Emile Haddad

Effects of Ti–W codoping on the optical and electrical switching of vanadium dioxide thin films grown by a reactive pulsed laser deposition

Thin films of thermochromic VO2, V1−xWxO2 and V1−x−yWxTiyO2 (x=0.014, and y=0.12) were synthesized onto quartz substrates using a reactive pulsed laser deposition technique. The films were then characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The W and Ti dopant effects on the semiconductor-to-metal phase transition of VO2 were investigated by measuring the temperature dependence of their electrical resistivity and their infrared transmittance. Remarkably strong effects of Ti–W codoping were observed on both the optical and electrical properties of V1−x−yWxTiyO2 films. The IR transmittance was improved, while the transition temperature could be varied from 36°C for W-doped VO2 film to 60°C for Ti–W codoped VO2 film. In addition, at room temperature, a higher temperature coefficient of resistance of 5.12%∕°C is achieved. Finally, both optical and electrical hysteresis are completely suppressed by Ti–W codoping the VO2 films.

Optical switching of vanadium dioxide thin films deposited by reactive pulsed laser deposition

The parameters of reactive pulsed laser deposition were successfully optimized for fabrication of vanadium dioxide thin films. It is observed that the O2 concentration in Ar gas and the total deposition pressure are critical in stabilizing the single VO2 phase. Thermochromic VO2 and V1−xWxO2 (x=0.014) thin films were synthesized on various substrates (silicon, quartz, and sapphire) at 5% of O2/Ar ratio gas and total pressure of 90 mTorr. The structural properties of the deposited films were analyzed by x-ray diffraction, while their semiconductor-to-metal phase transitions were studied by electrical resistivity using the four-point technique and infrared transmittance from room temperature up to 100 °C. The observed transition temperature was about 36 °C for W-doped VO2 compared to 68 °C for VO2 films. This transition temperature was then lowered by about 22.85 °C per 1 at. % of W added. The temperature coefficient of resistance was about 1.78%/°C for VO2 and about 1.90%/°C for W-doped VO2. Using the pump...

1 × 2 optical switch devices based on semiconductor-to-metallic phase transition characteristics of VO2 smart coatings

We have successfully fabricated two types of 1 × 2 optical switch devices, namely, all-optical switch (VO2/quartz) and electro-optical switch (VO2/TiO2/ITO/glass) based on the semiconductor-to-metallic phase transition characteristic of vanadium dioxide (VO2) smart coatings. The VO2 active layer, the TiO2 buffer layer and the ITO transparent conductive electrode used in these devices were achieved by reactive pulsed laser deposition. The optical switching of the fabricated devices was investigated at λ = 1.55 µm. The semiconductor (on) to metallic (off) phase transition was controlled by photo-excitation of VO2 in the case of the all-optical switch and by an external electric field applied between the ITO and the VO2 layer in the case of the electro-optical switch. The extinction ratio (on/off) is found to be much higher for the all-optical switch than for the electro-optical switch. For the all-optical switch, extinction ratios of about 22 and 12 dB are obtained in the transmission and reflection modes, respectively. In the case of the electro-optical switch, the extinction ratio is about 12 dB in the transmission mode and 5 dB in the reflection mode. Finally, to explain our optical switching results, we propose a simple model based on the energy band diagram of VO2 in which the charge density increases under an external excitation (either photo-excitation or an electrical field), and then induces the semiconductor-to-metallic phase transition in the VO2 active layer.

Self-Healing Materials Systems: Overview of Major Approaches and Recent Developed Technologies

The development of self-healing materials is now being considered for real engineering applications. Over the past few decades, there has been a huge interest in materials that can self-heal, as this property can increase materials lifetime, reduce replacement costs, and improve product safety. Self-healing systems can be made from a variety of polymers and metallic materials. This paper reviews the main technologies currently being developed, particularly on the thermosetting composite polymeric systems. An overview of various self-healing concepts over the past decade is then presented. Finally, a perspective on future self-healing approaches using this biomimetic technique is offered. The intention is to stimulate debate and reinforce the importance of a multidisciplinary approach in this exciting field.

Highly tunable-emittance radiator based on semiconductor-metal transition of VO2 thin films

This paper describes a VO2-based smart structure with an emittance that increases with the temperature. A large tunability of the spectral emittance, which can be as high as 0.90, was achieved. The transition of the total emittance with the temperature was fully reversible according to a hysteresis cycle, with a transition temperature of 66.5 °C. The total emittance of the device was found to be 0.22 and 0.71 at 25 °C and 100 °C, respectively. This emittance performance and the structure simplicity are promising for the next generation of energy-efficient cost-effective passive thermal control systems of spacecrafts.

