الفهرس 
Chromogenic materialsOnly 14 pages are availabe for public view
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Abstract
The excessive use of heating systems in cold climates and air
conditioning systems in hotter climates is resulting in the extensive use of electricity in order
to maintain such systems. This in turn leads to the greater use of fossil fuels and higher
emissions of carbon dioxide and other pollutant gases. The growing amount of carbon dioxide
emissions is contributing to the problem of global warming, increasing the need for alternative
technologies to heating and air conditioning systems.
One such alternative is the production of thin films which can be used as window glazing coatings
to construct ‘smart windows’. These windows have the greatest use within constant climates. In cold
climates, windows with high solar transmittance and low thermal emittance are needed; this
allows sunlight into the building to brighten the room but stops heat from escaping thus warming
the room. In constantly hot climates, materials that are transparent in the visible region but
reflective in the infrared, such as thin metallic coatings, can be used to ensure that the inside
of the building remains cool.
These solar control coatings, however, pose a problem in varying climates such as in northern and
central Europe. For these cases, materials that have altering properties owing to external
surroundings could be the solution. These ‘chromatic’ materials include several categories, such as
photochromic glasses and polymers, thermochromic metal oxides and electrochromic materials.
This thesis will deal with chemistry and physical concepts
behind the solar control coatings and chromic materials including ambient radiation and the ideal
of a blackbody object. It will then look in depth at chromic material MoO3 thin film as a
promising. The experimental results and theoretical insight as well as production techniques and
applications will be considered.
Chromogenic materials are considered as one of the best solution for reducing the energy
consumption. The basic function of chromogenic material is the reversible change of their optical
properties as a function of heat, electromagnetic radiation and electric potential. Generally these
materials are transparent and dim before after the exposure to the preceding fates respectively.
The thesis is concerned with photochromic materials which has high response to solar radiation.
The best form for coating the window glass for building and vehicles is the thin film.
Molybdenum oxide thin film is selected to be studied since it could be prepared with the available
facilities. Moreover the published mechanisms for the photochromic phenomenon still under debate.
Spray pyrolysis technique is used here to prepare the photochromic molybdenum oxide film. It is
simple and low cost and versatile technique. The demand of homogeneous, reproducible, scalable
process deposition of films is fulfilled by SP technique. Today spay pyrolysis finds use in a
variety of applications, ranging from biomedical to industrial, microelectronics to ceramics.
Spray pyrolysis technique involve the acceleration of precursor solution from a specially designed
atomizing nozzle to a very small droplets (fog or mist) incident of the surface of hot substrate.
Then pyrolysis takes place to the film. There are various parameters related
to the spray system such as: spray solution molarity, air and spray
solution flow rate, distance between the nozzle orifice and substrate surface, spray time and
substrate temperature. All these parameters one adjusted to their optimum value, which produce
homogenous, pine hole free and well adhered films, one at a time.
Molybdenum oxide film on glass substrates is formed by spaying 0.2 M solution prepared by
dissolving the appropriate weight of Molybdenum penta chloride pure (MoCl5) 99.98% in bidistalled
water. All spray parameter are kept constant the following values:
• Precursor, MoCl5.
• Solution molarity, 0.2 M.
• Solution flow rate, 0.3 ml /sec.
• Distance between nozzle orifice and substrate surface, 30 cm.
• Filtered and dried compressed air, 6 N cm-2.
• Substrate temperature varied from 200o C up to 400oC.
• Spray time varied from 1min. up to 5 min.
The run to deposit the film is repeated three times to ensure the repeatability of the same film.
Two groups of samples are prepared; the first one is prepared when the spray time in varied from
1-5 min. while each substrate temperature in the varied range 200oC to 400oC is fixed. The later
samples group is deposited when each spray time in the covered range is fixed while the substrate
temperature is varied from 200o C
up to 400oC.
The structure property of each obtained sample is examined by
GAXRD technique with respect to its preparation conditions.
The crystallinity and /or amorphousity nature of the deposited
film is elucidated from the obtained X- ray diffraction (XRD) pattern.
The present phases are identified referring to the standard JCPD cards. The crystallite size as
well as the induced strain in the film are calculated using WinFit software analysis of the XRD
pattern.
The XRD results show that the deposited films at substrate temperature ranging from 200oC up to
300oC and spray time ranging from 1 min. up to 5 min. are amorphous added to them the films
deposited at substrate temperature 325oC & 350oC and spray time 1
& 2 min.
Also, the sample prepared at substrate temperature 400oC and spray time range from 1min. up to 5
min. are crystalline added to them the samples prepared at substrate temperature 325oC & 350oC and
spray time 3, 4 & 5 min..
The characteristic peaks of only α-MoO3 phase are appeared with reasonable intensity in the XRD
patterns of all crystalline samples. This result ensure that the as deposited samples are single a-
MoO3 which is obtained by other techniques after heat treatment. The calculated values of
crystallite size and internal strain using WinFit program are found to be ranged from 10 – 28 nm
and 0.28 % to 0.05 % respectively.
Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) are used to depict the surface
morphology of the film. The calculated average surface roughness by AFM is found to be ranged from
1.3 to 1.5 nm which indicated the smoothing nature of the deposited films. The crystallite
distribution elucidated by AFM histogram indicates the homogeneity of the size almost at most
preparation conditions.
Transmission spectra in wavelength range from 0.2 to 2.5 µm of
all samples prepared at all conditions are obtained by double beam spectrophotometer. These (T - λ)
spectra are converted to absorption spectra (a − λ) to compatible with fitting by Lorentz model.
Each spectrum shows two absorption edge in short wavelength 400nm corresponding to the energy gap
of the prepared sample it show slight change with preparation conditions. The later is broad and
centered at
800 nm. This band is directly related to the sample color which is
dependent on preparation conditions. The fitting of this band gives rise to two clear subbands
dependent to great extent, on the preparation condition. The spectral weight of these resolved
bands is calculated and correlated with preparation conditions on the basis of the reported results
of x- ray photoelectrons spectroscopy (XPS) of α- MoO3 the deconvoluted bands may be due to
the itinerancy of
electron from paramagnetic Mo 5+ to Mo 6+ and /or Mo4+ to Mo5+ and
/or [Mo 5+ , Mo 5+]. These electronic transformations between the Molybdenum oxide states are
responsible for the coloration of the prepared samples.
It could be possible to adjust the preparation conditions to obtain a sample with the required
degree of blue color ranging from light blue to deep blue.
In order to study the photochromic phenomena the samples are exposed to UV light for different
duration 45 min. & 145 min..
A sample with light transparency 90% is selected to be exposed to the predetermined UV dose
and monitoring the appeared absorption band. This band is fitted and deconvoluted using the
Lorentz model. The characteristics of the resolved bands are correlated with the exposure
time.
The obtained results are discussed and interpreted on the light of
the reported results. It is found the main cause of the sample dim (high absorption) is the
electron transition between the different oxidation states of Molybdenum.
In general, the prepared sample dim (high absorption / or blue color) is attained by adjusting and
manipulating the preparation conditions to control the blue permanent color degree.
Also, the transparent amorphous α- MoO3 film sample changed to dim sample by exposure to UV
radiation. The degree of dim depends on the UV dose.