Showing papers in "Journal of Materials Science Letters in 2002"

Carbon dioxide absorption by lithium orthosilicate in a wide range of temperature and carbon dioxide concentrations

Abstract: The CO 2 absorption properties of lithium orthosilicate (Li 4 SiO 4 ) were evaluated and were compared with those of Li 2 ZrO 3

Conductivity measurement of electrospun PAN-based carbon nanofiber

Abstract: Although electrostatic generation, or electrospinning, of ultrafine fibers was invented as early as in 1930s [1], the technique has been used to produce conductive polymer fibers only recently [2]. Reneker and Chun et al. [3] electrospun polyaniline fibers from sulfuric acid into a coagulation bath. Chun et al. [4] electrospun polyacrylonitrile (PAN) nanofibers, and pyrolyzed them into carbon nanofibers. Because of their obvious high specific surface area, electrospun ultrafine fibers are expected to be used as high performance filters, or scaffolds in tissue engineering [5]. For the same reason, one may expect that chemisorbed gases may modulate the electrical conductivity, leading to the fabrication of sensor devices. Unfortunately, the electrical transport properties of these electrodeposited materials have caught the interest of few researchers except for Norris et al. [5], who used the indirect four-point probe method to measure the conductivity of the electrospun non-woven ultra-fiber mat of polyaniline doped with camphorsulfonic acid blended with polyethylene oxide (PEO). As the non-woven mat is highly porous and the “fill factor” of the fibers is less than that of a cast film, the measured conductivity tends to be lower than that of bulk [5]. In this paper, a direct method is used to measure the I -V curve and conductivity of the electrospun PAN-based carbon nanofibers. Commercial PAN powder and N,N-Dimethyl Formamide (DMF), in a ratio of 600 mg PAN to 10−5 m3 DMF, were used to prepare a solution. This mixture was vigorously stirred by an electromagnetically driven magnet at room temperature before it became a homogeneous polymer solution. The electrospinning was conducted in a homemade setup shown in Fig. 1. The DC power supply was an ES30-0.1P Model HV

Effects of the grain size on the corrosion behavior of refined AISI 304 austenitic stainless steels

Abstract: Although there have been many studies on fine grained ferritic steels, only a few research reports are available on refined austenitic stainless steels and, in particular, on the influence of the grain size on the corrosion resistance of this class of material [1, 2]. The grain size of ferritic steels can be easily induced by phase transformation, but in austenitic alloys, following the absence of a phase transformation, the grain diameter is usually controlled by recrystallization after cold working [3]. This method is mainly affected by the working temperature, amount of deformation and recrystallization temperature. Recrystallization after hot rolling is reported to have the effect of grain refining [4] but this method seems to be limited. In a previous paper [5] we examined the effect of subzero working on the grain refining of austenitic stainless steels. In particular, ultrafine grained AISI 304 stainless steel of ca. 1 μm average grain size was obtained by applying the reverse transformation of martensite to austenite on subzeroworked steel annealed at low temperatures. Up to now, the corrosion behavior of such ultrafinegrained austenitic stainless steels has not been reported. This paper deals with the corrosion behavior, especially general corrosion (GC), intergranular corrosion (IGC) and pitting corrosion (PC) of ultrafine-grained AISI 304 stainless steel. Results are compared with those of similar measurements on standard AISI 304 steel. The chemical composition of the AISI 304 stainless steel, obtained from a commercial batch, is shown in Table I. After subzero working down to 90% thickness reduction, the material was subjected to the following four heat treatments in order to obtain different microstructures: annealing at 800 ◦C for 160 s and 900 s (specimens A and B respectively) and at 1000 ◦C for 10 s and 600 s (specimens C and D respectively). The grain sizes corresponding to the above specimens, as measured by automatic image analyzer, are shown in Table II. The typical microstructures of the 1 μm and 50μm specimens are shown in Fig. 1. Tensile properties of the specimens are shown in Fig. 2. Ultimate tensile stress and 0.2% yield stress increase with decreasing grain size, according to the Hall Petch relation [6]. Steel materials were machined into corrosion test specimens of 15 × 15 × 1 mm. The specimen surface was polished by using increasingly finer abrasive papers, starting with a 300 grit paper and finishing up with

