Diana-Andra Borca-Tasciuc

Thermal properties of electrodeposited bismuth telluride nanowires embedded in amorphous alumina

Bismuth telluride nanowires are of interest for thermoelectric applications because of the predicted enhancement in the thermoelectric figure-of-merit in nanowire structures. In this letter, we carried out temperature-dependent thermal diffusivity characterization of a 40nm diameter Bi2Te3 nanowires∕alumina nanocomposite. Measured thermal diffusivity of the composite decreases from 9.2×10−7m2s−1 at 150Kto6.9×10−7m2s−1 at 300K and is lower than thermal diffusivity of unfilled alumina templates. Effective medium calculations indicate that the thermal conductivity along nanowires axis is at least an order of magnitude lower than thermal conductivity of the bulk bismuth telluride.

Altering Iron Oxide Nanoparticle Surface Properties Induce Cortical Neuron Cytotoxicity

Superparamagnetic iron oxide nanoparticles, with diameters in the range of a few tens of nanometers, display the ability to cross the blood-brain barrier and are envisioned as diagnostic and therapeutic tools in neuro-medicine. However, despite the numerous applications being explored, insufficient information is available on their potential toxic effect on neurons. While iron oxide has been shown to pose a decreased risk of toxicity, surface functionalization, often employed for targeted delivery, can significantly alter the biological response. This aspect is addressed in the present study, which investigates the response of primary cortical neurons to iron oxide nanoparticles with coatings frequently used in biomedical applications: aminosilane, dextran, and polydimethylamine. Prior to administering the particles to neuronal cultures, each particle type was thoroughly characterized to assess the (1) size of individual nanoparticles, (2) concentration of the particles in solution, and (3) agglomeration size and morphology. Culture results show that polydimethylamine functionalized nanoparticles induce cell death at all concentrations tested by swift and complete removal of the plasma membrane. Aminosilane coated particles affected metabolic activity only at higher concentrations while leaving the membrane intact, and dextran-coated nanoparticles partially altered viability at higher concentrations. These findings suggest that nanoparticle characterization and primary cell-based cytotoxicity evaluation should be completed prior to applying nanomaterials to the nervous system.

Anisotropic thermal properties of nanochanneled alumina templates

Anodic porous alumina templates contain highly ordered nanochannels that are frequently used for fabrication of thermoelectric nanowires. Thermal properties of the templates are of high interest in thermoelectric device applications. This paper reports the temperature-dependent, anisotropic thermal conductivity and thermal diffusivity of 55 μm thick anodic template Anodisc from Whatman, Inc. of 0.02 μm pore diameter and ∼30% porosity. The 3ω technique was employed to carry out measurements of the effective thermal conductivity and thermal diffusivity in the direction parallel to the nanochannels axis, while thermal anisotropy of the sample was estimated theoretically using the Maxwell model. Measured effective thermal conductivity is between 0.54 W/mK at 80 K and 1.3 W/mK at 300 K. Measured thermal diffusivity decreases from 2.1×10−6m2s−1 at 80 K to 1.08×10−6m2s−1 at 300 K. A photothermoelectric technique was employed to measure independently the components of the thermal diffusivity along the direction p...

Anisotropic thermal diffusivity of aligned multiwall carbon nanotube arrays

A photothermoelectric technique was employed to determine the anisotropic thermal diffusivity of thick arrays of multiwalled carbon nanotubes grown by chemical-vapor deposition. The thermal diffusivity along the alignment direction was also determined using a self-heating 3ω method. The agreement between the measured thermal diffusivities with the two techniques is between 2% and 13% in the tested temperature range. The thermal diffusivity along the alignment direction decreases slightly with temperature in the 80–300‐K temperature range and is ∼ two orders of magnitude smaller than the thermal diffusivity along the planes of graphite. The thermal diffusivity across the alignment direction is ∼25 times smaller than along the alignment direction and is between 50% and five times smaller than the thermal diffusivity across the planes of graphite in the measured temperature range.

Local temperature measurement in the vicinity of electromagnetically heated magnetite and gold nanoparticles

This paper describes a new technique employing fluorescent quantum dots as temperature probes for measuring the temperature rise in the proximity of nanoparticles heated by a radio frequency (rf) electromagnetic field. The remote heating of nanoparticles by an rf field is a promising approach to control biological transformations at the molecular level. In principle, the heat dissipated by each nanoparticle might produce a temperature increase in its proximity, facilitating a change in the molecules directly attached to it but not in the others. Although this method has been demonstrated to provide control over biological transformations, the proposed mechanism involves producing and maintaining large temperature differences across small distances, in the range of several degrees Celsius across tens of nanometers. Existing theories for heat generation and transfer in rf heated nanoparticle systems cannot account for these gradients. To better understand the limitations of local heating, the temperature in...

