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The concepts and opinions expressed in the articles are the exclusive responsibility of the authors. La Red Postal de Colombia, expires Dec. Design and construction of low shear laminar transport system for the production of nixtamal corn dough.
Dynamic stability of slender columns with semi-rigid connections under periodic axial load: theory. Dynamic stability of slender columns with semi-rigid connections under periodic axial load: verification and examples.
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Modeling of a simultaneous saccharification and fermentation process for ethanol production from lignocellulosic wastes by kluyveromyces marxianus. Sandro J. Discrete Particle Swarm Optimization in the numerical solution of a system of linear Diophantine equations. Thermodynamic analysis of Ra in an Organic Rankine Cycle for power generation from low temperature sources. A refined protocol for calculating air flow rate of naturally-ventilated broiler barns based on co2 mass balance.
Use of a multi-objective teaching-learning algorithm for reduction of power losses in a power test system. Bioethanol production by fermentation of hemicellulosic hydrolysates of african palm residues using an adapted strain of Scheffersomyces Stipitis. Vega-Posada, Edwin F. Medina, Juan M. Ramirez, Carlos A. Transformaciones de fase en recubrimientos de barrera térmica de zirconia estabilizada con yttria depositados mediante aspersión por plasma atmosférico. Efecto del sistema de calefacción en la calidad de la cama de galpones de pollos de engorde en condiciones de inverno.
Diseño y construcción de un sistema de transporte laminar de bajo cizallamiento para la producción de masa de maíz nixtamalizada. Modelado de un proceso de sacarificación y fermentación simultanea para la producción de etanol a partir de residuos lignocelulósico utilizando kluyveromyces marxianus.
Caracterización de la adherencia para películas de Ti6Al4V depositadas por medio de pulverización catódica magnetrón rf sobre acero inoxidable. Uso de un algoritmo de enseñanza-aprendizaje multi-objetivo para la reducción de pérdidas de energía en un sistema de potencia de prueba. Producción de bioetanol por fermentación de hidrolizados hemicelulósicos de residuos de palma africana usando una cepa de scheffersomyces stipitis adaptada. Aprovechamiento Energético de las Olas Costeras: Una propuesta de mejora de los convertidores de columna de agua oscilante.
Received in revised form: January 23th, Accepted: January 27th, Abstract Phase transformations in air plasma-sprayed thermal barrier coatings composed of ZrO2 — 8 wt. According to Rietveld refinement measurements, the content of the cubic phase in the top coat increases with time and temperature; it starts at 7. The presence of a cubic phase in high amounts is undesirable due its lower mechanical properties compared with the tetragonal phase. After hours of exposure to high temperature, the amount of Y2O3 in the tetragonal phase reduces to 6. The monoclinic phase reached Después de horas de exposición a alta temperatura, el contenido de Y2O3 en la fase tetragonal se reduce hasta 6.
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La fase monoclínica alcanza Introduction Thermal barrier coatings TBCs are multilayered systems widely used in gas turbines to increase efficiency and durability []. Base substrates are usually Ni-based superalloys that offer good mechanical strength and excellent corrosion, oxidation and erosion resistances at high temperatures []. They contain significant amounts of alloying elements such as Cr, Mo, Al, Ti, Fe and C, which favor intermetallic compounds precipitation [8,9].
Two types of BCs are commonly used: the Platinum- modified Nickel. DYNA 81 , pp. Figure 1. The TC faces combustion gases. The BS is air-cooled to increase the temperature gradient and, therefore, the efficiency. In addition, the tetragonal phase presents excellent mechanical and thermal properties compared with those of the monoclinic phase [2,19,26,27]. The tetragonal-to-monoclinic transformation is avoided with the addition of 6 to 8 wt. Consequently, a monoclinic phase appears from the Y-depleted tetragonal phase during cooling.
The presence of both, cubic phase in high amounts and monoclinic phase is undesirable due to their lower mechanical properties compared with the tetragonal phase. Also, the volumetric changes associated with the phase transformations favor crack generation and propagation which compromise the coating integrity. The paper is organized as follows: in Section 2, the experimental procedure, methods and materials are presented. The most important conclusions are summarized at the end.
