Optical Oxygen Sensor Using Silica Aerogel

Description:
Silica-aerogel-based optical sensors are inexpensive, non-invasive oxygen sensors that operate most accurately in the concentration range of zero to 30% oxygen or air pressure from 1 to 1,000 millibar.

Broad Fields of Use:
Measurement of oxygen partial pressure or air pressure if the fraction of oxygen is known.

Comparison with Current Technologies:
Most current technologies use a polymer-based sensing material that has limited life and must be replaced. The aerogel sensor has a long effective lifetime and a fast response time. Its use as a pressure sensor has the advantage of using a single gauge from 1 to 1,000 millibar. Several conventional gauges are required to cover the same pressure range.

Description of Current Application:
Aerogel is mostly air. It is the lightest existing solid material, and it can have a surface area as high as 1,000 m² per gram. It is one of the few existing materials that is both transparent and porous. It can be formed into almost any useful shape and makes an excellent thermal insulator. Although silica aerogel is the most familiar form, metal oxides such as iron and tin oxide, organic polymers, natural gels, and carbon can all form aerogels.

Figure 1. Silica aerogels are extremely lightweight insulators with a variety of industrially useful properties.

Silica aerogels are ideal materials for active and passive components in optical sensors. Their visible transparency, high surface area, facile transport of gases through the material, thermal and chemical stability, and ability to be filled with additional active phases are the key properties that aerogels bring to sensor applications. The Microstructured Materials Group at Berkeley Lab has recently discovered a new process that induces a permanent, visible photoluminescence in silica aerogels. Photoluminescence occurs when a material absorbs a photon of sufficient energy. The sensor is based on the observation that that the intensity of the photoluminescence is inversely proportional to the amount of gaseous oxygen within the aerogel.

The sensor is intended to perform as a low-cost, moderate sensitivity device operating most effectively in the concentration range of 0-30% oxygen. It operates independently of the nature of the other gases present in the feed gas, and of the feed gas flow rate. The prototype sensor has been successfully operated over a temperature range of -25 to +85°C (this range is based on other experimental limitations of the system, the actual usable range is larger). The highest sensitivity is observed at lower temperatures.

Figure 2. This is a prototype of an oxygen sensor that contains a special photoluminescent silica aerogel. Oxygen concentration is measured by recording the brightness of the aerogel, which is affected by the amount of gaseous oxygen around the aerogel.

The prototype sensor used a mercury-arc lamp for excitation (330 nm), and a silicon photodiode for detection of the emission (500 nm). The recent availability of ultraviolet light emitting diodes makes the instrument much more attractive in terms of size and power requirements. The prototype design can be miniaturized easily, and a device can be designed with built-in pressure and temperature compensation.

Contact:

Arlon Hunt, Senior Scientist
Phone: (510) 486-5370
E-mail:

MS 70-108B
Microstructured Materials Group
Lawrence Berkeley National Laboratory
Berkeley, CA 94720