Shaped selective thermal emitter

A geometrically shaped photonic crystal structure consisting of alternating layers of thin films is heated to emit light. The structure may include index matching layers or a cavity layer to enhance emissions. The layer thicknesses of the structure may be spatially varied to modify the emission spectrum versus emission angle. The self-focusing structure may be fabricated into a convex electrically heated wire filament light bulb, a concave visible thermophotovoltaic emitter, a concentric directional heat exchanger, an electronic display, or a variety of irregularly shaped remotely read temperature or strain sensors.

RELATED APPLICATIONS

This application claims the priority benefit of U.S. application Ser. No. 11/961639, entitled “Shaped Selective Thermal Emitter”, filed Dec. 20, 2008.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a geometrically-shaped wavelength-selective thermal emitter, more specifically to a visible light source, self-focusing thermophotovoltaic emitter, heat exchanger, electronic display, and sensor.

BACKGROUND OF THE INVENTION

Today's incandescent lighting is only about 5% efficient, yet enjoys a dominant market share due to a very low cost. A need has arisen for a high efficacy, low cost, non-toxic, fixture compatible, high quality light source. Various technologies have attempted to fill this need, but all have failed in at least one way or another. For example, U.S. Pat. No. 6,768,256 “Photonic crystal light source” and U.S. Pat. No. 6,611,085 “Photonically engineered incandescent emitter” disclose a woodpile PBG, which is a 3D layer by layer structure. The thermally stimulated PBG emitter has a significant increase efficiency over a black-body emitter. But, this woodpile approach requires an expensive enhanced state-of-the-art semiconductor fab for manufacture, hardly suited to commodity lighting. U.S. Pat. No. 7,085,038 “Apparatus having a photonic crystal” disclosed an inverse opal PBG, which is a 3D colloidal structure. Although the colloidal method reduces the cost, it is still costly to manufacture and the quality of the opal is difficult to maintain using the methods disclosed.

Another PBG structure has been disclosed in Ivan Celanovic, David Perreault, and John Kasskian, “Resonant-cavity enhanced thermal emission”, Physical Review B 72,075 (2005), DOI: 10,1103/PhysRevB, 72,075127, Notably, this structure incorporates a 1D PBG structure, a thermal cavity, and a mirror. The PBG structure is composed of alternating layers of 0.17 μm thick silicon and 0.39 μm thick silicon dioxide. The thermal cavity, composed of a 0.78 μm thick silicon dioxide, is a defect in the PBG and increases the emissivity of the device. The mirror is composed of tungsten or silver. Results show quasi-monochromatic thermal emission in the IR and 2.4 μm and good directivity. Only planar structures are disclosed.

ThermoPhotoVoltaic (TPV) electric generation offers the potential to recover vast amounts of waste heat, yet has failed to operate at reasonable efficiency or have a reasonable cost. PBG emitters have been proposed to increase the spectral efficiency of the thermal emitter in the TPV system. Also, photonic heat pumps have not yet been commercially realized due to cost/performance issues. A need has arisen for a narrow band thermally driven focused light source.

Electronic displays suffer from high cost, low brightness, poor contrast, perceptive color aberrations due to edge effects color combining of red, green, and blue pixels, various effects of backlighting, and failure of a main light source. A need has arisen for low cost, emissive displays.

Temperature and strain sensors are frequently limited due to; size, operating environment, resolution due to few measuring points and sensor size, multiplexing a larger number of measurement points, or connectivity issues due to movement of the point to be measured, electrical noise, and number of connections. Thus a need has arisen for tiny, remotely readable, rugged sensors.

Although Photonic Band Gap (PBG) technologies hold promise in all of these applications, they have been severely limited due to the choice of the desired structure, the manufacturability of the structure, and the cost of making the structure.

Traditional black body emitters are essentially isotropic. The light pattern is essentially determined by the shape of the fixture or bulb.

SUMMARY OF THE INVENTION

A geometrically-shaped spectrally-shaped thermal emitter is disclosed. The geometry of the emitter is key to many applications and fabrication thereof. Of special importance is that the directivity of the emitter coupled with the geometric shape of the emitter results in a self-focusing light emitter. Thus, unlike previous light sources, a separate reflector or lensing element is not required to produce a desired spatial light distribution.

