DEM - Departamento de Engenharia Mecânica

URI Permanente desta comunidadehttps://repositorio.ufrn.br/handle/1/131

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The effect of pulse width on the microstructure of d.c.-plasma-nitrided layers
(Elsevier, 1999-12-15) Alves Júnior, Clodomiro; Rodrigues, José de Anchieta; Martinelli, Antonio Eduardo
Modern ion nitriding equipment are powered by pulsed sources in order to prevent arcing and overheating. In industrial applications, the pulse frequency is usually adjusted to set the working temperature. However, frequency variations may drastically affect the plasma species population, and consequently, the microstructure of the final nitrided layer. In this study, AISI-409 steel was nitrided in a mixture of 20% N2–80% H2 using a d.c power source. The depth and microstructure of the nitrided layer were assessed as a function of the time that the voltage was off, toff, in the range of 1–4 ms during a 10 ms pulse cycle. The results suggested that both the microstructure and mechanical behavior of the nitrided layer were affected by toff. Long toff values imply a deleterious effect on the mechanical properties of the nitrided layers
Growth of nitrided layers on Fe–Cr alloys
(Elsevier, 2000-02-29) Alves Júnior, Clodomiro; Rodrigues, José de Anchieta; Martinelli, Antonio Eduardo
Chromium is an important alloying element present in numerous commercial steels. A systematic study on the nitriding behavior of Fe–Cr alloys is helpful in predicting the properties of nitrided Cr-alloyed steels. Aspects such as microstructural evolution, growth kinetics, and mechanical properties should be particularly emphasized. Fe–Cr alloys containing 5, 10, and 20 wt.% Cr have been arc melted and subsequently plasma nitrided under a N2–80% H2 atmosphere. The microstructure of the resulting nitrided layers was characterized and the microhardness profiles evaluated. Thicker layers developed on low chromium alloys. Differences in hardness profiles were also observed as a function of chromium contents. Nitriding Fe–5% Cr alloys resulted in two discrete fronts, refereed to as the diffusion front and the transformation front. Transformed regions sustained a decrease in hardness from 1000 down to 600 HV, associated with the conversion of homogeneously dispersed fine precipitates into coarser needle like particles immersed in the ferritic matrix. Similar behavior was not observed for the other alloys, where both fronts developed simultaneously
Effect of workpiece geometry on the uniformity of nitrided layers
(Elsevier, 2001-05-01) Alves Júnior, Clodomiro; Silva, Eronildes Felisberto da; Martinelli, Antonio Eduardo
The growth behavior of plasma-nitrided layers on workpieces with complex geometry was systematically investigated. AISI 316 stainless steel pellets with different heights were nitrided under a mixture of N2–80% H2 at different temperatures (673, 773 and 843 K) and pressures (100 and 500 Pa). Significant differences in thickness and hardness of the resulting nitrided layers were observed as a function of nitriding parameters. The thickness of nitrided layers increased with sample height, excepted those nitrided at 843 K. The diameter of eroded rings, commonly observed on nitrided samples, varied with coupon height. Changes in both layer thickness and eroded ring diameter are presently addressed based on the thermal balance and charge density that take place near the edges of the samples
Low-temperature synthesis of single-phase crystalline LaNiO3 perovskite via Pechini method
(Elsevier, 2002-03) Fernandes, Joana D. G.; Melo, Dulce Maria de Araújo; Zinner, Léa Barbieri; Salustiano, Clayton de Matos; Silva, Z. R.; Martinelli, Antonio Eduardo; Nasar, Marinalva Cerqueira; Alves Júnior, Clodomiro; Silva, Elson Longo da; Bernardi, Maria Inês Basso
LaNiO3 powders were prepared from polymeric precursors. The material was calcinated either under flowing oxygen or in air at temperatures ranging from 300 to 800 °C. The formation of new phases upon calcination was investigated using X-ray diffraction, thermal gravimetric analysis, Fourier-transform infrared spectroscopy and CHN analysis. The results revealed the formation of a single-phase powder having the perovskite structure at temperatures as low as 600 °C. The atmosphere did not significantly affect the final product
Nitriding of titanium disks and industrial dental implants using hollow cathode discharge
(Elsevier, 2005) Alves Júnior, Clodomiro; Guerra Neto, Custódio Leopoldino de Brito; Morais, Gustavo Henrique Silva de; Silva, Claudio Fernandes da; Hajek, Vaclav
