A superhydrophobic (SH) surface has many characteristics - of which are its self-cleaning and anti-corrosion
functionalities - that are desirable across various industries. A superhydrophobic surface utilizes the right combination
of surface chemistry and roughness that force water droplets to form high water contact angles (CA). This in turn allows
droplets to easily roll off and pick up dirt and debris across the surface while also preventing water from penetrating the
surface. We have developed a simple yet durable spray-on coating based on functionalized SiO<sub>2</sub> nanoparticles that can
easily be applied to surfaces including, but not limited to, optical sensors, photovoltaics, sights and lenses, textiles,
construction materials, and electronic devices. In addition, these coatings exhibit practical mechanical and environmental
durability that allow prolonged use of the coatings in harsh environments.
Most concentrating solar power (CSP) facilities in the USA are located in the desert southwest where open land and sunshine are abundant, but airborne dust is prevalent. The accumulation of dust, sand and other natural pollutants on collector mirrors and heliostats presents a significant operational problem and M&O cost for the CSP facilities in this region. The optical performance of the CSP collectors is key to achieving low electricity costs, where a 1% decrease in reflectance directly leads to a 1% increase in the levelized cost of electricity (LCOE) generated by these facilities. In this paper we describe the development of low cost, easy to apply anti-soiling coatings based on superhydrophobic (SH) functionalized nano silica materials and polymer binders that possess the key requirements necessary to inhibit particulate deposition on, and adhesion to, CSP mirror surfaces, and thereby significantly reducing mirror cleaning costs and facility downtime. The key requirements for these coatings are excellent optical clarity with minimal diffuse reflectance, and coating mechanical and exposure durability in harsh desert environments while maintaining SH and dirt shedding properties. The coatings developed to date have excellent SH properties with water contact angles >165° and rolling angles <5°. The solar weighted optical reflectance of the anti-soiling coating over the wavelength range 250 nm to 3μm is >99% that of uncoated mirror surfaces with coating diffuse reflectance being <1% over this wavelength range. Ongoing mechanical and accelerated solar UVA exposures also indicate these coatings will meet the required durability goals.
A superhydrophobic (SH) surface has many characteristics, one of which is its self-cleaning, anti-soiling functionality, that are desirable across various industries. A transparent, self-cleaning surface utilizes the right combination of surface chemistry and roughness that force water droplets to form high water contact angles (CA). This in turn allows droplets to easily roll off and pick up dirt and debris across the surface. In theory this is simple but in practice this can be very difficult as superhydrophobicity and optical transparency are competitive. We have developed a simple, spray-on coating based on functionalized SiO<sub>2</sub> nanoparticles that can easily be applied to surfaces whose application requires high transparency including, but not limited to, optical sensors, photovoltaics, sights, and lenses. In addition, these coatings exhibit practical mechanical and environmental durability that allow prolonged use of the coatings in harsh environments.
Proc. SPIE. 8377, Energy Harvesting and Storage: Materials, Devices, and Applications III
KEYWORDS: Energy efficiency, Capacitors, Dielectrics, Energy harvesting, Thermoelectric materials, Energy conversion efficiency, Thermal modeling, Temperature metrology, Pyroelectric materials, Instrument modeling
Harvesting electrical energy from thermal energy sources using pyroelectric conversion techniques
has been under investigation for over 50 years, but it has not received the attention that thermoelectric energy
harvesting techniques have during this time period. This lack of interest stems from early studies which
found that the energy conversion efficiencies achievable using pyroelectric materials were several times less
than those potentially achievable with thermoelectrics. More recent modeling and experimental studies have
shown that pyroelectric techniques can be cost competitive with thermoelectrics and, using new temperature
cycling techniques, has the potential to be several times as efficient as thermoelectrics under comparable
operating conditions. This paper will review the recent history in this field and describe the techniques that
are being developed to increase the opportunities for pyroelectric energy harvesting.
The development of a new thermal energy harvester concept, based on temperature cycled
pyroelectric thermal-to-electrical energy conversion, are also outlined. The approach uses a resonantly
driven, pyroelectric capacitive bimorph cantilever structure that can be used to rapidly cycle the temperature
in the energy harvester. The device has been modeled using a finite element multi-physics based method,
where the effect of the structure material properties and system parameters on the frequency and magnitude of
temperature cycling, and the efficiency of energy recycling using the proposed structure, have been modeled.
