The fluorescence behavior of Gd3+ and Tb3+ has been studied with the ions dissolved together in a Calibo-1 glass host. The two ions form a coupled system so that energy absorbed by Gd3+ is transferred to Tb3+ by a nonradiative mechanism and is then emitted as Tb3+ fluorescence. When Tb3+ is incorporated alone into this glass, concentration quenching effects are observable. These have been found to be due to the shortening of the Tb3+ 5D4 lifetime, which in turn is due to the Tb3+ ions being packed closer together as the concentration of Tb3+ is increased. It is proposed that this gives rise to ``chains'' of Tb3+ ions along which energy can migrate, by a nonradiative transfer mechanism, to sinks in the structure. The rate of internal conversion from Tb3+ 5D3 to Tb3+ 5D4 has also been studied as a function of concentration. The results have been interpreted as being due to the formation of Tb3+-Tb3+ resonance-coupled pairs, which causes the rate of internal conversion to increase as the concentration of Tb3+ increases. The effects of changes in temperature and base glass composition on the Tb3+ 5D4 lifetime have also been investigated and are reported.

OTH basic investigations into the nature of the glassy state and commercial applications in the field of hermetic sealing have aroused considerable interest in low-melting glassy materials. The purpose of this communication is to report the synthesis of a new group of low-melting inorganic glasses. The glasses thus concerned occur in systems involving the ternary (and quaternary) interactions of the elements arsenic, thallium, sulfur, and selenium.In addition to the low-melting characteristics, application interests lay in synthesizing glasses which would exhibit high fluidities at temperatures less than 400°C. Although binary arsenic-sulfur glasses have softening points of the order of 200'C.. most of the melts are still highly viscous at temperatures in excess of 400OC. Of twenty-four ternary additives studied, three were found to give systems having substantial areas of glass formation characterized by low softening points and high melt fluidities. These are the ternary systems As-TIS, As-TI-Se, and As-Se-S (Fig. 1 ).For the study of these systems, small (20-to 200-gm.) samples were prepared from the elements, by flame heating in lightly corked fused silica test tubes. The melts were allowed to cool slowly in order to emphasize any tendencies toward crystallization or separation of phases. X-ray diffraction methods and visual examination of fracture surfaces were used to determine whether each sample was a homogeneous single-phase glass.The glasses of these three systems have been found to be chemically durable and to exhibit extremely low water solubility. They have shown no tendencies toward devitrification. The resistivities range from 101 to 10'6 ohmcm.. depending on composition; most of the glasses fall in the range lo1* to lOl6 ohm-cm. The dielectric constants vary as a function of heavy metal concentration from 4 to 20.The softening temperatures in the ternary system As-TI-S, as determined by a dilatometer method, range from about 90' to 180'C. compared with the value of 195'C. for the previously reported glassformer AsSa. * However, the most remarkable effect of the addition of thallium to the system A S S is found to be in the lowering of the melt viscosity. The addition of 10 weight 70 (2.5 mole yo) of the metal to arsenic-sulfur mixtures reduces the temperature of 30-poise viscosity (a viscosity similar to that of castor oil a t room temperature) by up to 200OC. Generally, the 30-poise temperatures in this ternary system lie between 250' and 350°C. In the system As-TI-Se this viscosity is reached a t temperatures between 185" and 400°C.. and in the system As-S-Se from 300" to about 400'C., depending on the composition of the glass.The action of thallium in greatly reducing melt viscosities on addition to As-S compositions deserves consideration. By analogy with more conventional glasses it would be expected that 'the addition of large relatively low charged species would reduce melt viscosities, but the results obtained indicate that the action of thallium is unique among twenty-four metal additives...

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Glass formation has been found to be extensive in the systems As-TI-S, As-Tl-Se, and As-Se-S. Glasses in these systems exhibit unique lowmelting properties; some are highly fluid as low as 185OC. The marked effect of thallium in lowering the viscosity of binary As-S melts was found to be unique among all the metallic additives studied. The glasses are stable, covalent, chemically durable, and dielectric. They exhibit good wetting properties with respect to most metals, and the permeability of the seals formed as well as the volume permeability of the glasses themselves appears to be extremely low. Linear thermal expansion coefficients of some compositions are high, but associated thermal cracking problems are minimized to some extent by the low softening temperatures. The glasses can be evaporated and directly condensed (in uacuo) to form continuous, .grain-free glassy films. The glasses are of considerable interest in the study of the nature of the vitreous state. In addition, their over-all properties make them potentially useful for hermetic sealing of semiconductor and other moisture-sensitive electronic components which might be damaged by high-temperature sealing processes.

once the level of the organic species has been reduced by adsorption or complexing, allowing growth of CH crystals. Since the level of silicate ions in solution will be high, the initial hydration will be rapid.However, it is difficult to see how this theory, although it explains retardation, can explain complete inhibition of hydration, since there is no evidence, even after 50 days, that hydration products form in the paste made with 0.1 c7c sodium gluconate, nor does it explain the retention of organic material in pastes with no evidence of hydration products.The SEM micrographs show a distinct change in surface features and thus the inhibition of hydration could be due to a surface phenomenon. Tadros er a1.'* showed that, in the early stages of hydration, the surface of the hydrating C,S particles carries a positive charge due to adsorption of Caz+ ions onto the silica-rich layer left as the Ca2+ ions diffuse out. It seems logical that the sugar-acid anions could adsorb onto this positively charged layer, preventing the nucleation of C-S-H aciculae. Thus the sinks for all ions in solution are poisoned and hydration is halted. This hypothesis assumes that initial hydration does occur, at least to form a small amount of hydration product. This explanation appears to be reasonable, since adsorption onto C3S in an anhydrous system is very limited.:'Because sugars are not readily ionized, the greater effect of gluconic acid at low levels can be readily explained, i.e. adsorption of the negatively charged species on the positively charged surface will be much greater. The additional stability of sugar acids in alkali h a s also been suggested as a reason for their greater effectiveness as retardersi6 and also explains why the sugar-reduced lignosulfonate fractions are as effective as the sugar-containing fractions since it is not the sugars which are responsible for most of the delay in hydration. V. Conclusions(1) The delaying action of calcium lignosulfonates in the hydration of C,S is caused by sugar acids present in the lignosulfonates. The effect is that of a delayed accelerator rather than of a slowing of the hydration rate. (2) Sugar acids are more effective as delayed accelerators of C,S hydration than sugars because of their charge and stability.(3) The delay in hydration is caused by poisoning of the Ca(OH), nuclei and complexing of Ca2+ ions, preventing a growth of Ca(OH),. (4) Inhibition of hydration occurs when the C-S-H nucleation sites on the positively charged surface are poisoned by adsorbed sugar-acid anions.