2. Experimental Procedure

The experimental glasses were normally made in 500 g melts from batch materials of sufficient purity to satisfy the requirements for the production of optical glass. The standard procedure was to melt the batches in platinum crucibles, 6.5 cm in diameter by 7.5 cm deep. After the batch was melted, the melt was stirred for 2 h with a motor driven platinum-10-percent-rhodium, double-bladed propeller-type stirrer. The furnace used for melting was heated by silicon carbide resistance elements so that the furnace atmosphere was not contaminated by combustion products. After the melt was stirred, it was poured into a heated metal mold to form a block about 7.5 cm by 7.5 cm by 2.0 cm thick. When sufficiently rigid, the glass block was transferred to an electric muffle furnace, which was cooled to room temperature in approximately 18 h.

*Part of the work described in this report was sponsored by the Department of the Navy at the National Bureau of Standards, and by Project Themis, U.S. Air Force, at the University of Arizona.

**Address: Optical Sciences Center, University of Arizona, Tucson, Arizona 85721. The figures in brackets indicate the literature references at the end of the paper.

(3) Liquidus temperature [2]

(4) Infrared transmittances for 2 mm thickness from 1 to 6 μm.

For the more promising glasses certain other properties were measured. These included infrared transmittances for greater thicknesses, usually 8 mm, so that absorption coefficients could be computed; infrared refractive indices; linear coefficient of thermal expansion; and deformation temperature. For some glasses density, chemical durability, and elastic constants were also measured.

In addition, data on refractive indices (no), dispersions and specific volume were evaluated, using previously described methods [3]. One purpose of this evaluation is to develop quantitative property-composition relations for use by technolo gists in formulating glasses for optical uses. A further purpose is to clarify presently incomplete knowledge of the ternary phase diagrams of these glass-forming systems and to identify, if possible, "substructures" containing the cations present in the glasses.

3. Glass-Forming Systems
Investigated

Several ternary silicate systems were investigated as to the extent of the region of glass formation and the properties of the glasses obtained.

3.1. The BaO-TiO2-SiO2 System

In an effort to produce glasses having high values of refractive index at wavelengths of 2.0 to 2.5 μm, good infrared transmittances and good chemical durability melts were made in the ternary system BaO-TiO2-SiO2 [4]. Most high-index glasses presently available are either extra-dense flint glasses, which have a high PbO content, or rare-earth borate glasses [5]. The extra-dense flint glasses have fairly good infrared transmittances, cutting off, as do most silicate glasses, at about 5 μm. They have high refractive indices, but their chemical durability is rather poor and they have low deformation temperatures. The B2O3 content of most rareearth glasses makes them useless for infrared applications.

The phase equilibrium diagram for the ternary system BaO-TiO2-SiO2 has not been determined, but information is available on the binary sides

of the ternary system [6, 7, 8]. Rase and Roy have determined the liquidus temperatures and phase relations along the line BaO TiO2-SiO2 in the ternary diagram [9]. This information was very useful in selecting compositions in the ternary system that could be melted and cooled as glasses.

The composition of all melts made in the ternary system are given in table 1 and are plotted in the ternary diagram in figure 1. As may be seen from the figure, the longest BaO isopleth along which glasses were formed is the 25 mol percent line. Although glasses are not formed on this line to the BaO-SiO2 binary, glass formation begins at about the 20 mol percent of TiO2 and extends to relatively high concentrations of TiO2. This line of glass formation seems to follow a valley in the liquidus surface, as may be seen from table 1.

The color of the glasses changed very markedly as the TiO2 content was increased. Those containing up to about 15 mol percent of TiO2 were nearly colorless, whereas those containing intermediate amounts from 20 to 35 mol percent of TiO2, were orange colored, and the others having about 40 mol percent of TiO2 were dark brown to black. Evidently, as the TiO2 content is increased, the absorption increases at the shorter wavelengths in the visible region, and at higher TiO2 concentrations very little visible light is transmitted.