Standard plasma nitriding of commercially pure Ti or various Ti alloys for human body implants (e.g., hip, knee, shoulder and ankle implants) has already proven useful. However, its use in dental implantology is rather limited due to high nitriding temperatures. The small dental implants of complex geometries are frequently distorted. To solve this problem and benefit from the ability of the plasma treatment to modify the implant surface properties (needed for faster osseointegration process), such as creating different surface topographies, increasing surface roughness, changing local chemical properties by formation of different phases, cleaning/sterilizing contaminated surface and increasing the surface wettability, the titanium disk samples and industrial dental implants were nitrided using a hollow cathode discharge (HCD) configuration of a plasma nitriding system in a 20% N2–H2 atmosphere at pressures of 150 or 250 Pa and temperatures ranging from 400 to 500 °C for 1 or 2 h. The topography of the samples was characterized by optical and electron microscopy. Phases were determined by X-ray diffraction. The surface roughness and wettability were also quantified. Nitrided layer formation having better stability, increased surface roughness and higher wettability has been observed for samples treated at 450 and 500 °C and at a pressure of 150 Pa. Industrially fabricated dental implants were then nitrided at 500 °C/150 Pa for 2 h. The results show capability of HCD in treating dental implants. A significant change in the surface texture and superior wettability of the plasma-treated dental implants, with no geometric distortions, have been observed
Electrochemical behavior of steel-FeNbC composites used in the production of oxygen
(ABM, ABC, ABPol, 2005-04) Silva, Ariadne de Souza; Martinelli, Antonio Eduardo; Scatena júnior, Hélio; Silva, Jorge Herbert Echude; Alves Júnior, Clodomiro; Távora, Michele Portela
The electrocatalytic properties of steel-FeNbC composites were evaluated after potentiostatic ageing with the purpose of establishing the potential use of such materials as electrodes for the industrial production of oxygen. The effect of processing parameters, involved in the production of the composites by powder metallurgy, such as milling conditions and compaction pressure, on the performance of the electrodes was also studied. The electrodes were characterized according to their electrical resistance in NaOH 1M solution, considering that the purpose of the application requires this value to be ~ 0.5 W.cm2. This parameter was evaluated from repetitive electrochemical measurements carried out from 0.7 V to 1.5 V (vs. Ag/AgCl) by electrode anodization. Changes in the electrochemical behavior of the electrodes, especially concerning the O2 evolution in alkaline medium, were studied as a function of the applied potential. The results revealed that the electrocatalytic activity of the electrodes increased upon ageing, probably as a consequence of the growth of oxide layers and/or increase in the conductivity of the material, resulting in current gain and corresponding decrease in the polarization resistance to 0.75 W cm2
Production of hydrogen from methane using pulsed plasma and simultaneous storage in titanium sheet
(Elsevier, 2006-01) Silva, C. F. da; Ishikawa, Tomaz; Santos, Sydney Ferreira; Alves Júnior, Clodomiro; Martinelli, Antonio Eduardo
High-frequency pulsed plasma (HFPP) was applied to produce hydrogen by reforming natural gas. The process is based on the molecular dissociation of the gas into different ionized species, such as H+ and H+2 . The plasma was generated applying a pulsed voltage in an atmosphere of methane. The efficiency of the system was optimized by adjusting the frequency of the pulsed plasma aiming at maximum selectivity to hydrogen. The reforming of natural gas was accompanied by the simultaneous storage of hydrogen in a hollow cathode set-up made of titanium. Hydrogen contents higher than 900 ppm (in weight%) were stored in the Ti cathode. The efficiency of the method was compatible to industrial applications. Pyrolitic carbon condensed on the lower walls of the reactor but did not interfere with the kinetics of hydrogen absorption. In fact, the carbon produced in such manner could be considered a byproduct of the process and can be used as natural gas adsorbent once adequately activated
Surface modification of Ti implants by plasma oxidation in hollow cathode discharge
(Elsevier, 2006-01-24) Silva, M. A. M.; Martinelli, Antonio Eduardo; Alves Júnior, Clodomiro; Nascimento, Rubens Maribondo do; Távora, Michele Portela; Vilar, Carlos da Silva
