Aerogels, piezoelectric devices, and uses therefor

Details Aerogels, piezoelectric devices, and uses therefor Aerogels, piezoelectric devices, and uses therefor

2000-03-06

2004-01-13

Kiliman, Leszek (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Including a second component containing structurally defined...

C428S354000, C428S402000, C181S286000, C181S293000, C102S303000, C089S036020

Reexamination Certificate

active

06677034

ABSTRACT:

This invention relates to aerogels, piezoelectric devices, and uses therefor. More particularly, the invention relates to novel constructions made from aerogels, to the use of aerogels as resistive terminations in acoustic transducers, to the use of aerogels as translators in electric motors and transducers, to the use of aerogels as pistons in acoustic transducers, to the use of aerogels as transducer elements in visual display units, and to novel constructions of piezoelectric devices, some with integral positioning and control mechanisms, some for use as print-head devices, and some for use as electromechanical drivers.
AEROGELS AND THEIR USES
A conventional gel is a mixture of a rubbery solid substance, such as gelatine, which forms a continuous phase within which is dispersed, and trapped, a liquid, such as water. The result is a jelly (or jello). An aerogel might be described as a solid/gas analogue of a gel; it is a substantially solid porous (vesicular) material in which some suitable solid substance forms a continuous phase holding within itself, in pores rather like open-cell “bubbles” (vesicles), a gas (typically air). Pumice, which is a fine solid foam of air in solidified lava (technically it is a “vesicular glass”) usually having a density less than one (so it floats in water), is a natural material similar in some ways to an aerogel, though the pores in the latter tend to be of random form and disposition rather then “bubble”-like.
Many synthetic, man-made, aerogels are known. Most are based on silica (silicon dioxide), but there are many made from other materials, such as metal oxides, and a variety of plastics (like polyurethanes) and natural and synthetic rubbers. They are commonly manufactured by making a gel of the solid and some suitable liquid (such as water), and then removing the liquid in such a way that the surface tension forces of the liquid do not collapse the gel structure, as usually happens if a gel is allowed to dry out without special precautions. Typically, this involves displacing the gel's liquid component with alcohol, then displacing the alcohol in turn with liquid CO
2
. The gel is then subjected to a temperature and pressure high enough to take the CO
2
into its super-critical state (when it is effectively neither liquid nor gas), at which point is may be vented from the gel leaving behind the solid phase. The absence of surface-tension forces in the super-critical fluid ensures that the solid phase is not collapsed during the venting stage; thus, this leaves the solid phase of the gel largely intact and interconnected, and in a highly porous state—an aerogel.
In general, aerogels are highly porous. They have pore sizes typically in the range 5-50 nanometers, and porosities in the range 50-99.9%.
Depending on what they're made from, and exactly how they're made, so aerogels can have a wide range of physical properties. They can be light (like that silica variety available as SP-50 from Matsushita, which weighs somewhat less than 185 g/l while the silica from which it is formed is itself much heavier, at about 2.2 kg/l) or extremely light (like the silica material prepared by Larry Husbresh, of the Lawerence Livermore National Laboratory, USA, in the late 1980s, which weighs about 3 g/l, or only about three times the density of air at NTP, despite the silica from which it is made weighing as much as 2.2 kg/l). They can have large or small cells (vesicles, or pores)—ranging from a few hundred nanometers down to as little as a few nanometers in average diameter. They can be magnetic (or, at least, easily magnetised or affected by a magnet), such as the silica aerogel/transition metal composites made by the Microstructured Materials group at the Berkley Lab (USA), or they can be substantially non-magnetic, as is the case with most others. They can be opaque (to visible light) or even transparent (such as Matsushita's SP-50).
Many aerogels are described in some detail, with their methods of preparation, in
Aerogels
5, Proceedings of the Fifth International Symposium on Aerogels, (ISA-5), Montpellier, France, 8-10 September 1997: Editors J Phalippou, R Vacher (North Holland Elsevier, 1998). Specific aerials there described include those mentioned above (Matsushita at pages 369, Hoeschst at page 24), together with (on page 36) an ICI polyurethane/polyisocyanate aerogel in both monolithic and particle form with a density of 80-100 kg/m
3
and a pore size of 11-18 nanometers—
AEROGELS AS SOUND ABSORBERS
One aspect of this invention relates to the use of aerogels as substantially resistive terminations in acoustic transducers.
An acoustic transducer—as typified by either a loudspeaker or a microphone—is a device that converts electrical energy into audible sound energy (or, in the case of a microphone, vice versa). They are generally electromechanical devices; in a loudspeaker, for example, a diaphragm is driven by a “translator”—a moving coil or a magnet itself driven by an applied magnetic field—in a manner dependent on the electrical signal to be transduced, and the result is that the diaphragm causes air pressure changes that reproduce the sound represented by the electrical signal.
Loudspeakers—transducers for acoustic use like this—are most commonly of the light-weight moving-coil design, and can be made to operate up to and beyond the limit of human hearing (20,000 Hz or more). However, at the low end of the range, e.g. below 100 Hz, there arise acoustic problems unrelated to the electromechanical nature of the transducer, which problems are due to the dipole nature of any simple electroacoustic transducer. Thus, because the sound radiation—the air pressure waves—from the rear of the transducer is nominally of equal amplitude but in antiphase to that from the front of the transducer, then—and this is especially a problem at low frequencies, where the wavelengths involved are on a par with, or larger than, the dimensions of the transducer—the essentially omnidirectional rear radiation destructively interferes with the front radiation, causing a significant loss of useful output.
One standard solution to this problem is to mount the transducer in the front wall of a closed box, thus stopping the rear radiation from escaping from the box, and thus preventing the destructive interference. Unfortunately, conveniently-sized (generally, small) boxes have a volume that is only a relatively small multiple of the volume displaced by motion of the transducer, and thus with such a closed box the air within, having nowhere to escape, becomes significantly compressed and decompressed as the transducer moves back and forth a significant distance (as it must at low frequencies if it is to produce any real acoustic power). The net effect is that the closed volume of trapped air acts as a capacitative load on the transducer, and reduces the stroke at high displacements for a given input power to the transducer. This in turn produces non-linearity and distortion in the output sound.
An alternative way of looking at the problem is to consider the dimensions of the box. Where the internal dimensions of the box are small compared to a wavelength of sound from the rear of the transducer, the box, being rigid, reflects the sound back to the transducer. This reflected sound is largely in-phase with the rear-radiated sound, and this therefore loads the transducer so as greatly to reduce its amplitude of motion (compared to that without the box), thus reducing the useful sound output from the front of the transducer.
Additionally, closed boxes have relatively strong resonances, and much effort has to be expended to reduce these resonances in practical loudspeakers.
A variation of the closed box approach is to fit a bass-reflex port—that is, essentially an acoustic tuned circuit that aims to reverse the phase of the rear-radiation over a narrow frequency band before it reaches the front of the transducer. This solution is used in some practical loudspeakers, but has drawbacks due at least in part to its resonance-related basis, and is known to

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