The Development of β-Magnesia Windows and an Apparatus for Using Them

The properties of optically clear β-magnesia crystals have been investigated with an eye toward using than as windows in a vacuum furnace. β-magnesia was found inert to most metallic vapors below 1000° C, and well adapted for use as windows in spectroscopic problems. Windows have been made and tested satisfactorily, and a technique for cutting and polishing the windows was developed by Mr. Pearson in his optical shop. The optical constants of a large clear crystal of β-magnesia were measured and a preliminary investigation made to see whether or not it is suitable tor use in lenses transparent out to 2150 Angstroms. It was concluded that the hygroscopic properties of the substance render it unsuitable. for such lenses. even though it has advantageous optical constants. Many experiments were made in an attempt to find some technique for sealing the windows into steel tubes so that an absorption furnace could be built where metal vapors could be investigated under conditions of thermodynamic equilibrium, One successful method was found and is described in detail in this thesis. Seals were made by this method and found satisfactory up to at least 587° C. Some of the more important unsuccessful trials are described in the appendix. In order to utilize the windows and their properties, a furnace was constructed with a specially designed needle valve that permitted control of evacuation through a port in the side of the absorption chamber wall, but which did not introduce any cold spots in the furnace. The design of the needle valve is discussed at length. The specific accomplishments reported in this theses are: (1) The determination of the optical constants of β-magnesia. (2) The development of a technique for sealing β-magnesia windows into a furnace. (3) The design of a needle valve which, together with the windows, permits the realization of thermodynamic equilibrium in an absorption chamber containing caustic metal vapors.