اثر توزیع ZrB2 و SiC روی فرسایش کامپوزیت های کربن/کربن اصلاح شده

Effect of ZrB2 and SiC distributions on the ablation of

modified carbon/carbon composites

اثر توزیع ZrB2 و SiC روی فرسایش کامپوزیت های کربن/کربن اصلاح شده

ABSTRACT

To understand the effect of introduced phases’ distribution on the ablation of carbon/carbon (C/C) composites, C/C–ZrB2–SiC composites containing isolated (SZ-1) and coupled (SZ-2) ZrB2–SiC particles were prepared via controllable precursor infiltrations and pyrolysis, respectively. Ablation under oxyacetylene torch at both Z and XY directions showed that SZ-1 had better ablation resistance than SZ-2, while the surface temperature rose up to about 2400 °C. The ablated surface morphologies showed that a relative integrate layer of ablation products was formed on SZ-1, whereas the layer on SZ-2 was defective. It was believed that the dissimilar distributions of ZrB2–SiC particles in composites resulted in distinct structures of ablation products on the surfaces and consequently their different ablation behavior.

چاپ سه بعدی کامپوزیت کربن/کربن توسط زینتر لیزر انتخابی

Three dimensional printing of carbon/carbon composites

by selective laser sintering

چاپ سه بعدی کامپوزیت کربن/کربن توسط زینتر لیزر انتخابی

ABSTRACT

A novel three dimensional (3D) printing method is developed to fabricate complex carbon/carbon (C/C) composite components. The C/C composites with the high mechanical performance are achieved by combining the selective laser sintering and the chemical vapor infiltration-thermal treatment process. The C/C composites with a maximum density of 1.5 g/cm3 and bending strength of 100 MPa are obtained. Complex C/C composite parts with the minimum thickness of 0.5 mm are accurately fabricated with the computer-aided design technique. This developed 3D printing method can be well applied to the production of complex C/C composite parts with high precision and good mechanical performance.

 

رسوب دهی لیزر پالسی لایه های نازک CdTe و اثر دمای زیرلایه

CdTe thin films grown by pulsed laser deposition

using powder as target: Effect of substrate temperature

لایه های نازک CdTe رشد کرده به روش رسوب دهی لیزر پالسی و با استفاده از

پودر به عنوان ماده هدف : اثر دمای زیرلایه

ABSTRACT

CdTe thin films were deposited by pulsed laser deposition on Corning glass slides using CdTe powder as target. Films were grown at substrate temperatures ranging from room temperature (~25 °C) to 300 °C. The structural, compositional and optical properties were analyzed as a function of substrate temperature. X-ray diffraction shows that CdTe films grown at room temperature have hexagonal phase, while for higher temperatures the films have cubic phase. Raman and EDS indicate that films grew with Te excess, which suggests that CdTe films have p-type conductivity.

 

رسوب نشانی الکتروشیمیایی کادمیوم تلوریم نانوکریستال

Synthesis and characterization of electrochemically

deposited nanocrystalline CdTe thin films

شناسایی و سنتز لایه های نازک نانوکریستال CdTe

به روش رسوب نشانی الکتروشیمیایی

ABSTRACT

Electrodeposition is emerging as a method for the synthesis of semiconductor thin films and nanostructures. In this work we prepared the nanocrystalline CdTe thin films on indium tin oxide coated glass substrate from aqueous acidic bath at the deposition temperature 50 ± 1 °C. The films were grown potentiostatically from −0.60 V to −0.82 V with respect to saturated calomel reference electrode. The structural, compositional, morphological and optical properties were investigated using X-ray diffraction (XRD), energy dispersive analysis by X-rays (EDAX), atomic force microscopy (AFM), and UV–vis spectroscopy respectively and cyclic voltammetery. The structural and optical studies revealed that films are nanocrystalline in nature and possess cubic phase, also the films are preferentially oriented along the cubic (1 1 1) plane. The effect of cadmium composition on the deposited morphology was also investigated. The size dependent blue shift in the experimentally determined absorption edge has been compared with the theoretical predictions based on the effective mass approximation and tight binding approximation. It is shown that the experimentally determined absorption edges depart from the theoretically calculated values.

 

خواص لایه های رسوب نشانی شده CdTe به روش تبخیر باریکه الکترونی

Properties of CdTe films deposited by

electron beam evaporation

خواص لایه های رسوب نشانی شده  CdTe به روش تبخیر باریکه الکترونی 

ABSTRACT

Cadmium telluride thin films were prepared by electron beam evaporation on glass substrates kept at different temperatures in the range 30 - 300 °C. The films were characterized by X-ray diffraction, scanning electron microscopy and optical absorption measurements. The conductivity of the films was measured in the temperature range 100 - 300 K. While the low temperature data (100 - 200 K) could be explained by the variable range hopping process, the high temperature data (200 - 300 K) could be explained on the basis of Seto's model for thermionic emission of the carriers over the grain boundaries. Transmission spectra have indicated a direct band gap around 1.55 eV.

