Abstract

Recently, with the rapid advancement of wireless communication, the microwave absorbing materials are becoming increasingly important for applications in some special fields such as silent rooms, radar systems and military applications. Among the candidates for EM wave absorbers, magnetic nanocapsules, i.e. magnetic nanoparticles coated with an insulator shell including graphite, oxidation etc., are now becoming a study focus of researchers. The reason is that this type of EM wave absorbers not only possess a high saturation magnetization and Snoek's limit at high frequencies, but also decrease eddy current phenomenon induced by electromagnetic wave.

Thin film technologies allow the preparation of materials that are compositionally modulated down to the nanometer scale. When the grain size of crystalline materials approaches the ten nanometer range, the properties known from their large-scale counterparts often change substantially. Such changes affect electronic and optical properties as well as the mechanical behavior of nanoscaled thin films. Examples include hard materials with built-in functionalities such as coloration, designed frictional properties through a variety of mechanisms as well as hardness-enhancement in wear-resistant coatings. Nanocomposite coatings, which consist of nanocrystalline nitrides or carbides can provide combinations of advantageous properties such as enhanced hardness and low friction for the case of TiC/a-C:H, high hardness and optical transparency in Al-Si-N thin films or designed thermal conductivity with oxynitride coatings for wear protection on cutting tools. Various examples of nanostructured thin film systems will be presented and the underlying principles for the successful operation of such thin film materials will be discussed.

High thermal conductivity films with electrically insulating properties have a great potential for the effective heat transfer as substrate and thermal interface materials in high density and high power electronic packages. There have been lots of studies to achieve high thermal conductivity composites using high thermal conductivity fillers such alumina, aluminum nitride, boron nitride, CNT and grapheme, recently. Among them, boron nitride(BN) and aluminum nitride(AlN) ceramics are promising candidate for high thermal conductivity with electrically insulating filler materials. This work presents an enhance heat transfer properties of ceramic/polymer and ceramic/ceramic nanocomposite films using BN/AlN nanosheets(nanoflake) and polymer resins. BN nanosheet was prepared by a chemical expoliation using organic media and subsequent ultrasonic treatment. High thermal conductivity over 5W/mK in transverse and 10W/mK in in-plane direction of cast films were achieved for BN nanosheet/polymer nanocomposites that were prepared under 250°C. Also, thermal conductivity over 5W/mK were achieved for BN/AlN nanosheets/glass ceramic composites which can be sintered under 900°C. Further improvement of thermal conductivity up to 15W/mK was achieved by applying high thermal conductivity polymers and adequate alignment of nano size ceramic sheets and high density packing through multimodal powders and two stage film forming process including first cast, resin infiltration and pressured-roll cast at elevated temperatures.

In recent years has increased the interest for developing multifunctional materials which can be used for diagnosis and therapies for major diseases such as cancer. The purpose of this work is the synthesis and characterization of luminescent/magnetic nanoparticles (LMNPs) made of magnetite (Fe₃O₄) and their simultaneous combination with europium doped lanthanum oxide (La₂O₃:Eu), terbium doped lanthanum oxide (La₂O₃:Tb) or thulium doped lanthanum oxide (La₂O₃:Tm) for possible applications as contrast and thermal ablation agents in cancer treatment.

The method used to obtain the LMNPs is spray pyrolysis due it allows the synthesis of nanoparticles with narrow size distribution, adjustable size, high crystallinity and good stoichiometry. Luminescent nanoparticles were prepared with different concentration of dopant material and were post annealed at different temperatures in order to find the best luminescent response. Once obtained the luminescent materials with the highest luminescence, the synthesis of magnetoluminescent materials was followed. This second step was done for different concentration of luminescent and magnetic nanomaterials in order to obtain the best multifunctional properties material. The determination of the best material was made by characterizing the nanoparticles with different techniques such as spectrophotometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and measurement of magnetic properties using a vibrating-sample magnetometer (VSM) and a magnetic induction device.

Partial support from CONACyT (Grant 100555) and DGAPA-UNAM (Grant IN-109913) and technical work provided by E. Aparicio, F. Ruíz and I. Gradilla is gratefully acknowledged.

Hydrophobicity, water repellency, is a desirable surface property for materials in many industries. Water repellent surfaces are used to prevent deleterious effects of water such as; corrosion and degradation, on various materials. There are several methods which can be used to modify the properties of surfaces. Some of these techniques include sol-gel method, plasma, chemical vapor deposition, atomic layer deposition, and also traditional wet chemistry methods. Some novel materials for surface modification are Polyhedral oligomeric silsesquioxanes (POSS) silanols. These materials possess some of the surface modification characteristics of other silicon-based materials.In this project, Polyhedral Oligomeric Silsesquioxanes (POSS) were used to treat hydrophilic materials such as cotton and glass. Two POSS silanols, TriSilanollsooctyl POSS and TrisilanolPhenyl POSS, were deposited by immersion methods on glass and polymeric surfaces. Several concentrations and curing temperatures were evaluated. Contact angle measurements of water and other liquids were used to calculate the surface properties and wettability. Dynamic capture mode was used to calculate the hysteresis of advancing and receding contact angles. The hysteresis provided information regarding the wettability properties of the obtained surfaces. Statistical Analysis was performed on the contact angle data collected from the glass, cotton, and PVA samples. The analysis of the PVA films coated with TrisilanolIsooctyl POSS demonstrated an effect of the curing temperature in the water repellency. Whereas the PVA coating with the TrisilanolPhenyl POSS had a lower variability in the contact angle as the curing temperature was increased. The cotton samples coated with POSS where compared with contact angles measured at 10 seconds and 60 seconds after placing the droplet. The results for 0.1% TrisilanolPhenyl POSS cotton samples proved that only water repellency is achieved at higher curing temperatures. The results of 4% TrisilanolPhenyl POSS cotton samples showed a higher repellency even at lower curing temperatures at 10 seconds. The results of the contact angles at 60 seconds demonstrated that as the curing temperature is increased, the coating prevents the absorption of water. The results demonstrated that POSS can impart hydrophobic properties to these materials even at low concentrations. The unique properties of POSS silanols for surface modification are also discussed.

Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films are aggregates consisting of ultrananocrystalline diamond (UNCD) grains of less than 10 nm in diameter and an a-C:H matrix [1]. In our previous study, we have realized the film growth without the pretreatment of substrates using diamond powder by pulsed laser deposition (PLD) and coaxial arc plasma deposition (CAPD), and experimentally proved the formation of p-type and n-type conductions accompanied by enhanced electrical conductivities by the doping of boron and nitrogen, respectively [2,3].

In this work, heterojunctions diodes comprising B-doped UNCD/a-C:H films and n-type Si substrates were fabricated by PLD and CAPD and they were evaluated as photodetectors. The B-doped UNCD/a-C:H showed obvious photoconduction in the UV and visible ranges. Heterojunction diodes exhibited a typical rectifying action. Photocurrent was evidently observed, and the external quantum efficiency was estimated to be more than 70% for UV monochromatic light under negative biases.

[1] T. Yoshitake et al., Jpn. J. Appl. Phys. 48, 020222 (2009).

[2] S. Ohmagari et al., Jpn. J. Appl. Phys. 50, 035101 (2011).

[3] S. Al-Riyami et al., Appl. Phys. Express, 3, 115102 (2010).