The BC is a metallic layer that initially provides adherence between the TC and the substrate [10,11]. During operation at high temperatures, aluminum diffuses from the BC and reacts to form a barrier layer, known as the TGO. The TGO provides the barrier to oxygen diffusion to avoid substrate oxidation. The TC is usually composed by Yttria Y2O3 stabilized Zirconia ZrO2 , and serves as the main defense mechanism of gas turbines against erosion and corrosion. It must be stabilized in order to maintain its tetragonal structure at room temperature and also to keep thermal properties constant conductivity and thermal expansion coefficient in the range of working temperatures.
To accomplish this stabilization, some elements such as Hafnium Hf and Yttrium Y , are commonly added []. These changes stabilize the tetragonal phase and decrease the thermal conductivity. In both cases, thermal conductivity is reduced due to an un-harmonic scatter of the charge carriers in the ceramics at high temperature, i. Zirconia without stabilizers can exist in three different phases [18,22]: cubic, tetragonal and monoclinic, see the phase diagram [23] in Fig. The tetragonal-to- monoclinic The BC layer was deposited onto a nickel-base substrate, namely Inconel The dimensions of the TBC samples were 10 cm x 10 cm, extracted from plates of 30 cm x 30 cm.
The samples were cut with a precision saw operating at RPM. Sample preparation included grinding with No. A complete characterization of this material with similar heat treatment conditions has been reported in previous works [32,33]. Rietveld semi-quantitative measurements were made to account the phase changes at different temperature conditions. To ensure reliability of the results, two replicas of each sample were also measured. It is well known that the tetragonal lattice parameters change depend on the Y2O3 content. Therefore, Rietveld measurements were also performed to determine the lattice tetragonal parameters a and c, in order to determine the amount of Y2O3 in this phase.
Figure 3.
Measurements performed at room temperature, after the heat treatments. Some researchers have reported a TC consisting exclusively of the tetragonal phase [31] and others have found considerable amounts of the monoclinic phase in the as-sprayed condition [34,36]. It can be observed that the monoclinic phase appears in the sample treated for hours. The tetragonal phase decreases from At hours, the monoclinic phase rapidly starts to form and, after hours, it reaches This is observed both in.
This expression was derived by H. Scott in [22], based on the change of lattice parameters of Yttria Stabilized Zirconia powders with different YO1. Results and analysis In the as-sprayed condition, the TC was a mixture of In this condition, the Y2O3 content in the tetragonal phase was 7. Figure 4. Figure 5. The peak corresponding to monoclinic phase first appears in samples treated for hours. Measurements were performed at room temperature, after the heat treatments.
Figure 6. The uncertainty in the phase content was determined using the standard deviation. The uncertainty value was around 0. Then, the cubic phase growth proceeds at a slower rate. After hours, the monoclinic phase increases quickly, overpassing the cubic content before hours. However, for crystallographic purposes, both tetragonal structures can be analyzed as the same tetragonal polymorph in the zirconia solid solution [28,38].
As can be observed in Fig. After hours, the Y2O3 content in t' phase decreases to 6. Then, the Y-depleted tetragonal phase transforms to a monoclinic phase during cooling and the monoclinic phase becomes more stable, favored by the Yttrium reduction in the tetragonal phase. Another factor which promotes the tetragonal-to-monoclinic phase transformation is the grain size [36,39,40]. From thermodynamic formulations, some researchers [39] have found that the surface energy of the tetragonal phase is lower than that of the monoclinic phase for a grain size smaller than nm. Then, for a grain size smaller than nm, the tetragonal phase is more stable.
In addition, greater grain sizes favor the diffusion rate through the grain boundaries [36]. Therefore, it can be concluded that the Yttrium diffusion from the tetragonal phase increases and the amount of monoclinic phase formed from the Yttriumdepleted tetragonal phase increases. The results of Rietveld measurements performed in the samples treated at different temperatures for 50 hours are presented in Fig. The cubic phase increases slightly around 3 wt.