Federal Scientists Develop New Ways to Measure Hydrogen in Metals and Other Materials

Failed turbine blades, fouled silicon chips, faltering superconductors.  Blame them on hydrogen.

Hydrogen poses problems when too much of it gets into processed materials.  Manufacturers would like to eliminate as much hydrogen as possible from a wide variety of materials, but cannot do so reliably without the ability to quantify very small amounts of the element.

Chemists at the Cold Neutron Research Facility of the National Institute of Standards and Technology have developed an instrument for nondestructive measurement of hydrogen and other elements.  Their method, called cold neutron prompt gamma activation analysis, determines trace hydrogen content of a sample without the disadvantages of other measurement techniques. For example, conventional methods for measuring hydrogen in metals require heating and destroying the sample.  Prompt gamma activation analysis does not.

"Our technique is particularly well suited for rapid qualitative and quantitative analysis of materials,"  NIST research chemist Rick L. Paul said at the 206th National Meeting of the American Chemical Society in Chicago today.

Neutron beams from a nuclear reactor have emerged as one of the fundamental tools of materials science.  Highly penetrating, they can probe deep within materials.  They are unrivaled for studying the structure and behavior of large complex molecules in polymer, chemical and biological research.

Striking a sample with such beams is the first step in prompt gamma activation analysis. As soon as the neutrons hit the nuclei of atoms in the sample, energy is given off in the form of gamma rays.  This energy varies depending on the elements from which it is emitted.  So by measuring the number of gamma rays released and the energy of each, scientists can identify and quantify the sample's elements.

NIST researchers are utilizing cold neutrons to improve the sensitivity of prompt gamma activation analysis.  Cold neutrons move at far slower speeds than thermal neutrons, so they are absorbed more efficiently by trace amounts of elements in a sample.  As neutrons leave the NIST Research Reactor, they pass through a chamber of deuterated ice (D2O at about -235 degrees C). This reduces their average speed from 2,200 meters per second to just a few hundred meters per second.

Paul and his colleague Richard Lindstrom have used the new technique to measure hydrogen in broken jet engine turbine blades, a variety of fullerene compounds (also known as buckyballs), diamond thin films and silicon wafers.  They have also measured hydrogen and other elements in the Allende meteorite which landed in Chihuahua, Mexico, on Feb. 8, 1969. 

Too much hydrogen in metals can make them brittle.  The U.S. Air Force recently asked NIST to determine the levels of hydrogen in titanium alloy turbine blades which had broken.  Cold neutron prompt gamma activation analysis revealed that one turbine blade contained two to three times as much hydrogen as another blade from the same engine.

In other tests of the new method, NIST chemists have measured hydrogen in advanced materials such as fullerenes, quartz crystals and silicon wafers.  Many of these measurements were done on request for U.S. companies such as AT&T; Bell Labs, Intel and Eastman Kodak.  In another effort to assist industry, NIST is working toward better reference standards for hydrogen in metals.  Metal processors and manufacturers will be able to use the standards to develop and calibrate their own methods for measuring hydrogen in their products.

The NIST Cold Neutron Research Facility, the first such laboratory in the United States dedicated to advanced materials research, is a resource for scientists in industry and academia.

NIST, an agency of the U.S. Commerce Department's Technology Administration, aims to help industry strengthen its competitiveness.  Through research, services, grants and outreach program, NIST helps industry in developing, adapting and commercializing technologies that lead to greater productivity, higher quality, and new and improved products and services.