 

سنتز شیمیایی لایه های نازک نانوکریستال کادمیوم تلوریم

Structural and optical properties of CdTe-nanocrystals

thin films grown by chemical synthesis

خواص نوری و ساختار لایه های نازک نانوکریستال CdTe رشد کرده به روش سنتز شیمیایی

ABSTRACT

By mans of a chemical synthesis technique stoichiometric CdTe-nanocrystals thin films were prepared on glass substrates at 70 °C. First, Cd(OH)2 films were deposited on glass substrates, then these films were immersed in a growing solution prepared by dissolution of Te in hydroxymethane sulfinic acid to obtain CdTe. The structural analysis indicates that CdTe thin films have a zinc-blende structure. The average nanocrystal size was 19.4 nm and the thickness of the films 170 nm. The Raman characterization shows the presence of the longitudinal optical mode and their second order mode, which indicates a good crystalline quality. The optical transmittance was less than 5% in the visible region (400–700 nm). The compositional characterization indicates that CdTe films grew with Te excess.

 

شناسایی و سنتز نانوکریستال های CdTe شفاف فوتولومینسنت

Synthesis and characterization of

brightly photoluminescent CdTe nanocrystals

شناسایی و سنتز نانوکریستال های CdTe شفاف فوتولومینسنت

ABSTRACT

A new synthesis procedure for the preparation of spherical shaped CdTe nanocrystals (NCs) is presented, exhibiting bright luminescence with exceptionally high quantum efficiency (up to 85%). The growth of these NCs occurs in a non-coordinating solvent, octadecene, with the addition of oleic acid/tri-octylphosphine stabilizers, CdO as a precursor for the Cd monomers and additional Cd metal particles as a supplementary Cd reservoir source. The dependence of the crystalline quality and the optical properties of the CdTe NCs, on the initial Cd:Te precursors’ molar ratio, and the reaction duration were investigated. It was demonstrated that the NCs’ properties improved significantly as the initial Cd:Te molar ratios are increased. The obtained NCs’ properties were correlated with measurements of the Cd0 concentration in Cd metal particles, CdTe NCs and in Cd monomer solutions.

 

خواص اپتیکی و سنتز لایه های نازک نانوکریستال CdTe

Synthesis and optical characterization of nanocrystalline CdTe thin films

خواص اپتیکی و سنتز لایه های نازک نانوکریستال CdTe

ABSTRACT

From several years the study of binary compounds has been intensified in order to find new materials for solar photocells. The development of thin film solar cells is an active area of research at this time. Much attention has been paid to the development of low cost, high efficiency thin film solar cells. CdTe is one of the suitable candidates for the production of thin film solar cells due to its ideal band gap, high absorption coefficient. The present work deals with thickness dependent study of CdTe thin films. Nanocrystalline CdTe bulk powder was synthesized by wet chemical route at pH≈11.2 using cadmium chloride and potassium telluride as starting materials. The product sample was characterized by transmission electron microscope, X-ray diffraction and scanning electron microscope. The structural characteristics studied by X-ray diffraction showed that the films are polycrystalline in nature. CdTe thin films with thickness 40, 60, 80 and 100 nm were prepared on glass substrates by using thermal evaporation onto glass substrate under a vacuum of 10−6 Torr. The optical constants (absorption coefficient, optical band gap, refractive index, extinction coefficient, real and imaginary part of dielectric constant) of CdTe thin films was studied as a function of photon energy in the wavelength region 400–2000 nm. Analysis of the optical absorption data shows that the rule of direct transitions predominates. It has been found that the absorption coefficient, refractive index (n) and extinction coefficient (k) decreases while the values of optical band gap increase with an increase in thickness from 40 to 100 nm, which can be explained qualitatively by a thickness dependence of the grain size through decrease in grain boundary barrier height with grain size.

 

سنتز لایه های CdTe به روش آنیل کردن در جعبه گرافیتی

Synthesis of high quality CdTe films by graphite box annealing

سنتز لایه های CdTe کیفیت بالا به روش آنیل کردن در جعبه گرافیتی

ABSTRACT

CdTe films were prepared by graphite box annealing of an evaporated CdTe precursor in different ambient conditions in the quartz annealing chamber. Large grain growth was observed for films synthesized at 710 K in argon atmosphere. The films thus produced were characterized by measuring the optical, microstructural and electrical properties.

 

سنتز لایه های نازک نانوکریستالی CdTe به روش تبخیر حرارتی در خلأ

Synthesis, electrical properties and transport mechanisms

of thermally vacuum evaporated CdTe nanocrystalline thin films

سنتز، خواص الکتریکی و مکانیزم های انتقال لایه های نازک نانوکریستالی CdTe

به روش تبخیر حرارتی در خلأ

ABSTRACT

A stoichiometry CdTe nano-structured powder was synthesized by chemical process. Thin films of different thicknesses (40, 60, and 100 nm) of CdTe were prepared by thermal evaporation method onto silicon substrates. Current–voltage (I–V) and capacitance–voltage (C–V) characteristics of CdTe nanocrystalline thin films deposited on p-Si as heterojunction have been investigated. At low voltages, current in the forward direction was found to obey the diode equation and the conduction was controlled by thermionic emission mechanism. Also, various electrical parameters were determined from the I–V and C–V analysis. The thickness dependence of the obtained capacitance–voltage (C–V) characteristics was also considered.