Limit between stages I and II of a seed imbibition curve

O conhecimento das fases de absorção de água das diferentes espécies é importante em pesquisas objetivando melhorar a qualidade das sementes, utilizando tratamentos como condicionamento osmótico, pré-hidratação e uso de bioreguladores. O objetivo do presente trabalho foi estabelecer uma metodologia para determinar o limite entre a primeira e a segunda fase do processo, considerando o modelo W = f(t) - (a - w0)exp(-kt), utilizando testes estatísticos. O limite entre as duas primeiras fases do processo de absorção de água pelas sementes, foi determinado utilizando-se a distribuição assintótica de uma função de estimadores. O ponto a partir do qual esta diferença (W*) deixa de ser significativa foi determinado utilizando-se o teste estatístico T de Student. Para os dados utilizados como exemplo, tem-se o modelo ou =(0,434 + 0,00162 t) - (0,434 - w o)exp(-0,121 t), com r² = 0,98 e W* = (0,434 - w o) exp(-0,121 t). O valor de t encontrado (27,2 horas) é menor do que o valor determinado, considerando-se como critério para mudança de fase a diferença de 1% entre a assíntota e o valor estimado pelo modelo ajustado. Essa diferença de duas horas corresponde a 0,28% de água absorvida.

Sintering of tin oxide and its applications in electronics and processing of high purity optical glasses

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Addition of sedimentary rock to kaolinitic clays: influence on sintering process

Foram avaliadas, durante o processo de sinterização, as propriedades mecânicas de peças cerâmicas a base de argila com adição de rocha sedimentar. Foram preparados corpos de prova com 0, 20, 40, 60 e 80% em peso de rocha adicionada ao material argiloso. As peças foram sinterizadas nas temperaturas de 500, 800, 900, 1000, 1100 e 1200 °C e, posteriormente, submetidas à análise de difração de raios X e a ensaios tecnológicos Os resultados de difração de raios X mostram que a rocha sedimentar apresenta argilominerais micáceos enquanto o material argiloso possui a caulinita como fase principal. Técnicas de análises térmicas e difração de raios X das diferentes misturas mostram reações que indicam transformação (inversão do quartzo), decomposição (perda de hidróxidos) e formação de fase (mulita) durante o aquecimento das amostras. Os ensaios tecnológicos mostram que a adição da rocha sedimentar melhora algumas propriedades do material sinterizado, auxiliada pela presença de fundentes. Entretanto, a presença de quartzo na rocha dificulta a formação da fase mulita. A formação de novas fases e as transformações ocorridas no aquecimento e resfriamento das amostras ajuda explicar as propriedades tecnológicas dos materiais cerâmicos.

On Ti-18Si-6B and Ti-7.5Si-22.5B powder alloys prepared by high-energy ball milling and sintering

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Synthesis and sintering of PZT ceramics

Lead zirconate powder, with Zr/Ti ratio of 50/50 was prepared by polymeric precursor method and doped with 3, 5 and 7 mol% of Sr+2 Or Ba+2, as well as by 0.2 to 5 mol% of Nb+5. The powder was calcined at 750 degrees C by 4 hours and milled during 1.5 h in isopropilic alcohol. Powders were characterized by surface area measurements (BET method), by infrared spectroscopy and by X-ray diffraction to characterize the crystal structure. Isostatically pressed samples were sintered in a dilatometer furnace by using a constant heating rate of 10 degrees C/min from ambient to 1200 degrees C. Synthetic air and air with water vapor were used as atmospheres. Both Sr+2 and Ba+2 substitute Pb+2 and favor the formation of rhombohedral phase. Otherwise, Nb+5 substitute preferentially Zr+4 favoring tetragonal phase. The concentration of dopants and the atmosphere influence the densification and the microstructure of the PZT, which alters the dielectric and piezoelectric properties of the ceramics.

