Method of manufacturing a humidity sensing material
Inventors
Rolin, Terry D. • Small, Ian K.
Assignees
National Aeronautics and Space Administration NASA
Publication Number
US-9987658-B1
Publication Date
2018-06-05
Expiration Date
2037-03-29
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Abstract
A method is provided for manufacturing a humidity sensing material. Particles of a trivalent rare earth hydroxide or oxide (such as lanthanum hydroxide) are mixed with particles of barium oxide and titanium dioxide in specified proportions. The particle mixture is heated to generate a sintered mixture that is milled. The resulting milled particles are mixed with glass particles, an organic surfactant, a solvent, an organic vehicle, and an alkali hydroxide. The resulting liquid mixture is deposited as a layer thereof onto a substrate. The substrate and layer thereon are processed to remove liquid portions of the liquid mixture. Such liquid removal processing includes at least one cycle of heating the layer followed by a corresponding cycle of cooling the layer in a nitrogen atmosphere containing less than 25 parts per million of oxygen.
Core Innovation
The invention provides a method for manufacturing a humidity sensing material based on a lanthanum-doped barium titanate ceramic composition. The process involves mixing particles of a trivalent rare earth hydroxide or oxide (such as lanthanum hydroxide) with particles of barium oxide and titanium dioxide in specified weight proportions. This mixture is heated to form a sintered mixture, which is then milled to achieve particles of a controlled nanometer scale.
The milled particles are mixed with glass particles, an organic surfactant, a solvent, an organic vehicle, and an alkali hydroxide to form a liquid mixture. This mixture is deposited as a layer on a substrate and processed through controlled heating and cooling cycles in a low oxygen nitrogen atmosphere to remove liquid components and achieve the desired solid humidity sensing material layer.
The problem addressed by this invention relates to the need for highly sensitive yet compact humidity sensing materials to detect changes in environmental humidity that affect electronic devices. Existing sensors need to decrease in size alongside shrinking electronic devices while maintaining or improving sensitivity to humidity changes manifesting as changes in electrical properties like capacitance.
Claims Coverage
The patent includes three independent claims, each directed to a method of manufacturing a humidity sensing material with defined compositions and processing steps. The following are the main inventive features extracted from these claims.
Method of preparing a lanthanum-doped barium titanate particle mixture
Providing particles of lanthanum hydroxide, barium oxide, and titanium dioxide each having average grain diameters of 50-700 nanometers, and mixing them in proportions of 0.7-5.0 weight percent lanthanum hydroxide, 60-65 weight percent barium oxide, with the remainder titanium dioxide, to generate a particle mixture.
Formation and milling of the sintered particle mixture
Heating the particle mixture to a temperature between 1000-1300° C. to generate a sintered mixture, followed by milling the sintered material to produce milled particles having diameters of 50-700 nanometers.
Preparation of a liquid mixture and deposition on substrate
Mixing 50-70 weight percent of the milled particles with 5-15 weight percent glass particles (0.5-10 micrometers), 0.1-5.0 weight percent organic surfactant, 5-25 weight percent solvent, 5-25 weight percent organic vehicle, and 1-5 weight percent alkali hydroxide, then depositing the resulting liquid mixture as a layer on a substrate, with layer thickness 1-50 micrometers.
Thermal processing steps including low oxygen atmosphere treatment
Processing the deposited layer through cycles of heating to 850-900° C. for up to 15 minutes and cooling in a nitrogen atmosphere containing less than 25 parts per million oxygen, to remove liquids, reflow glass particles, and finalize the humidity sensing material as a solid specimen.
The independent claims collectively cover the specific combination of nanometer-scale starting particles in defined proportions, sintering and milling steps, formulation of a composite liquid mixture incorporating glass and organic components, layer deposition, and controlled thermal processing, including low-oxygen atmosphere treatment to produce a highly sensitive lanthanum-doped barium titanate humidity sensing material.
Stated Advantages
The method produces a highly sensitive humidity sensing material that exhibits rapid and large electrical response (capacitance changes) to small humidity variations.
The sensing material is effective in very small amounts, allowing thin layers (1-50 micrometers) with small surface footprints, suitable for miniaturized electronic devices.
The capacitive response is stable and repeatable over multiple humidity cycles, demonstrating improved sensor reliability.
The sensitivity spans a broad range of relative humidity levels and remains consistent at different temperature conditions, simplifying sensor calibration.
Documented Applications
Sensitive hygrometers used in semiconductor manufacturing facilities.
Electronic weather stations.
Humidity sensing aboard aerospace vehicles requiring fast and sensitive responses with low mass and small footprint sensing devices.
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