A high efficiency light source is disclosed, including a 1D PBG structure deposited on a wire substrate. Use of a wire substrate has several key advantages, including low-cost reel-to-reel processing, spatial integration of the directivity of a PBG light source, and compatibility with existing commodity manufacturing processes, bulbs, sockets, and fixtures. Use of a PBG structure allows shaping of the thermal emission spectra, without the use of hazardous materials, such as Mercury. Use of a 1D PBG structure, with or without a cavity, allows the simplicity and cost effectiveness of thin-film processing. A further advantage of a 1D structure is that the center emission wavelength is a function of layer thickness; a gradient in the thickness allows easy profiling of the emission spectra. A spatial gradient in PBG wavelength is essentially impossible with colloidal techniques. Modification of color temperature and color rendering are as simple as changing the thickness gradient.

A TPV generator is disclosed, including a concave PBG emitter. The 1D PBG emitter is simple and cost-effective to manufacture. Also, PBG emitters are directional. This provides a key advantage in providing a self-lensing effect, optically concentrating the power density on the PV cell. This is advantageous as the power per area of PBG emissions may be considerably lower than the power handling capability of commercial PV cells.

An electronically tunable PBG is disclosed. One of the PBG materials is replaced with a piezoelectric material. Application of a voltage changes the resonant wavelength of the structure. Again, the PBG is thermally stimulated to emit light. Optionally, this structure may not be heated and operated in a reflective or transmissive mode. A pixilated pattern is etched into the PBG on a flexible substrate. This arrangement has the advantage of a single layer of low-cost color-tunable pixels on an emissive flexible display.

Temperature and strain sensors are also disclosed. Use of high CTE materials shifts the central wavelength as a function of temperature. A strain sensor prefers use of higher modulus materials and must be oriented in the direction of applied strain. A 3D PBG may be used in this application to sense elongation. Key advantages include: these sensors may be deposited on irregular shaped surfaces, such as turbine blades: the sensor is read remotely by sensing the emitted wavelength, eliminating effects of electromagnetic interference: is emissive, thus relatively immune to dirt buildup: and requires no electricity or other wiring.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention and their advantages are best understood by referring toFIGS. 1 through 10of the drawings, in which like numerals refer to like parts. The figures are not to scale, especially the apparent thickness of the thin films

FIG. 1is a diagram illustrating a cross section of a filament100of a selective thermal emitter. Substrate110is a metallic wire or other filamentous substrate capable of resistively heating PBG120to emit light. Refractory metals are preferred for their melting point and low diffusion. Alternatively, a non-conductive material may be used for substrate110and PBG120itself is resistively heated. Use of a wire substrate has several key advantages, including low-cost reel to reel processing, integration of the directivity of a PBG light source, no light intensity non-uniformity or radiative losses from the backside of a planar structure, and compatibility with existing commodity manufacturing processes, bulbs, sockets, and fixtures.

PBG120consists of a multitude of thin film layers of low dielectric121and high dielectric122forming a ID PBG structure. Optionally, a well-known defect layer123is added to enhance thermal emissivity in conjunction with a high reflectivity substrate. Only 8 layers are shown as an example. PBG120need not actually possess a photonic band gap. A Photonic Crystal is sufficient (for this embodiment and all alternate embodiments), as it modifies the photonic density of states within the structure, and thus shapes the thermal emission spectrum. This structure has the advantage of low-cost thin film processing. Alternatively, PBG120may employ a 2D or 3D PBG structure.

Low dielectric121and high dielectric122are selected for a high contrast in dielectric function, to give the best spectral shaping; material stability at operating temperature, no melting, no alloying, no chemical decomposition; and a matched CTE, to avoid layer de-lamination. A key advantage is operating temperature of a selective emitter is much less than for an equivalent lumen output of a black body. This allows a much broader selection of materials and a much longer emitter life. For illustrative purposes, some materials which meet these criteria and associated advantages are: Silicon/Alumina for low CTE; Titanium Dioxide/Magnesia for a high CTE; Titanium Dioxide/Silicon Dioxide for higher CTE and piezoelectric effects; and Tungsten/Tungsten Carbide for very high temperature operation. Many other materials are readily envisioned for use in particular applications or to give particular performance.