Synthesis and sintering of ZrO2-CeO2 powder by use of polymeric precursor based on Pechini process

Nanocrystalline ZrO2-12 mol % CeO2 powders were synthesized using a polymeric precursor method based on the Pechini process. X-ray diffraction (XRD) patterns showed that the method was effective to synthesize tetragonal zirconia single-phase. The mean crystallite size attained ranges from 6 to 15 nm. The BET surface areas were relatively high reaching 97 m(2)/g. Studies by nitrogen adsorption/desorption on powders, dilatometry of the compacts, and transmission electron microscopy (TEM) of the powders, were also developed to verify the particles agglomeration state. Both citric acid : ethylene glycol ratio and calcination temperature affected the powder morphology, which influenced the sinterability and microstructure of the sintered material, as showed by scanning electron microscopy (SEM). (C) 2001 Kluwer Academic Publishers.

Study on the orientation degree of Pb1-xLaxTiO3 thin films by the rocking curve technique and its morphological aspects

Thin films of perovskite-type materials such as PbTiO3, BaTiO3, (Pb,La)TiO3, (Pb, La)(ZrTi)O-3, KNbO3, and Pb(Mg,Nb)03 have been attracting great interest for applications like non-volatile memories, ultrasonic sensors and optical devices. Thin film should be epitaxially grown or at least highly textured since the properties of this anisotropic material depend on the crystallographic orientation. For optical devices, in particular, an epitaxial thin film without defects are essential to reduce optical propagation losses. Pb1-xLaxTiO3 (PLT) where x=0, 13 and 27% thin films were prepared by a chemical method (polymeric precursors method), and deposited by the spin coating technique onto substrates of SrTiO3 (STO) and LaAlO3 (LAO). The films were then beat treated at 500 degrees C in a controlled atmosphere of 0,. The orientation degree of the thin films was obtained from rocking curve technique, by means of X-ray difftaction analysis. A microstructural study revealed that the films were crack-free, homogeneous and have low roughness. (c) 2006 Elsevier B.V. All rights reserved.

Effects of powder preparation and sintering procedure on microstructure and dielectric properties of PLZT ceramics

PLZT ceramics belong to one of the very important groups of functional materials that make a basis for the production of a large range of electronic devices. The microstructure and properties of ceramics depend on the powder preparation and thermal processing conditions. Various techniques have been used to obtain chemically homogeneous and fine starting powders. PLZT powders have been prepared by two different production routes: by a modified Pechini method, using a polymeric precursor method (PMM) and by a partial oxalate method. A two-step sintering process, including a hot pressing, was carried out at 1100 and 1200degreesC Distinct phases obtained during the sintering process have been investigated by SEM and EDS techniques and dielectric properties such as permittivity and dielectric loss were measured in a frequency range from 1 to 20 kHz.. A significant difference in microstructure and dielectric properties, depending on powder origin and sintering procedure, has been noticed.

Sintering of tin oxide using zinc oxide as a densification aid

The physicochemical electronic characteristics of SnO2 render it useful in many technical applications, including ceramic varistors, stable electrodes used in electric glass-melting furnaces and electrometallurgy of aluminum, transparent windows and chemical sensors. The use of ZnO as a sintering aid was explored in this study to obtain SnO2 as a dense ceramic. Compacts were obtained by mechanical mixing of oxides, isostatic pressing at 210 MPa and sintering in situ inside a dilatometer at heating rates of 10degreesC/min. The grain size and microstructure were investigated by scanning and transmission electron microscopy (SEM/TEM). The phases and chemical composition were analyzed by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results indicated that ZnO acts as a densification aid for SnO2, improving its grain growth with additions of up to 2 mol%. ZnO forms a solid solution with SnO2 UP to 1 mol%, above which SnZnO3 precipitates in the grain boundary, potentially inhibiting shrinkage and grain growth. (C) 2004 Kluwer Academic Publishers

Microstructural evolution during sintering of CoO doped SnO2 ceramics

Addition of 0.5 mol% of CoO into SnO2 promotes densification of this oxide to 99% of the theoretical density during sintering. TEM in this system reveals that after sintering at 1210 degrees C a secondary phase of Co2SnO4 is precipitated at the SnO2 grain boundaries during cooling. This phase is formed by diffusion of Co ions from the bulk to the grain boundary during sintering leaving needle-like defects at the grain bulk. The high resolution TEM micrograph of this system sintered at 1210 degrees C and 1400 degrees C showed an amorphous grain boundary region low in cobalt, indicating that the Co2SnO4 phase is precipitated from this region. (C) 1999 Elsevier B.V. Limited and Techna S.r.l. All rights reserved.

