The viscosity anomaly observed above the critical points of a series of sodium borosilicate melts is investigated by ultrasonic shear relaxation spectroscopy. The measurements were conducted at frequencies between 3 and 25 MHz and temperatures between 800 and 1300deg;C. The samples tested have different critical temperatures as a result of small doping additions to the critical composition of the immiscibility surface in the phase diagram of the sodium borosilicate system. This permitted measurements over a wide range of reduced temperatures. Analysis of the data yields three structural relaxation parameters which are: the instantaneous shear modulus, the most probable relaxation time, and the width of the distribution of shear relaxation times. These parameters are related to the viscosity. It is found that while the instantaneous modulus and the most probable relaxation time appear unaffected by the presence of superhyphen;critical composition fluctuations, the width of the spectrum of shear relaxation times broadens drastically as the critical temperature is approached from above. Similar and parallel curves are formed from the widths of the distributions of relaxation times of all four samples when the temperature is properly normalized by the critical temperature. The general behavior of these curves exhibits the three reduced temperature regions predicted in a previously published relaxation model. The supercritical broadening of the width of the distribution of relaxation times accounts for the observed viscosity anomaly.
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