FIG. 2is a perspective diagram illustrating a longitudinal cross section of a filament of a tapered selective thermal emitter200. Substrate210and PBG220are similar to substrate110and PBG120. PBG220incorporates a gradient and/or stepped longitudinal variation in layer thickness. The emission wavelength directly scales with layer thickness. The emission bandwidth without any layer thickness variation is relatively narrow. Variation of the layer thickness of a 1D PBG provides a key advantage: the entire emission spectra can easily be changed, as the integral of the emission spectra of each segment. Modification of color temperature and color rendering are as simple as changing the thickness gradient. Various quasi-monochromatic light colors can also easily be produced. Significant variation in the emission spectra of existing fluorescent and LED based sources requires new material systems with years of research to identify these systems. Advantages over a 2D or 3D PBG are the ease of changing lattice constant and a continuously variable lattice constant. Multiple periods of variation230may be included on a single filament. Some portion235of filament200may have a minimal PBG220, leading to low emissivity segments. An advantage is the adjustment of the total light output of emitter200.

FIG. 3is a diagram illustrating a cross section of a filament of an offset selective thermal emitter300. Offset emitter300is analogous to tapered emitter200, but with the emission wavelength varied radially instead of longitudinally. Substrate320is analogous to substrate220and PBG320is analogous to PBG220. Radial variation is accomplished by offsetting the substrate from the material source. Substrate320may be coiled or a coil of coils before coating.

FIG. 4illustrates an apparatus400for reel-to-reel fabrication of a filament. Multiple deposition chambers431to438(only 4 are shown for clarity, and 8 layers is only as an example) each deposit one layer of PBG420before the substrate410is moved to the next chamber. The drawing is not to scale as the chambers are relatively large and the deposited films are very thin. Walls between the chambers each contain one hole only just large enough to allow the filament to pass and to minimize cross contamination between the chambers. Substrate410is passed through the chambers in a reel-to-reel fashion. The source and take-up reels are not shown for clarity. Variation in layer thickness, as illustrated with tapered selective thermal emitter200, is readily accomplished by varying the rate at which substrate410is drawn between the chamber segments while keeping the deposition rate constant. Alternatively, the deposition rate may be varied and the feed rate held constant, or any combination in between. The may be varied and the feed rate held constant, or any combination in between. The deposition process may employ well known evaporative deposition, ion beam assisted deposition, chemical vapor deposition, molecular beam epitaxy, sputter deposition, or atomic layer deposition methods. An advantage is large lengths of filament can be made very quickly, very inexpensively, and with minimal user interaction. Furthermore, multiple strands of filaments may be fabricated simultaneously, given enough distance between the strands to ensure an even layer thickness.

FIG. 5is a diagram illustrating a screw type light bulb500. Filament510is analogous to tapered emitter200; mounted to filament support wires520; in stem530; packaged in an A style or other bulb540; with Edison or other style base550. An advantage is compatibility with very common Edison base sockets and fixtures. The advantage is a high efficacy, low cost, non-toxic, fixture compatible, high quality light source.

Optionally, multiple redundant filaments of the same color510a-cmay be packaged in a single bulb. Additional control electronics are required to sense the failure of one filament and switch to a new filament. Optionally, the control electronics are smart and report filament failure via a communications link, monitor used and remaining filament life, and provide dimming or color control functions. Alternatively, any number of filaments of differing color may be packaged together and independently controlled to dynamically change the color output. Electrical connection may be in either a wye, delta, or independent configuration.

Bulb540may be plastic. All plastics are permeable to oxygen. In a typical bulb, a hot tungsten filament is very flammable and would be severely life-limited with any oxygen intrusion. Filament510may include an outer layer incorporating a ceramic or glass. Hot ceramic or glass is not subject to rapid attack by permeated atmospheric oxygen. Filament510may also incorporate a metal, as selective emission allows an operating temperature much lower than a typical black-body emitter.

FIG. 6is a diagram illustrating a light bulb in a tubular package600. Filament610with period630is analogous to one or more periods230of tapered emitter200; supported along its length by multiple filament support rings640; packaged in a bulb650; with end caps660; and pins670. Period630may have low-emissivity regions to lower the total output power. Bulb650may be linear or circular. An advantage is socket compatibility with existing fluorescent bulbs, although the ballast no longer provides a useful function.

FIG. 7is a diagram illustrating directionally focused light bulb700. Energy is focused into a pattern by the directional emission spectrum and the shape of filament710. Filament710is a rounded ribbon, instead of a wire, to provide the desired light distribution pattern. Although the substrate of filament710is already low emittance, the backside may be covered with a low-emissivity coating. Filament710is mounted to filament support wires720; in stem730; packaged in bulb740; with Edison or other style base750. Several key advantages exist over well known Parabolic Aluminized Reflector (PAR) bulbs: the PBG provides a spectrally shaped emission, efficacy is much higher; and the PBG emissions are directional, thus a reflector is not required, allowing use of much cheaper commodity A style packaging.

