation characteristics in addition to low thermal expansion coefficient (CTE). To remove the accumulated heat generated by high performance electronic devices is crucial for proper operation and would contribute to the improvement of their capabilities. Moreover, the enhancement of these properties may fulfill the demands in other spreading out industries as power, thermal energy storage, electrical, light emitting diodes (LEDs), sensors, aerospace, automotive or naval engineering among others.
Epoxy resins are a kind of thermosets extensively used to coat and encapsulate elements of electronic devices such as laptops, smartphones, digital cameras, televisions, and all types of modern appliances. Epoxies are used for their excellent properties such as exceptional adhesion ability to a huge range of different surfaces, the good protection capacity against moisture and physical and chemical aggressive environments, notable temperature resistance, electrical insulative character to avoid short circuits in electronic devices and their easy processability and relative low cost. However, as is common in polymers, epoxy-based thermosets lack good thermal conductivity (in the range of 0.2 W/m·K). The most economic and simple technique to face this issue is still today through the addition of high thermal conductive fillers.
Two different epoxy resins have been used as the starting material. One based on a diglycidyl derivative (DGEBA) and the other on a cycloaliphatic epoxy (ECC). During the research, we have developed the optimization of two different latent cationic epoxy systems, which leads to the homopolymerization of the epoxy resins used. The initiator selected in both cases was a commercial benzylanilinium salt. In the case of ECC monomer, it was required the addition of little amounts of triethanolamine (TEA) to confer the latency to the formulation. In order to facilitate the curing of the DGEBA resin, a little proportion of glycerol was added favoring the activated monomer (AM) mechanism. Alternatively, a tetrafunctional thiol hardener (PETMP) was used to cure ECC by a polycondensation mechanism using 4-(N,N-dimethylamino)pyridine (DMAP) as the base acting as the catalyst.
The fillers added into the matrix in this work can be divided in two groups: ceramic fillers and carbon-based materials. Among the ceramic particles used, stands out the hexagonal form of boron nitride (BN). This material gathers a set of properties that make it ideal to be used as a filler to meet the objectives. BN provides high thermal conductivity, low dielectric constant and high electrical resistivity, low CTE and density, high mechanical strength and chemical and thermal stability. Different sizes and shapes of this filler were tested. Other ceramics were also essayed, such as alumina (Al2O3), aluminium nitride (AlN) and silicon carbide (SiC). Among carbon-based materials, carbon nanotubes (CNTs) and expanded graphite (EG) were used. Carbon nanotubes are the most studied nanomaterials to date, motivated by the intrinsic high thermal conductivity as well as the mechanical behavior they present. Expanded graphite has received much attention in recent years in the field of composites for its properties, emphasizing its low cost and extremely low density. Due to the electrical conductivity of the carbonaceous materials, their proportion in the composite had to be limited and they were mixed with BN to maintain the electrical insulation of the composites, improving the thermal conductivity.
Differential scanning calorimetric (DSC) analyzes were performed to optimize the curing of the epoxy formulations and determine the influence of the filler addition during the reaction. The rheological tests allowed to determine the gel point of DGEBA formulations and the influence of a variable proportion of added BN. Also, this technique provides information of the viscoelastic properties of the formulations before curing. These studies allowed us to determine the percolation threshold concentration.
Once cured, the thermosets were characterized by many different techniques. The thermogravimetric analysis (TGA) determined the stability of the composites against temperature. The dynamic mechanical thermal analysis (DMTA) evaluated the dependence of the mechanical characteristics with temperature. Knoop microindentations were performed to evaluate the hardness of the materials. The composites were inspected by environmental scanning electron microscopy (ESEM) to observe the fracture surfaces and the final dispersion of the particles in the matrix. Thermo mechanical analysis (TMA) allowed to determine the thermal expansion coefficient of the materials obtained. Some materials were examined by X-ray diffraction to know the purity of the fillers and their crystalline structure.
DGEBA thermosets were evaluated both in their adhesion to metallic surfaces by tensile lap-shear strength of bonded assemblies’ and in their resistance to fracture by impact test. Moreover, their dielectric breakdown strength was determined. In addition, the electrical resistivities of the materials with BN and carbon particles were determined, to discern their possible use as electrical insulating materials. More importantly, the thermal conductivity of all the performed materials were determined by the transient hot bridge (THB) method.
In general, the fillers act as reinforcement inside the epoxy matrix and improve the mechanical behavior of the materials. The CTE is gradually reduced with the addition of fillers. Up to a certain concentration of particles, the toughness of epoxy resins is improved. Slight differences in thermal stability were found between the neat and composite materials, only differentiated by the less proportion of matrix that could be degraded.
Low proportions of carbon materials are enough to cause a decrease of several orders of magnitude in the electrical resistivity of the thermosets. Nevertheless, the combination with BN particles leads to the possibility to use EG and CNT at higher concentration since BN acts as a barrier for the transmission of the electrons. The materials with the best performances in the proposed objectives were those of homopolymerized ECC with the combined addition of 70 wt. % of BN platelets and 2.5 and 5 wt. % of EG. The values of thermal conductivity improved by more than 1600 % in reference to the neat epoxy and were 2.08 and 2.22 W/m·K, respectively. These materials also kept sufficient electrical insulation, in the range of 1010 and 106 Ω·m, respectively.
