We developed an automatic system in which we can prepare one long piece
of glass sample library and anneal it successively under a temperature
gradient (200∼1050^∘C/40cm). A glass capillary tube (ID 1.5
mmφ× OD 8 mmφ) is used as a cell of the library. Zinc
tellurite melt at 800 ^∘C is sucked into the cell by applying a
vacuum pressure to the other side of the capillary. Crystallization is
observed as a white segment in the library under an appropriate annealing
condition. A set of 2D sample array is obtained by preparing several
sample libraries with different annealing time. Since this array itself
corresponds to a T-T-T (Time-Temperature-Transform) diagram, a tangent
which touches the crystallized region and pass through the melting
temperature at t=0 gives critical cooling rate, Q.

Compositional dependence of Q shows that the minimum of Q is not
located at the eutectic point of ZnO-TeO2 system, which has been
believed to be most stable composition on the basis of the known glass
forming regions, but is shifted to ZnO-rich region. Considering the
high reproducibility of this automated system and the large error bar of
previous results, this result is more reliable.

We also investigated the dependency of Q on thermal hysteresis. For
the melt once quenched below the glass transition temperature
(T\mathrmg) and annealed at above T\mathrmg, the Q value
becomes larger than that without the quenching. This is because the
former glass melt went through the nucleating temperature region located
just above T\mathrmg. It is concluded that this automated system
makes it easier than ever to examine quantitatively the thermal
stability of glass melt having low melting temperature, which is one of
the important factors for judging the feasibility of fabricating new
glass devises with desired quality.