NASA CASE NO.

PRINT FIG. __1_

NOTICE

LAR 15266-1-CU

The invention disclosed in this document resulted from research in aeronautical and

space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the NASA Patent Licensing Regulation (14 Code of

Federal Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to

grant licenses to commercial concerns. Although NASA encourages nonexclusive

licensing to promote competition and achieve the widest possible utilization, NASA

will provide the necessary incentive to the licensee to achieve early practical

application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 143, Hampton, Virginia 23681-0001.

Serial No.: 08/238,040

04/02/94 LaRC

(NASA-Case-LAR-15266-I-CU) SURFACE

ACOUSTIC WAVE OXYGEN PRESSURE

SENSOR Patent Application (NASA.

Lan_ley Research Center) 14 p

N95-13837

Unc|as

G3/35 0029864
AWARDS ABSTRACT 

SURFACE ACOUSTIC WAVE OXYGEN PRESSURE SENSOR 

NASA Case No.: LAR 15266-1 -CU 

Surface acoustic wave (SAW) pressure measuring devices of the prior art are based on 

the change in resonant frequency caused by the mechanical force of pressure on the 

piezoelectric material of the SAW device. If the piezoelectric material is fabricated as a 

thin membrane and arranged in such a way that the force of pressure causes the 

membrane to distort or flex, the sensitivity of the device can be enhanced somewhat. 

However, SAW pressure sensors of this type lack sensitivity for the measurement of 

pressures below 100 Pa. 

By the present invention, a transducer is provided which is useful in measuring absolute 

gas-state oxygen pressure from low pressures (pressures less than 100 Pa) to 

atmospheric pressure (1 .O1 x 1 o5 Pa). This was accomplished by providing a SAW 

device having a piezoelectric material which is coated with a chemical that will 

selectively and reversibly bind oxygen. 

The novelty of the invention resides in the coating which is used on the piezoelectric 

material of the SAW device. This coating is a chemical which will selectively and 

reversibly bind oxygen. When oxygen is bound by the coating on the SAW device, the 

mass of the coating increases by an amount equal to the mass of the bound oxygen. 

An increase in the mass of the coating caused by the bound oxygen causes a 

corresponding decrease in the resonant frequency of the SAW device. 

Inventors: Donald M. Oglesby Billy T. Upchurch 

Address: 3213 Little Island Road 1213 Bluebird Drive 

Employer: NASA LaRC 

Inventors: Bradley D. Leighty 

Address: 943 Lowry Place 

Newport News, VA 23602 

Employer: NASA LaRC 

Initial Evaluator: George Wood 

Serial No.: 081238,040 

Filed: May 2, 1994 
LAR 15266- I-CU - i- PATENT APPLICATION

SURFACE ACOUSTIC WAVE OXYGEN PRESSURE SENSOR

Origin of the Invention

5

10

The invention described herein was jointly made by employees of

the United States Government and a NASA Contractor employee in the

performance of work under NASA Contract NAS1-19858 with Old

Dominion University and is subject to the provisions of Public Law 96-517

(35 U.S.C. 202)in which the contractor has elected not to retain title.

Background of the Invention

1___. Field of the Invention

15 This invention relates generally to oxygen pressure sensors. It

relates particularly to an oxygen pressure sensor prepared by coating a

surface acoustic wave (SAW) piezoelectric device with an oxygen binding

agent tailored to provide variable sensitivity.

20 2___. Description of the Related Art

The theory and applications of surface elastic waves have been

reviewed. R. M. White, "Surface Elastic Waves", Proceedings of the

IEEE, Vol. 58, No. 8, August 1970, pp. 1238-1276. Since publication of

25 a work by Wohltjen and Dessy (H. Wohltjen and R. Dessy, "Surface

Acoustic Wave Probes for Chemical Analysis", Analytical Chemistry,

Vol. 51, No. 9, August 1979, pp. 1458-1464), considerable interest in

the use of surface acoustic wave (SAW) devices for chemical sensing has

developed, and a review thereon has been published. D. S. Ballantine, Jr.

30 and H. Wohltjen, "Surface Acoustic Waves", Analytical Chemistry,
10

LAR 15266-I-CU -2- PATENT APPLICATION

Vol. 61, No. 11, June 1989, pp. 704A-715A.

SAW pressure measuring devices of the prior art are based on the

change in resonant frequency caused by the mechanical force of pressure

on the piezoelectric material of the SAW device. If the piezoelectric

material is fabricated as a thin membrane and arranged in such a way that

the force of pressure causes the membrane to distort or flex, the

sensitivity of the device can be enhanced somewhat. U.S. patent

4,100,811. However, SAW pressure sensors of this type lack sensitivity

for the measurement of pressures below 100 Pa.