As a result of superior biocompatibility and mechanical adequacy, titanium has been widely used in the manufacture of dental implants. On the downside, relatively long periods are normally required to fully integration of the bone into the implant. Surface modification techniques have the potential to shorten the osseointegration time of implants significantly contributing to patient comfort. In this work, plasma discharge in oxidizing atmospheres was used to modify the surface of Ti implants by the production of rough surfaces that consists of a mixture of Ti-oxides. The results showed that considerable improvement on surface roughness and mechanical stability of the oxidized layers could be achieved by confining the effect of the plasma discharge by shielding the cathodic region of the reactor. Processing parameters including temperature, time and pressure were optimized and applied to commercially available Ti implants. Improved wetting was obtained, which is potentially associated to shorter osseointegration periods
Effect of cooling rate on properties of plasma nitrided AISI 1010 steel
(Elsevier, 2007-02-15) Alves Júnior, Clodomiro; Lima, José de Anchieta; Hajek, Vaclav; Cunha, João Batista Marimon; Santos, Carlos Alberto
Application of Monte Carlo method in the solution of radiation heat transfer in participating media
(Revista da Engenharia Térmica, 2007-06-30) Maurente, André Jesus Soares; Bayer, P. O; França, Francis Henrique Ramos
The temperatures of the gases produced in combustion processes are very high so thermal radiation constitutes an important heat transfer mechanism in industrial furnaces. Most furnaces can be modeled as gray enclosures containing non-gray gases. The radiation heat transfer can be obtained with the aid of the weighted-sum-of-gray-gases model, determining the zonal exchange areas for each of the gray gases considered in the sum. For some enclosures with simple geometries, there are correlations to obtain the direct exchange-areas which can be used to determine the total exchange areas. However, for enclosures with complex geometries, determining the direct exchange areas can become a difficult task. In this case, the use of the Monte Carlo method is advantageous, for it allows one to approach geometric complexities without additional complications. Therefore, the method was applied to compute total exchange areas in enclosures containing participating media. Two cases were considered: cylindrical enclosures and enclosures that have generic geometries formed from cube combinations. The results presented a good agreement with solutions available in the literature
A Monte Carlo implementation to solve radiation heat transfer in non-uniform media with spectrally dependent properties
(Elsevier, 2007-11) Maurente, André Jesus Soares; Vielmo, Horácio Antonio; França, Francis Henrique Ramos
This paper presents the application of the Monte Carlo method to solve the radiative heat exchange in non- homogeneous, non-isothermal gases with spectrally dependent properties. Among others models, the absorption-line blackbody (ALB) distribution function, originally defined and derived for the spectral line-based weighted-sum-of-gray-gases (SLW) model, allows an immediate, simple implementation of the Monte Carlo method to account the spectral dependence of the radiative properties. This work shows how the Monte Carlo method can be combined to the ALB distribution function, and provides results for heat transfer in a mixture of water vapor, carbon dioxide and nitrogen that have satisfactory agreement with the SLW method and with line-by-line integration. Finally, the solution technique is employed to solve two examples aiming at demonstrating the effect of the absorbing species concentration on the thermal radiative exchanges. The method is of great interest for the computation of radiative transfer in combustion systems where the chemical species concentration and the temperature are not uniform
Caracterização de filmes de poliéster modificados por plasma de O2 a baixa pressão
(Universidade Federal do Rio de Janeiro, 2008-05-12) Costa, Thércio Henrique de Carvalho; Feitor, Michelle Cequeira; Alves Junior, Clodomiro; Bezerra, Clovis de Medeiros
