THERMAL CHARACTERISTICS OF FUSED SILICA CERAMIC WASTE
AND ITS APPLICATION IN CASTING JEWELRY
and Qun Zeng
Translated from Novye Ogneupory, No. 12, pp. 30 – 32, December, 2013.
Original article submitted September 25, 2013.
can be obtained from fused silica ceramic wastes formed at factories that make
polycrystalline silicon, where fused silica is converted to cristobalite during the production of polycrystalline
ingots. High-purity cristobalite powder is obtained by grinding these wastes. A study was made of the coeffi
cient of linear thermal expansion of molds which are composed of cristobalite with a gypsum binder and are
used to make jewelry. It was established that the thermal expansion which takes place during the transition to
the cristobalite phase can effectively compensate for the shrinkage which the gypsum undergoes in the tem
perature range from 200 to 400°C. Experiments involving the casting of a copper alloy showed that the use of
cristobalite can improve the thermal performance of molds employed in the production of jewelry.
Keywords: cristobalite, thermal expansion, mold for making jewelry.
The high-temperature phase of silica is stable from
1470°C to its melting point of 1728°C. It is known by the
name b-cristobalite. This phase remains metastable down to
275°C and is then transformed into the low-temperature
phase — a-cristobalite . Polysilicon factories use large
numbers of fused silica ceramic crucibles as containers for
high-temperature processing operations. When the tempera
ture decreases, cracks are formed in the walls of the crucible
due to transformation to the phase b/a-cristobalite . Thus,
this ceramic becomes available as a solid waste product .
Worldwide consumption of fused silica ceramic crucibles is
substantial, which makes their recycling an important issue.
Cristobalite powder plays an important role in obtaining
the exact dimensions needed for precision castings and the
surface roughness that is required . The molds tradition
ally used for producing such castings are made from granules
that are obtained with the use of a binder — usually
. Defects may be formed in the casting as a result
of shrinkage of the mold during the thermal decomposition
of gypsum. Thermal expansion during the cristobalite phase
transition can effectively compensate for gypsum shrinkage
, thus improving the thermal stability and mechanical
strength of the mold and keeping it from cracking .
Traditional cristobalite materials are made by calcining
selected grains of treated quartz at high temperatures . The
production process consumes large quantities of energy and
quartz minerals. Studies we have made of methods of recy-
cling fused silica ceramic wastes have allowed us to develop
a new technology for using such wastes in making precision
castings for the jewelry industry. The present article exam
ines the thermal expansion of solid wastes and other materi
als used to make molds employed in the production of preci
sion castings. A casting is obtained in the form of a copper
alloy and the quality of the finished product is evaluated.
Cristobalite powder was obtained by grinding fused sil
ica ceramic wastes (Jiangxi LDK Solar Hi-Tech Co., Ltd.).
The phases of the wastes were studied at room temperature
on an X’Pert PRO x-ray diffractometer operated with copper
-radiation (l = 0.15406 nm). The diffraction angle ranged
from 10° to 50° and scanning speed was 0.02 deg/sec. The
concentrations of the main impurity elements in the wastes
were determined by ICP mass spectrometry (PerkinElmer
Elan 6000). The fused silica ceramic wastes were studied by
differential thermal analysis (DTA) with the use of an HCR-1
analyzer made by the company “Beijing Hengjiu.” The anal
ysis was performed in an air atmosphere and the wastes were
heated from 50 to 500°C at a rate of 10°C/min. The thermal
Refractories and Industrial Ceramics Vol. 54, No. 6, March, 2014
1083-4877/14/05406-0490 © 2014 Springer Science+Business Media New York
Laboratory of Nanophotonic Functional Materials and Devices,
South China Normal University, Guangzhou, China.
Jewelry Institute of Guangzhou Panyu Polytechnic, Guangzhou,