الفهرس 
Introduction and Aim of the Present WorkOnly 14 pages are availabe for public view
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Abstract
PREPARATION AND PHYSICAL PROPERTIES CHARACTERIZATION OF LANTHANUM-BASED
OXIDE FILMS
Lanthanum-based oxides are technologically important category of metal oxides with diverse chemical and physical properties as will as wide range of applications. Of these oxides, lanthanum oxide, La2O3 (LO), and lanthanum nickel oxide, LaNiO3 (LNO) are very attractive function materials that are being investigated for numerous potential industrial and technological applications. The perovskite structure of LNO thin films combined with their metallic behavior, which agrees with most of ferroelectric thin films; make them a very attractive material to be studied. On the other hand, LO is considered a dielectric material that has the largest dielectric constant and band gap energy between the rare earth oxides and is used as a dielectric layer in various electronic applications. Its very interesting to prepare and characterize these contrary materials using relatively easy inexpensive chemical based technique such as spray pyrolysis (SP). SP is a versatile processing technique being considered in research to prepare thin and thick films, ceramic coatings, and powders. Furthermore, it does not require high quality substrates or chemicals, and operates at moderate temperatures of (100-500ºC).
In addition to the training and experience in the field of thin film preparations and characterizations the student gained, the presented work was aimed to prepare some technologically important lanthanum-based oxides films
of LO and LNO using SP technique. The structure, morphology and composition of the sprayed films were studied using XRD, SEM, and EDX. Also, the effect of the annealing temperature on the formed phase, surface morphology, optical properties, and electrical properties of the prepared films were investigated.
LO thin films were prepared from hydrated lanthanum chloride (LaCl3-7H2O) precursor on fused silica substrates at substrate temperatures of 300, 350, 400, and 450oC. On the other hand, LNO films were prepared from equimolar ratio of hydrated lanthanum chloride (LaCl3.7H2O) and nickel chloride (NiCl2.6H2O) at 450oC on the same type of substrate. This is because no LNO films were formed or obtained at temperatures below 450oC. XRD of sprayed LO films showed the formation of multiphase polycrystalline films at temperatures from 300oC up to 400oC. Samples deposited at 450oC were amorphous in nature similar to LNO films deposited at 450oC. Sprayed LO films deposited between 300oC and 400oC were found to compose of lanthanum chloride (LaCl3), lanthanum chloride hydroxide La(OH)2Cl, and cubic LO (c-La2O3). This indicates incomplete pyrolysis of the precursor during as deposited films. Vanishing of LaCl and La(OH)2Cl peaks at 450ºC suggests a complete pyrolysis of the precursor and formation of amorphous LO films.
DTA and TGA of a dried LO precursor showed two stable thermal pyrolysis temperature ranges; between 200ºC and 370ºC and above 470°C, corresponding to incomplete pyrolysis and stable LO phase formation, respectively. Also, DTA and TGA analysis of dried LNO precursor showed the formation of LNO phase after 560ºC. Since the deposition temperature at 450ºC is considered to be low for the formation of both hexagonal LO and the perovskite structure of LNO; samples deposited at this temperature were amorphous. EDX measurements showed that Cl and C are of the composition of as-deposited LNO sample prepared at 450oC, which were missing in the high temperature annealed samples.
As-deposited amorphous LO and LNO films were annealed between 550 and 850°C for 1 hr. in air. Single-phase hexagonal LO films textured with c-axis normal to the substrate surface were produced after the annealing of the amorphous samples. As well as, XRD of annealed LNO films revealed the formation of cubic perovskite LaNiO3structure at 750 and 850oC. LNO films annealed at 550 and 650oC remained in amorphous or poor crystalline phases. EDX measurements showed the disappearance of Cl and C peaks from the composition of films annealed at 750 and 850oC. EDX analysis of these films showed that the ratio of La: Ni: O close to its stoichiometric value when annealed at 750 and 850oC. Increasing the annealing temperature enhanced the LO and LNO texture structure. A small decrease in the unit cell volume and dimensions of both LO and LNO films were observed with increasing the annealing temperature. SEM of the prepared films showed the formation of almost crack free LNO films with rough surfaces, while LO films have rough surfaces filled with randomly distributed micro-cracks that increased with annealing temperature.
Optical parameters of the annealed LNO films showed a great difference between the amorphous and perovskite LNO samples. The optical band gap of the annealed LNO films was measured assuming a direct transition, which showed relatively low values decreased with increasing the annealing temperature. The electrical resistivity and conductivity of the films were also measured at room temperature to submit a low resistivity films down to 0.22 x 10-4 Ω.cm at 850⁰C. The electrical behavior of the perovskite LNO samples annealed at 750 and 850⁰C was found to be a semiconducting one. The thermal activation energy of the crystalline films was in the range of 0.17- 0.35 eV, which is slightly lower than the literature data.
The main conclusions that could be drawn from this work can be listed bellow:
• Low-cost spray pyrolysis technique can be successfully used to prepare technologically important oxide materials of dielectric LO and conductive perovskite LNO following a two step process, spraying and annealing.
• XRD along with DTA and TGA showed that as-deposited films were either incomplete pyrolysis phases or have amorphous structures. Single phases of dielectric h-LO and conductive perovskite LNO were obtained only after annealing the films at temperatures higher than 600oC in air. We believe that this process can be shorten into one step process if the heating unite of the SP system can reach a temperature higher than the required pyrolyesis temperature e.g. > 600oC. In spite of that, this temperature is consider lower than that is been used if these oxides to be formed from direct oxide powder mixing as in glass preparations.
• Increasing the annealing temperature decreases the unit cell dimensions and volume indicating an increasing crystallinity and densifications. Furthermore, the chemical composition of the formed oxides showed that the La:Ni is in its stoichiometric value, however the oxygen content increases as the annealing temperature increased.
• Crack free film formation remained a problem in sprayed films. The formed cracks depend on the nature of the sprayed procures (concentrations, surface tension, pH value, and viscosity), spraying parameters (liquid flow rate, droplet size, and substrate temperature). This could be a point of further investigations and future work, in addition to using organic precursors to lower the pyrolysis temperature. These two points are of considerable importance for utilizing these functional films in several industrial applications.