Filmes de poliéster possuem um vasto campo de aplicação, devido algumas propriedades que são inerentes desse tipo de material, tais como boa resistência mecânica, resistência química a ácidos e bases e baixo custo de produção. Entretanto possuem limitações, como baixa tensão superficial, baixa rugosa, pouca afinidade com corantes, e pouca adesão, que impedem o uso dos mesmos para algumas finalidades principalmente quando esse uso requer que o material possua uma boa molhabilidade. Dentre as varias técnicas existentes para aumentar a tensão superficial, as técnicas que utilizam o plasma, como fonte energética, são as mais promissoras pela versatilidade e por não apresentar agentes nocivos ao meio. Então, tendo em vista a boa aceitabilidade do uso do plasma em materiais poliméricos, trataram-se filmes de poliéster usando plasma com atmosfera gasosa de oxigênio variando-se o tempo de tratamento de 10 - 60 min com incremento de 10 min a cada tratamento subseqüente. Após o tratamento por plasma as amostras foram caracterizadas por medidas de ângulo de contato, tensão superficial, espectroscopia Raman, Espectroscopia de infravermelho de reflexão total atenuada (IR-ATR), e microscopia de força atômica (AFM), onde foi caracterizado o aumento na molhabilidade dos filmes tratados por plasma bem como as variáveis que contribuíram para tal efeito. A técnica de plasma mostrou-se bastante eficiente na modificação de filmes de poliéster, uma vez que se obteve aumento na molhabilidade de todos os filmes tratados e sem nenhuma variação significativa com relação ao tempo de exposição das amostras pelo plasma, o que leva a propor que o tratamento por plasma tanto contribuiu para o aumento da molhabilidade através da implementação de grupos funcionais na superfície dos mesmos como com a criação de rugosidade superficial nas amostras
Comparison of the standard weighted-sum-of-gray-gases with the absorption-line blackbody distribution function for the computation of radiative heat transfer in H2O/CO2 mixtures
(Elsevier, 2008-07) Maurente, André Jesus Soares; Vielmo, Horácio Antonio; França, Francis Henrique Ramos
This paper presents the computation of radiation heat transfer in a cylindrical enclosure in which the dimensions, the chemical species concentrations and the temperature fields make a realistic representation of an actual combustion chamber. Two gas models are applied and compared: the absorption-line blackbody distribution function (ALBDF), and the standard weighted-sum-of-gray-gases (WSGG) based on coefficients and correlations that are widely used in engineering. While the standard WSGG is restricted to the assumption of homogeneous gas mixture, the ALBDF can be applied to both homogeneous and non-homogeneous media. For the two gas models, the radiative exchanges are computed with the aid of the Monte Carlo method. The results show considerable discrepancies between the WSGG and the ALBDF models for the homogeneous medium. In addition, the importance of considering the non-homogeneity of the medium for an accurate computation of the radiative heat transfer is shown. r 2008 Elsevier Ltd. All rights reserved
Osseointegration evaluation of plasma nitrided titanium implants
(Surface Engineering, 2009) Guerra Neto, Custódio Leopoldino de Brito; Silva, Marco Aurélio Medeiros da; Alves Júnior, Clodomiro
There is a need for new techniques that modify metallic implant surfaces, resulting in good bone response for low density areas. Among the different techniques, plasma nitriding is being applied in biomedicine with excellent results. However, its use in titanium dental implants is very limited owing to high process temperatures (between 700 and 800°C) that result in distortions. In order to solve this problem, the authors used a new plasma method with a hollow cathode discharge configuration. Ti grade II dental implants were nitrided in a 20% N2–H2 atmosphere at a temperature of 450°C, 150 Pa (1·5 mbar), for 1 h and inserted into rabbit tibias to assess osseointegration. The results showed that plasma nitriding caused a significant change in the surface texture of the nitrided implants and greater removal torque than that of the control implants
Experimental study of plasma nitriding dental implant surfaces
(Taylor and Francis, 2009) Guerra Neto, Custódio Leopoldino de Brito; Silva, Marco Aurélio Medeiros da; Alves Júnior, Clodomiro
Titanium samples have been used as the substrate that was submitted to a plasma discharge nitriding for the surface treatment of dental implants. Two different arrangements regarding the electrode shapes were tested: planar and hollow cathode. The treated samples were evaluated regarding surface phases, texture, roughness, layer thickness, wettability and visual appearance. The roughness of the surface of all samples submitted to the plasma nitriding process was lower compared to those treated by commercially available technology plasma spray, on the other hand, better wettability was achieved. Some commercially available implants were also treated under the best conditions found with the experiment at titanium samples. After plasma nitriding these implants, tests were carried out to evaluate both wettability and surface texture. As a result the treated implants had both their surface texture and wettability improved