The effect of ZnO on the sintering and stabilization of ZrO2 center dot MgO system

The sintering of ZrO2. MgO . ZnO powder has been investigated by TMA (Thermal Mechanical Analyser) and its phases analysed by XRD (X-ray diffraction pattern). The data obtained from sintering was studied by the Bannister equation and its dominant sintering mechanism was calculated. It was observed that the ZnO addition in the ZrO2. MgO solid solution lead to increased zirconia stabilization, According to the vacancies model, the ZnO addition did not lead to zirconia phases stabilization (PSZ). An analysis of the rate control in the initial stage of the sintering (region I) showed a mechanism of volume diffusion type. In other regions (regions II and III), the grain growth did lead to the Bannister equation deviation, which was observed by SEM (Scanning Electron Microscopy). These results were different from those demonstrated by other authors who studied the ZrO2. Y2O3 solid solution and obtained a mechanism of grain boundary diffusion type. (C) 1999 Published by Elsevier B.V. Ltd and Techna S.r.l. All rights reserved.

Sintering of ultrafine undoped SnO2 powder

The sintering process of nanometric undoped SnO2 powder was studied. No macroscopic shrinkage was observed during the sintening process. Grain growth kinetics investigation showed that surface diffusion is the dominant mechanism in the temperature range 500-1300 degreesC. For temperatures higher than 1300 degreesC, high weight loss was measured, suggesting evaporation-condensation as the dominant mass-transport mechanism. Thermogravimetric analysis (TG) and mass spectroscopy studies showed that the surface contamination of the SnO2 particles by chemical species like H2O, OH- and CO2, has a strong influence on the role of mass transport controlled by surface diffusion. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Sintering mechanisms of ZrO2 center dot MgO with addition of TiO2 and CuO

Substitutions of Ti and Cu in ZrO2.MgO (Z), cause transformation from monoclinic (m) to cubic (c) and tetragonal (t). According to the vacancy model and solid Solution formation models, neither CuO nor TiO2 cause zirconia stabilization, which derives front other phenomena. Data analysis by TMA using the CRH (constant rate of heating) method shows a solid state reaction of ZrO2.MgO.TiO2 (Z.TiO2) demonstrating a dominant mechanism of volume diffusion (n = 1). However, the sintering of ZrO2.MgO.CuO (Z.CuO) shows a viscous flow mechanism (n = 0), a similar phenomena to that of by sintering of glass. Transformations, such as: CuO to Cu2O at 1000 degreesC, ZrO2 (m) to ZrO2 (t) at 1100 degreesC and Cu2O (s) to Cu2O (l) at 1230 degreesC cause successive rearrangements of microstructure inside of region I (sintering process) and lead to interpretation errors when the Bannister equation is used. (C) 2003 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Speedup and scalability analysis of Master-Slave applications on large heterogeneous clusters

Although cluster environments have an enormous potential processing power, real applications that take advantage of this power remain an elusive goal. This is due, in part, to the lack of understanding about the characteristics of the applications best suited for these environments. This paper focuses on Master/Slave applications for large heterogeneous clusters. It defines application, cluster and execution models to derive an analytic expression for the execution time. It defines speedup and derives speedup bounds based on the inherent parallelism of the application and the aggregated computing power of the cluster. The paper derives an analytical expression for efficiency and uses it to define scalability of the algorithm-cluster combination based on the isoefficiency metric. Furthermore, the paper establishes necessary and sufficient conditions for an algorithm-cluster combination to be scalable which are easy to verify and use in practice. Finally, it covers the impact of network contention as the number of processors grow. (C) 2007 Elsevier B.V. All rights reserved.