Cottrell and Farrington Daniels began a collaboration in 1939 to find a method to make nitric acid.
In a 1948 obituary for Cottrell written by Vannevar Bush and published in “National Academy Biographical Memoirs,” Bush noted:
“In 1939, through his advocacy, the University of Wisconsin initiated experiments which might lead to the fixation of nitrogen on an industrial scale. These involved application for the heating and quick chilling of air of the Royster pebble-bed furnace method which was prominent among Dr. Cottrell’s studies in earlier years.”
With support from the Wisconsin Alumni Research Foundation and the Food Machinery and Chemical Corporation for research and development, Cottrell and Daniels pursued the project until it became clear that the Haber process was more economical.
Daniels remained interested in nitrogen fixation throughout his life, though he is best remembered as a leading expert on solar energy. He directed the Solar Energy Laboratory at Wisconsin, exploring, among other issues, practical applications such as cooking, space heating, agricultural and industrial drying, distillation, cooling and refrigeration, and photo- and thermo-electric conversion. He was also interested in energy storage. Many people consider him one of the “pioneers of modern direct use of solar energy.”
Cottrell and Daniels remained friends and associates. Cottrell was sitting next to Daniels at a scientific meeting of the National Academy of Sciences in Berkeley in November 1948 when Cottrell slumped over in his chair and died of a heart attack. In an obituary in “Science,” written by Daniels in 1949, he observed: “This is the way he probably would have chosen for the ending of his vigorous, unselfish life.”