In vitro study of cell behaviour on plasma surface modified titanium
(Taylor and Francis, 2009) Guerra Neto, Custódio Leopoldino de Brito; Silva, Marco Aurélio Medeiros da; Alves Júnior, Clodomiro
In recent years, several technologies that modify implant surfaces have been emerged. Among these techniques, the plasma nitriding process has been successfully applied in biomedical field. Nevertheless, its use in dental implants is quite limited owing to the high temperatures of the process (between 700 and 800°C), which causes distortion. In order to solve this problem, a new approach is proposed in the present paper, by which nitriding under a hollow cathode discharge is used to modify surfaces. Grade II Ti plates were submitted to nitriding under hollow cathode discharge conditions and treated at a temperature 450°C and pressure of 150 Pa for 1 h. These showed that plasma nitriding helped bring about a significant change in the surface texture of the treated plates. Furthermore, cell proliferation was 2⋅5 times as high as that of the untreated plates
Nitriding in cathodic cage of stainless steel AISI 316: inﬂuence of sample position
(Vacuum, 2009) Sousa, R. R. M. de; Araujo, F. O. de; Costa, J. A. P. da; Dumelow, T.; Oliveira, R. S. de; Alves Jr, C.
Análise do coeficiente de perfomance de um chiller doméstico operando com o R-401A em regime transiente
(Instituto Federal do Rio Grande do Norte - IFRN, 2009) Medeiros, Pedro Samuel Gomes; Barbosa, Cleiton Rubens Formiga
Este artigo apresenta uma análise do coeficiente de performance (COP) de um chiller doméstico quando operado em regime transiente. Os dados foram coletados por instrumentos medidores de pressão e temperatura, instalados em pontos estratégicos no equipamento. A análise do COP foi realizada pelo software CoolPack, onde foi feito também um comparativo entre o COP do ciclo ideal e do ciclo real a partir de gráficos pressão versus entalpia específica. Os resultados mostram que o incremento do COP é devido ao aumento considerável da pressão de evaporação do ciclo e que os valores da taxa de incremento são aproximadamente os mesmos entre o ciclo real e o ideal. Através deste trabalho é possível obter conclusões a respeito do comportamento de um ciclo de refrigeração durante o processo de resfriamento a partir do fluido refrigerante utilizado
Análise exergética do ciclo transcrítico de um refrigerador de garrafas operando com CO2
(Instituto Federal de Educação, Ciência e Tecnologia do Rio Grande do Norte (IFRN), 2009) Almeida, Igor Marcel Gomes; Barbosa, Cleiton Rubens Formiga; Fontes, Francisco de Assis Oliveira
A segunda lei da termodinâmica trata da qualidade da energia. Mais especificamente, ela está preocupada com a degradação de energia durante um processo, a geração de entropia, e as perdas de oportunidade da realização de trabalho, o que oferece muita margem para melhorias. O objetivo deste trabalho é identificar os principais fatores que afetam a performance do sistema de refrigeração com CO2. Devido ao impacto global dos CFC´s e HFC´s o uso de refrigerantes naturais tem recebido atenção mundial. O refrigerante natural, dióxido de carbono (CO2/R744) é promissor para uso em sistemas de refrigeração, especialmente no ciclo transcrítico. Uma análise exergética através do ciclo de um refrigerador de garrafas (expositor) adaptado para uso comercial com dióxido de carbono foi realizada onde as efetividades dos componentes do sistema foram estimadas e classificadas, permitindo direcionar os esforços para o aperfeiçoamento dos componentes do ciclo transcrítico. A análise revelou que o compressor e a válvula de expansão são as grandes fontes de perdas no sistema e, os esforços devem ser direcionados para o aprimoramento destes componentes
A Comparative Study of Mechanical and Tribological Properties of AISI-304 and AISI-316 Submitted to Glow Discharge Nitriding
(Materials Research, 2009-04-17) Nascimento, Fabiana Cristina; Foerster, Carlos Eugênio; Silva, Silvio Luiz Rutz da; Lepienskia, Carlos Mauricio; Siqueira, Carlos José de Mesquita; Alves Júnior, Clodomiro

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