Results show that thermal contact conductance and heat source temperature differences play key roles in
dominating the cantilever resonant frequency and efficiency of the energy conversion technique. This paper
outlines the modeling, fabrication and testing of cantilever and pyroelectric structures and single element
devices that demonstrate the potential of this technology for the development of high efficiency thermal-toelectrical
energy conversion devices.
Proc. SPIE. 8035, Energy Harvesting and Storage: Materials, Devices, and Applications II
KEYWORDS: Fabrication, Microelectromechanical systems, Energy efficiency, Solar energy, Capacitors, Dielectrics, Computing systems, Energy conversion efficiency, Thermal modeling, Temperature metrology
The efficient conversion of waste thermal energy into electrical energy is of considerable interest due to
the huge sources of low-grade thermal energy available in technologically advanced societies. Our group at the
Oak Ridge National Laboratory (ORNL) is developing a new type of high efficiency thermal waste heat energy
converter that can be used to actively cool electronic devices, concentrated photovoltaic solar cells, computers and
large waste heat producing systems, while generating electricity that can be used to power remote monitoring
sensor systems, or recycled to provide electrical power. The energy harvester is a temperature cycled pyroelectric
thermal-to-electrical energy harvester that can be used to generate electrical energy from thermal waste streams
with temperature gradients of only a few degrees. The approach uses a resonantly driven pyroelectric capacitive
bimorph cantilever structure that potentially has energy conversion efficiencies several times those of any
previously demonstrated pyroelectric or thermoelectric thermal energy harvesters. The goals of this effort are to
demonstrate the feasibility of fabricating high conversion efficiency MEMS based pyroelectric energy converters
that can be fabricated into scalable arrays using well known microscale fabrication techniques and materials.
These fabrication efforts are supported by detailed modeling studies of the pyroelectric energy converter
structures to demonstrate the energy conversion efficiencies and electrical energy generation capabilities of these
energy converters. This paper reports on the modeling, fabrication and testing of test structures and single
element devices that demonstrate the potential of this technology for the development of high efficiency thermal-to-electrical energy harvesters.
Significant advances have recently been made to develop optically interrogated microsensor based chemical sensors with specific application to hydrogen vapor sensing and leak detection in the hydrogen economy. We have developed functionalized polymer-film and palladium/silver alloy coated microcantilever arrays with nanomechanical sensing for this application. The uniqueness of this approach is in the use of independent component analysis (ICA) and the classification techniques of neural networks to analyze the signals produced by an array of microcantilever sensors. This analysis identifies and quantifies the amount of hydrogen and other trace gases physisorbed on the arrays. Selectivity is achieved by using arrays of functionalized sensors with a moderate distribution of specificity among the sensing elements. The device consists of an array of beam-shaped transducers with molecular recognition phases (MRPs) applied to one surface of the transducers. Bending moments on the individual transducers can be detected by illuminating them with a laser or an LED and then reading the reflected light with an optical position sensitive detector (PSD) such as a CCD. Judicious selection of MRPs for the array provides multiple isolated interaction surfaces for sensing the environment. When a particular chemical agent binds to a transducer, the effective surface stresses of its modified and uncoated sides change unequally and the
transducer begins to bend. The extent of bending depends upon the specific interactions between the microcantilever's MRP and the analyte. Thus, the readout of a multi-MRP array is a complex multidimensional signal that can be analyzed to deconvolve a multicomponent gas mixture. The use of this sensing and analysis technique in unattended networked arrays of sensors for various monitoring and surveillance applications is discussed.
Multispectral Imaging has recently made considerable improvements to the sensitivity, uniformity and
dynamic range of infrared FPAs based on capacitively read, bimorph microcantilever sensor technology. The
company is presently prototyping 160x120 imaging arrays with 50 μm pitch pixels and is actively pursuing the
development of next generation 25 μm pitch pixel arrays. Measured peak NETD values for recently fabricated 50μm pitch focal plane arrays are in the 40-50mK range, with individual pixels in the 10-15mK range. The modeled
and measured tradeoffs discussed in this paper lead to a possible 2-3 times further improvement in average NETD.