Summary of the Invention

15

20

25

It is accordingly a primary object of the present invention to

overcome difficulties presented by prior art devices and provide a

transducer which is useful in measuring absolute gas-state oxygen

pressure from low pressures (pressures less than 100 Pa) to atmospheric

pressure (1.01 x 10 s Pa).

It is also a primary object of the present invention to provide a

highly sensitive air pressure sensor.

These and other objects and their attending benefits are achieved

by providing a surface acoustic wave (SAW) device, the piezoelectric

material of which is coated with a chemical that will selectively and

reversibly bind oxygen. Since air is 20.9% oxygen, this device will serve

as an air pressure sensor with high sensitivity.

Brief Description of the Drawing.s

3O

For a more complete understanding of the present invention,

including its objects and attending benefits, reference should be made to

the Description of the Preferred Embodiments, which is set forth in detail
5

10

LAR 15266- I-CU - 3 - PATENT APPLICATION

below. This Description should be read together with the accompanying

drawings, wherein:

FIG. 1 is a schematic diagram of a surface acoustic wave (SAW)

oxygen pressure sensor according to the present invention;

FIG. 2 is a structural formula showing the oxygen binding Co(11)Cl/2

picnic-basket porphyrin with poly(octylmethacrylate-co-vinylpyridine

providing the axial ligand;

FIG. 3 is a typical frequency oxygen pressure response curve for an

SAW oxygen pressure sensor according to the present invention; and

FIG. 4 is the actual frequency/oxygen pressure response curve for

the SAW oxygen pressure sensor described in the Example, infra.

Description of the Preferred Embodiments

15 This invention relates to a new transducer for the measurement of

absolute gas-state oxygen pressure from low pressures (less than 100 Pa)

to atmospheric pressure (1.01 x 105 Pa), and is based on a surface

acoustic wave (SAW) device with a coating that will selectively and

reversibly bind oxygen. This invention may also be used as a sensitive

20 gaseous oxygen detector.

The piezoelectric material of an SAW device (see, e.g., U.S.

4,100,811 is coated with a chemical which will selectively and reversibly

bind oxygen. When oxygen is bound by the coating on the SAW device

the mass of the coating increases by an amount equal to the mass of the

25 bound oxygen. The resonant frequency of the SAW device is given by

the equation:

m

_f = (k I + k2)f2(_)

30 (1)
5

LAR 15266-I-CU

where

_f =

k 1 =

f

m =

A =

-4- PATENT APPLICATION

SAW resonance frequency change (Hz)

-9.33 X 10 .8 m2s/kg, k2 = -4.16 x 10 .8 m2s/kg for YX

quartz oscillator

SAW device resonance frequency (Hz)

mass of the coating (kg)

coated area (m2).

An increase in the mass of the coating caused by the bound oxygen

10 causes a corresponding decrease in the resonant frequency of the SAW

device. A diagram of the device is shown in FIG. 1. The fundamental

resonant frequency of the uncoated SAW device is determined by the

piezoelectric material and fabrication of the electrodes, but greater

sensitivity is achievable if it has a resonant frequency greater than 100

15 megahertz (MHz). The upper frequency is limited by electrode fabrication

constraints. Devices having resonant frequencies of 158 MHz or 200

MHz were used for the embodiment given herein.

The coating may be prepared from, but is not limited to, any one of

numerous oxygen binding compounds, including the oxygen-binding

20 porphryins, Salen Schiff-bases, and lacunar cyclidines. The requirements

of the oxygen-binding compound in the coating on the SAW device are:

The coating should have the appropriate equilibrium constant

(binding constant) for the reaction: Binding Agent + 02 = Binding

Agent • 02.

25 Oxygen-binding must be rapid and reversible.

The coating must be stable and not undergo oxidation.

The compound must be amenable to coating or bonding onto the

surface of the SAW device.

It is convenient to define the binding constant as the partial

:30 pressure of oxygen when one-half the available oxygen sites in the
LAR 15266- I-CU

coating are filled with oxygen.

Po 2

P1/2o2

-5 - PATENT APPLICATION

The following designations are used:

= partial pressure of oxygen (torr)

= the partial pressure of oxygen when one-half of

the binding sites are occupied.

5 The relationship between P1/2o2 and the standard equilibrium constant of

Pl/2o 2 = 1/K.,.