FIG. 8is a diagram illustrating a cross section of a focused selective thermal emitter800. Substrate810is heated by a heat source from tube815and uniformly heats PBG820. Radiated emissions from PBG820are predominantly normal to the substrate. The shape of substrate810focuses the radiated emissions from PBG820onto PV cell830without any optical elements other than the shape of the emitter. Another advantage is an increase in the power density at the PV cell, to match the power capability of the cell, thus increasing the efficiency of the cell and requiring a smaller cell area. Mirror835returns energy not incident on PV cell830to PBG820.

FIG. 9is a diagram illustrating a heat exchanger900. Substrate910and PBG920are similar to substrate810and PBG820. PBG920covers the interior concave surface of substrate910and energy is focused on heat collection element930. Heat collection element is preferred to be a black body absorber or a PBG absorber and cannot be substantially reflective at the emission wavelengths of PBG920. The resulting temperature of930is higher than substrate910due to the geometric concentration of optical energy and due to the selective emission and reflectance spectrum of PBG920. Substrate910and heat collection element930are heated and cooled by a thermal transfer fluid.

FIG. 10is a diagram illustrating an electrically color tunable emitter1000. One or more layers1021,1022, or1023include piezoelectric materials, forming an actuated PBG1020. Individually addressable contacts1010form an electrode and a rear mirror for actuated PBG1020. Insulator1011prevents reference electrode1030from shorting PBG1020. Collimator1060limits color change due to viewing angle, as the light chromaticity of any PBG is not constant with viewing angle. Substrate1050must be heated for a thermally emissive device. Alternatively, the substrate may not be heated and the device operated in a reflective mode. Multiple pixels1040are patterned to form an electronic display. Optionally, each pixel may be individually heated for brightness control. Advantages include manufacture of only one color of pixel, no masking between colors, no phosphors, increased resolution of monochromatic displays, true color displays, and IR displays. In an alternate embodiment, a magnetic material is substituted for a piezoelectric material. In an alternative embodiment, tunablity is accomplished by shifting the dielectric constant. Alternatively, the display may not be pixelated and used as an electronically color changeable light source. Uses include architectural or decorative lighting.

The device is operated by varying the color of a selected pixel to a desired value by piezoelectrically changing the dimensions of the PBG. Thus, red, green, and blue pixels, and any other desired color, are readily produced. A black may be produced by shifting the color to the IR or UV. Pulse width modulation between a visible color and black shifts the apparent brightness. In an alternative embodiment, each pixel is individually heated to control brightness level.

FIG. 11is a diagram illustrating a sensor1100. Reflective layer1110is attached to the object to be sensed. For sensing temperature, one or more layers1121,1122, and1123of PBG1120is selected to include a material with a high Coefficient of Thermal Expansion (CTE). Thus, the thickness of PBG1120, and thus its central emissive wavelength is a function of temperature. This wavelength is read by spectrometer1130. Substrate1140may be an irregular shape. Sensor1100is remotely readable, capable of withstanding high temperatures, presents little mass or aerodynamic load to the device being measured, and is capable of operation in corrosive, toxic, or explosive environments. Another advantage is the sensor is emissive, eliminating the need for a probe beam and allowing operation under dusty and dirty conditions.

In a further embodiment, one spectrometer1130reads multiple sensors1100. For example, a sensor may be placed on each blade in a turbine. Rotation of the blades switches the view of spectrometer1130between multiple sensors1100. Covering an entire blade with sensor1100allows a complete measurement of the temperature profile across a blade. Complete accurate temperature profiling of irregular moving surfaces is simply not possible with other technologies. Different wavelengths of sensors may be placed on different blades, to key the collected data to a particular blade.

One potential drawback of temperature sensitive PBG1120is that it also inherently measures strain. This limitation may be overcome by placing 2 sensors nearby, one with a large CTE and the other with a small CTE, giving two measurements to solve for both temperature and strain. Using a spectrometer modified for high speed operation, vibration is indicated by changes in strain. In yet another alternate embodiment, electric, magnetic, or chemicals may be sensed by selection of the desired materials.