Fabrication of polydimethylsiloxane composites with nickel nanoparticle and nanowire fillers and study of their mechanical and magnetic properties

This work presents the fabrication and characterization of mechanical and magnetic properties of polydimethylsiloxane (PDMS) nanocomposites with nickel nanoparticles and nanowires as fillers. To enhance filler dispersion and polymer-filler interface bonding, allyltrimethoxysilane was used for nanofiller coating. Sample preparation was carried out by speed mixing and curing at 100 °C. Nanowire-PDMS composites were exposed to magnetic field prior to full curing in order to facilitate nanowire alignment. Composites with concentrations of 5, 10, and 15 vol % of nanoparticles and 5 vol % of nanowires were prepared and tested. An increase in elastic modulus of ∼30% was observed for composites with 5 vol % nanoparticle concentration. A much higher increase in elastic modulus, of ∼80%, was observed for nanowire-based composites of same concentration. The measured elastic modulus agrees well with predictions that assume strong interface bonding between the polymer and the filler. Magnetic anisotropy and higher rem...

Potential Sources of Errors in Measuring and Evaluating the Specific Loss Power of Magnetic Nanoparticles in an Alternating Magnetic Field

Heat-generating magnetic nanoparticles suspensions are being explored in research and clinical settings as hyperthermia treatment for cancer or as adjuvant in established cancer therapies. In these applications it is essential to use low nanoparticle dosage to prevent any potential side effects including those associated with their accumulation in liver or spleen. Hence, developing particles with superior heating properties continues to remain an active area of research. Specific loss power (SLP), also referred to as specific absorption rate (SAR), represents the power dissipation per unit mass of magnetic nanoparticles in alternating magnetic fields. Accurate measurement of SLP is the key for understanding the parameters that control the heat generation rate, which is required to optimize these systems. However, at presents there are no standards for performing SLP measurements and no accepted calibration materials, making it difficult to compare the performance of various systems reported in literature. Previous work from this group discussed the effect of sample volume and geometry on the SLP data accuracy. In this study, additional analysis and experiments are carried out to investigate the effect of the power dissipation rate, the magnetic properties and the method for temperature slope calculation on the accuracy of the reported power density. Results indicate that when the same heating time is used, the volume at which heat losses become negligible decreases with decreasing sample heating rate. Furthermore, it is shown that for calculating initial temperature slope, a larger error occurs with a longer heating time and higher power level regardless of the curve fitting methods, hence, when power density or heating time increases, a higher order curve fitting (e.g., 2nd polynomial and exponential) is more desirable. In addition, when the magnetization of a nanoparticle suspension is low, the SLP is independent of the sample geometry.

Analysis and Optimization of Asynchronously Controlled Electrostatic Energy Harvesters

Mechanical to electrical energy conversion employing variable capacitors is assisted by electronic circuits that can have synchronous or asynchronous architectures. The later does not require synchronization of electrical events with mechanical motion, which eliminates difficulties in gate clocking and the power consumption associated with intelligent control circuitry. However, implementation of asynchronous energy harvesting circuits with the mechanical-to-electrical converter can be detrimental to the performance of the converter when done without concurrent optimization of the mechanical device and the circuit, an aspect mainly overlooked in the literature. This paper carries out system level analysis of electrostatic micro-generators with asynchronous control and charge fly-back mechanism to optimize the useful energy generated by the harvester. Our theoretical and experimental investigations show that there is an optimum value for either the storage capacitor or cycle number for maximum scavenging of ambient energy via asynchronous electrostatic transduction. The analysis also indicates that the maximum power is extracted from the system when approaching synchronization of mechanical and electrical events. However, there is a region of interest where the storage capacitor can be optimized to produce almost 70% of the ideal power, taken as the power harvested with synchronous converters when neglecting the power consumption associated with synchronizing control circuitry. Theoretical predictions are confirmed by measurements on an asynchronous energy harvesting circuit implemented with a macro-scale electrostatic converter prototype.

Thermal and electrical transport along MWCNT arrays grown on Inconel substrates

This work reports on thermal and electrical conductivities and interface resistances for transport along aligned multiwalled carbon nanotubes (CNT) films grown on a nickel superalloy (Inconel) substrate. The measured specific thermal resistance of the combined Inconel–CNT and indium–CNT interfaces is of the same order as reported for CNT and silicon or SiO 2 interfaces but much higher than theoretical predictions considering perfect contact between the tubes and substrate. Imperfect mechanical contact with the substrate and a large contribution caused by indium–CNT interface are thought to be mainly responsible for the high interface resistances and the low effective values of thermal and electrical conductivities. However, reported results represent an incentive for further research on CNT synthesis on metallic substrates for thermal management applications and pave the way for much easier integration of carbon nanotubes in electronic applications.

Low-Power ASIC for Microwatt Electrostatic Energy Harvesters

This paper presents a technique for enhancing the power output from vibration-based charge-constrained electrostatic energy harvesters (EEHs). Starting from a diode-based charge pump and a buck converter stage with an inductive energy flyback scheme, a novel controller integrated circuit is proposed to monitor and control the charging/discharging events of the electrostatic converter. The fundamental challenges of designing self-powered event-synchronization circuits for charge-constrained EEHs are addressed. Synchronous energy conversion is accomplished with minimal power dissipation without the complexity of precise timing circuitry. Experimental results are presented for the design, implemented in AMI 0.7-μm high-voltage CMOS process, using a macroscale electrostatic converter prototype. The energy-harvesting circuit produces a measured 1.688 μW of power for a power investment of 417 nW. The measured power consumption associated with the on-chip controller unit is 24nW, approximately 1.42% of the total power harvested.