The preferred embodiment of the invention uses coatings

containing the oxygen-binding porphyrins. The picket-fence porphyrins

and the picnic-basket porphyrins (PBP) (as understood by those of skill in

10 the art) are preferred because they are stable, bind oxygen reversibly, and

may be synthetically modified to control the binding constant. The

formula and structure of a typical picnic-basket porphyrin is shown in FIG.

2. In addition to the possibility of changing the central metal ion (Co in

FIG. 2), the length of the bridge or "handle" can be changed to change

15 the oxygen binding constant. The bridging group of the PBP used for this

invention consists of a -OCH 2- unit between the isophthalate rings which

are joined to the porphyrin ring by the amide linkages. This PBP is

designated as C1/2PBP. The ability to modify the binding constant of the

active ingredient of the coating is important because the binding constant

20 determines the optimum pressure-measurement range for the invention.

The central metal ion such as Co(ll) or Fe(ll) of an oxygen-binding

porphyrin has six bond sites. Four are occupied by nitrogens from the

porphyrin molecule. The fifth must be occupied by an electron pair donor

molecule called the axial ligand, and the sixth site can bind oxygen

25 reversibly. SAW device coatings based on oxygen-binding porphyrins

must contain a substance that will serve as the axial ligand. This

substance may be but is not limited to, pyridine, methyl imidazole,

1,5-dicycloimidazole or a substance which will furnish these types of

chemical groups. An example of a substance which will furnish an axial

30 ligand to the oxygen-binding porphyrin is a copolymer containing an axial
5

10

LAR 15266-1-CU 6

ligand along the polymer chain, such as

poly(octylmethacrylate-co-vinylpyridine).

A typical frequency/oxygen-pressure response curve is illustrated

by FIG. 3. The optimum pressure-measurement range for the coating

depicted in FIG. 3 is between 1.0 kPa and 100 kPa. For a given pressure

range there is an optimum binding constant for the activity ingredient of

the coating. Table I gives the optimum values of P1/2 for different oxygen

(and air) pressure ranges. The minimum pressure of each range is based

on the pressure required to produce a signal change of 500 Hz and the

maximum pressure for each range is that required to saturate 70% of the

binding sites. Since air has a constant concentration of oxygen this

invention may be used to measure air pressure.

PATENT APPLICATION

15

20

TABLE I

OPTIMUM BINDING CONSTANTS FOR SAW DEVICE COATINGS

Oxygen Pressure Air Pressure Binding Constant,

Range (kPa) Range(kPa) P112(kPa)

0.002 0.3

0.010 1.3

0.130 13.0

0.700 53.0

1.300 133.0

2.700 267.0

0.013 - 1.3

0.070 - 6.7

0.070 - 67.0

3.300 - 267.0

6.700 - 667.0

13.000 -1333.0

0.11

0.57

5.70

29.00

57.00

115.00

25

3O

Example

A 158 MHz SAW device (Microsensor Systems, Incorporated, 62

Corporate Court, Bowling Green, Kentucky 42103) was spray-coated in a

nitrogen atmosphere. The coating solution was 99.24% toluene, 0.35%

1,5-dicyclohexylimidazole, and 0.41% cobalt(ll)Cl_2 picket-fence

porphyrin. The SAW device was then cured at 60°C under nitrogen for
5

LAR 15266- I-CU - 7- PATENT APPLICATION

30 minutes. The frequency response to oxygen (or Air) pressure was

measured with a CEM-158-B dual delay line oscillator system

(Microsensor Systems, Incorporated). The device was suitable for

measuring oxygen pressure in the range of 0.25 KPa-26 KPa with a

sensitivity ranging from 436 HZ/kPa to 6 Hz/kPa. The response of the

device is illustrated by FIG. 4.

What is claimed is:
EJLR15266 -1 -CU -_- PATENT APPLICATION

SURFACE ACOUSTIC WAVE OXYGEN PRESSURE SENSOR

Abstract of the Disclosure

5 A transducer for the measurement of absolute gas-state oxygen

pressure from pressures of less than 100 Pa to atmospheric pressure

(1.01 x 10 s Pa) is based on a standard surface acoustic wave (SAW)

device. The piezoelectric material of the SAW device is coated with a

compound which will selectively and reversibly bind oxygen. When

10 oxygen is bound by the coating, the mass of the coating increases by an

amount equal to the mass of the bound oxygen. Such an increase in the

mass of the coating causes a corresponding decrease in the resonant

frequency of the SAW device.

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