Micro-optical switch device based on semiconductor-to-metallic phase transition characteristics of W-doped VO2 smart coatings

The authors have successfully fabricated planar micro-optical switch device exploiting the transmitting semiconductor (on) to the reflecting metallic (off) phase transition of thermochromic W(1.4at.%)-doped VO2 smart coatings and driven by an external voltage. The starting W-doped VO2-coated Al2O3 exhibited infrared transmittance switching about 45%. After the microfabrication, the temperature dependence of electrical resistance of the micro-optical switch showed clearly its well-known semiconductor-to-metallic phase transition at a transition temperature of 36°C. A reversible transmittance switching (on/off) as high as 28dB was achieved with this device at λ=1.55μm. In addition, the transmittance switching modulation of the device was demonstrated at 1.55μm by switching the micro-optical switch simultaneously with dc and ac voltages.

Fluidic patch antenna based on liquid metal alloy/single-wall carbon-nanotubes operating at the S-band frequency

This letter describes the fabrication and characterization of a fluidic patch antenna operating at the S-band frequency (4 GHz). The antenna prototype is composed of a nanocomposite material made by a liquid metal alloy (eutectic gallium indium) blended with single-wall carbon-nanotube (SWNTs). The nanocomposite is then enclosed in a polymeric substrate by employing the UV-assisted direct-writing technology. The fluidic antennas specimens feature excellent performances, in perfect agreement with simulations, showing an increase in the electrical conductivity and reflection coefficient with respect to the SWNTs concentration. The effect of the SWNTs on the long-term stability of antennas mechanical properties is also demonstrated.

Fabrication of stationary micro-optical shutter based on semiconductor-to-metallic phase transition of W-doped VO2 active layer driven by an external voltage

The authors have successfully fabricated stationary micro-optical shutter arrays based on the well-known transmitting semiconductor (on) to the reflecting metallic (off) phase transition of thermochromic W-doped VO2 active layers operating at room temperature and driven by an external voltage. This shutter consists of 16 active planar micro-optical slits for which the optical switching (either transmittance or reflectance) can be controlled individually. This allows performing any desirable on-off switching combinations. The current-voltage characteristic of the micro-slit shows that the current jumps when the phase transition occurs. Transmittance switching as high as 25 dB and reflectance switching of about 6 dB were achieved with this device at λ=1.55 μm. Therefore, this electrically controllable VO2-array can be used as a stationary Hadamard shutter to increase the sensitivity of infrared spectrometers.

Micromechanical characterization of single-walled carbon nanotube reinforced ethylidene norbornene nanocomposites for self-healing applications

We report on the fabrication of self-healing nanocomposite materials, consisting of single-walled carbon nanotube (SWCNT) reinforced 5-ethylidene-2-norbornene (5E2N) healing agent?reacted with ruthenium Grubbs catalyst?by means of ultrasonication, followed by a three-roll mixing mill process. The kinetics of the 5E2N ring opening metathesis polymerization (ROMP) was studied as a function of the reaction temperature and the SWCNT loads. Our results demonstrated that the ROMP reaction was still effective in a large temperature domain (???15?45??C), occurring at very short time scales (less than 1?min at 40??C). On the other hand, the micro-indentation analysis performed on the SWCNT/5E2N nanocomposite material after its ROMP polymerization showed a clear increase in both the hardness and the Young modulus?up to nine times higher than that of the virgin polymer?when SWCNT loads range only from 0.1 to 2?wt%. The approach demonstrated here opens new prospects for using carbon nanotube and healing agent nanocomposite materials for self-repair functionality, especially in a space environment.

Advanced MEMS and Integrated-Optic Components for Multifunctional Integrated Optical Micromachines

Optical technologies can play a strategic role in improving the performance, functionality, and reducing the mass of various spacecraft technologies, such as true time-delay T/R modules for phased-array antennas and optical sensor systems for satellite navigation and systems status. However, current photonic and fiber-optic systems tend to be bulky relative to the requirements for space applications. Micro integrated-optic circuits increase the integration of optical components on a single substrate, to provide multi-function optical processing and switching similar to electronic integrated circuits. This minimizes the number of external optical interconnections required and sensitivity to external vibrations; maximizing the system information capacity, optical throughput, and reliability, while minimizing the overall system size and weight. This paper considers a systematic development of MEMS integrated-optic circuits on SOI for various space application. A unique blend of MEMS, smart-material and photonic technologies is employed to miniaturize the size of the basic components, while improving on the attainable performance.