A number of factors influence the performance of these devices which includes the optimization of
sometimes competing design requirements. For example, the tuning and optimization of the infrared optical
resonant cavity structure while maximizing the change in sensor capacitance during IR irradiance. Similarly there
are tradeoffs between structural rigidity, which increases the structure resonant frequency improving noise
immunity, and thermal response times. These tradeoffs are discussed with reference to real world sensor
structures. Results from detailed thermo-electromechanical-optical modeling of the operation of the 25 μm pitch
pixels will be discussed in reference to the design and fabrication of 25 μm pitch test pixels. The most recent
infrared sensitivity and other performance measurements from the development of the company's first commercial
160 x 120 pixel imaging array product will also be presented.
This paper reports on the development of small pixel pitch infrared FPAs based on the capacitively read bimorph microcantilever sensor technology. The heat sensing bimorph microcantilever structures are fabricated directly onto the CMOS control and amplification electronics to produce a high performance, low cost imager that is compatible with standard silicon IC foundry processing and materials. Positional responsivities of greater than 0.3 μm/K have been modeled and measured for 50 μm pitch pixels, corresponding to a temperature coefficient of capacitance, &Dgr;C/C, (equivalent to TCR for microbolometers) above 30%/K. This responsivity, along with noise capacitances in the sub-attofarad range and nominal sensor capacitances of 15 fF, give modeled NEDT < 20 mK for these devices.
At smaller pixel pitches, the positional responsivity decreases rapidly with feature size resulting in increased system NEDTs. Modeling the performance of microcantilever based IR sensors with innovative sensor structures and pixel pitches down to 17 μm indicates NEDTs < 20 mK and thermal time constants in the 5 msec range, are feasible with this technology. Results from detailed thermo-electro-opto-mechanical modeling of the operation of the 25 μm pitch pixels are presented.
The design and operation of an advanced bimorph microcantilever based infrared imaging detector are presented. This technology has the potential to achieve very high sensitivities due to its inherent high responsivity and low noise sensor and detection electronics. The sensor array is composed of bimaterial, thermally sensitive microcantilever structures that are the moving elements of variable plate capacitors. The heat sensing microcantilever structures are integrated with CMOS control and amplification electronics to produce a low cost imager that is compatible with standard silicon IC foundry processing and materials. The bimorph sensor structure is fabricated using low thermal expansion, high thermal isolation silicon oxide and oxynitride materials, and a high thermal expansion aluminum alloy bimetal. The microcantilever paddle is designed to move away from the substrate at elevated imaging temperatures, leading to large modeled sensor dynamic ranges (~16 bits). A temperature coefficient of capacitance, ▵C/C, (equivalent to TCR for microbolometers) above 30% has been modeled and measured for these structures, leading to modeled NEDT < 20 mK and thermal time constants in the 5-10 msec range giving a figure-of-merit  NEDT.Tau = 100-200 mK.msec. The development efforts to date have focused on the fabrication of 160x120 pixel arrays with 50 micron pitch pixels. Results from detailed thermo-electro-opto-mechanical modeling of the operation of these sensors are compared with experimental measurements from various test and integrated sensor structures and arrays.
The structure and operation of a new uncooled thermal infrared imaging detector is described which is composed of bimaterial, thermally sensitive microcantilever structures that are the moving elements of variable plate capacitors. The heat sensing microcantilever structures are integrated with CMOS control and amplification electronics to produce a low cost imager that is compatible with silicon IC foundry processing and materials. The bimorph sensor structure is fabricated using amorphous hydrogenated silicon carbide (a-SiC:H) as the low thermal expansion coefficient material, and gold as the high thermal expansion coefficient bimaterial (14 x 10<sup>-6</sup>/K). Amorphous hydrogenated silicon carbide is an ideal material in this application due to its very low thermal conductivity (0.34 W/m-K) and low thermal expansion coefficient (4x10<sup>-6</sup>/K). High resistivity (200-400 Ω/sq) thin Ti/W films are used as the infrared resonant cavity absorber and low thermal loss electrical interconnect to the substrate electrical contacts. A temperature coefficient of capacitance, ΔC/C, (equivalent to TCR for microbolometers) above 20% has been measured for these structures, and modeling of the performance of these devices indicates sensor performance in the range NETD < 5 mK and thermal time constants in the 5 -10 msec range are feasible with this technique. Our development efforts have focused on the fabrication of 320 x 240 imaging arrays with 50 micron pitch pixels. A number of these arrays have been fabricated with performance characteristics that are predicted by a detailed thermo-electro-optical-mechanical model of the sensor. The sensor design and the results from measurements of the thermo-electromechanical and optical properties of the detector arrays will be discussed.