| Summary: | 碩士 === 逢甲大學 === 材料科學研究所 === 85 === ABSTRACTThe use of chemical vapor deposition (CVD) process as a
method of producing large scale and compact components for the
infrared transmitting materials has been developed in the last
two decades. Among those materials, Zinc Sulphide is applied in
domes and windows which requires the transmittance in the
wavelength of 8 to 12μm. The low pressure chemical vapor
deposition(LPCVD) process's offering advantages over
conventional hot-press methods are the high purity, high density
the ability of producing large cponents with complex shapes.
Properties of CVD materials can be controlled by adjusting
process parameters such as working temperature, pressure, degree
of supersaturation, flow rate, growth rate, etc. Among which the
deposition temperature may be very critical to the subsequent
properties including optical properties and mechanical
properties. In this study, polycrystalline ZnS infrared
transmitting material was prepared by using a horizontal LPCVD
reactor. All the process parameters are fixed and deposition
temperature is changed from 560 ℃ to 850 ℃ to reveal the
effect on microstructure and the subsequent optical and
mechanical properties. X-ray diffractometry(XRD), energy-
dispersive spectrometry(EDS), optical microscopy(OM) and
scanning Electron Microscopy(SEM) were used to reveal the
microstructure of the deposits. Hardness value and fracture
toughness were determined using Vicker's indentation technique.
Erosion rate was measured by solid particle erosion method. The
optical properties were measured by using the spectrophotometer
with wavelength from 0.35μm to 16μm. Experimental results show
that the color of deposited ZnS changes as the deposition
temperature changes. The color of the deposit is brown at low
temperature while it becomes white and clear with increasing
deposition temperature from 560℃ to 850℃. A type of zinc
blende structure(β- phase) was found of the deposits all over
deposition temperatures with (111) texture at high deposition
temperatures. The grain size of ZnS increases with increasing
deposition temperature. Both hardness and fracture toughness are
a function of grain size and follow Hall-Petch equation. Solid
particle erosion test show that the deposits obtained at high
deposition temperature have more serious damage feature on the
surface.The spectrum shows that the deposition temperature does
not affect the transmittance in the range over 14μm as well as
the long wavelength cut-off, it is caused by the extrinsic
absorption in the ZnS. The IR spectrum shows that the deposit
obtained at lower deposition temperature exhibit a significant
6.3μm absorption band which is caused by the Zn-H stretching in
the ZnS. The Zn-H stretching could decrease the transmittance in
the range of 4 to 16μm. As the deposition temperature is
increased over 700℃, the 6.3μm absorption band disappears and
the transmittance in the range of 4 to 16μm increases. The
spectrum in short wavelength region of 0.35 to 2.5μm is
affected by the grain size and pores of the ZnS. This results in
that the deposits obtained at high deposition temperature has
shorter cut-on wavelength and larger transmittance in the range
of 0.35 to 2.5 μm.
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