Nano-CaCO3/polypropylene composites made with ultra-high-speed mixer

Abstract: In polymer-based nanocomposites, because of the large fraction of the nanoparticle atoms that reside at the interface, there would be a strong interface interaction between filler and polymer. The mechanical properties of nanocomposites would improve significantly at a low filler content if the ultrafine phase dimension was retained [1]. However, the homogeneous dispersion of nonaparticles is very difficult because nanoparticles with high surface energy are easy to agglomerate. To break up the agglomerates, studies have been carried out on the approaches of in situ polymerization of monomers in the presence of nanoparticles [2–4] and other intercalation polymerization techniques [5]. Although nanoparticles can be dispersed uniformly, the methods with complex processes, special conditions and high cost are limited to laboratory scale. Melt blending nanoparticles with polymer is the best compounding technique in the case of mass production of nanocomposites with low cost. In the literature [6–9], it is reported that the problem of melt blending is that nanoparticles tend to agglomerate due to their high surface energy during mixing even when the nanoparticle content in the composites is small. The key is to decrease the surface energy of the nanoparticles, however, the uniform surface coverage of nanoparticles is hard to realize by simply physical and mechanical means. In order to resolve this problem, an ultra-high-speed mixer has been designed to treat the surface of the nanoparticles [10]. The mixer has high speed (6000 rpm), special rotors that are designed to deal with nanoparticles. Under high shear force, the nanoparticle surface is modified with coupling agent, which is added by means of an atomizer. The purpose of this letter is to show how efficiently the mechanical performance of the nanocomposites can be improved by the above approach. Isotactic polypropylene (PP) homopolymer (melting flow index (MI) = 5g/10 min) was used as matrix and CaCO3 (diameter = 70 nm, density = 2400 kg/m3) was selected as the filler. Stearic acid was used as coupling agent. Nano-CaCO3 and stearic acid were mixed in the high speed mixer for 5 min. Because of high shear force, the temperature of mixtures could increase to 140 ◦C, and the powders were ready for compounding. PP and fillers were melt mixed in a twin-screw extruder at 200 ◦C and 100 rpm. The specimens for

Development of ultra fine grain structure by martensitic reversion in stainless steel

Abstract: Austenitic stainless steels have good corrosion resistance and good formability but they have also relative low yield strength. It is well known that the mechanical properties of austenitic stainless steels are very sensible to the chemical composition (which can induce hardening by both substitutional and interstitial solid solution) and to microstructural features (such as grain size and δ-ferrite content). Recently there have been commercial developments to exploit the effect of these variables in stainless steel taking advantage of changes in the chemical composition induced by nitrogen addition [1, 2]. Another effective way to increase yield strength without impairing good ductility is grain refining. Although this approach has induced the development of ultrafine grain carbon steels (e.g. [3]), no attempts have been still reported on this approach for austenitic stainless steels. In fact, austenitic stainless steels do not undergo phase transformation at typical annealing temperatures and then the only way to refine the grain is recrystallization after cold rolling. However, the strengthening by grain refining is limited, due to the high recrystallization temperature of this stainless steel grade. For instance, the recrystallization temperature of the AISI 301 steel is above 900 ◦C and the minimum grain size obtained is in the range 10–30 μm [4]. In austenitic stainless steels, plastic deformation of austenite creates the proper defect structure which acts as embryo for martensite deformation: the successive reversion of deformation-induced martensite (α′) enables a marked grain refining [5, 6]. In this letter the production of an ultra fine microstructure in an AISI 301 stainless steel by martensitic reversion is reported. The chemical composition of the steel used is shown in Table I. The procedure used to refine the grain is the following (see Fig. 1): • Metastable γ is almost entirely transformed to α′ by heavy cold rolling: in fact the retained γ cannot be refined during the subsequent annealing. • α′ reverts to recrystallized austenite γR during annealing at low temperature.

Complete set of material constants of 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3 domain engineered single crystal

Abstract: The relaxor based Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZNPT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) domain engineered single crystal systems exhibit superior electromechanical property compared to the conventional PZT ceramics, which have been dominating piezoelectric applications for more than 40 years. The theoretical studies for the physical mechanism of this multi-domain system have generated a great deal of interests in scientific community [1–6]. From both fundamental study and device design purpose, it is necessary to get the complete set of matrix properties for those crystals. Recently, such a complete set of elastic, piezoelectric and dielectric constants has been reported for 0.95Pb(Zn1/3Nb2/3)O3-0.045PbTiO3 (PZN-4.5%PT) and 0.92Pb(Zn1/3Nb2/3)O3-0.08PbTiO3 (PZN-8.0%PT) single crystal systems [7–9]. It is known that PZT4.5%PT single crystal system [2] is fairly far away from the morphotropic phase boundary (MPB) composition, therefore, it does not represent the maximum electromechanical capability of this solid solution system. It has been shown that the PZN-8.0%PT single crystal system possesses much larger piezoelectric strain constant d33 than that of PZT-4.5%PT [9]. The electromechanical coupling coefficient k33 of PZN-8.0%PT could reach 0.94, which is higher than that of PZT-4.5%PT. However, PZN-PT single crystal systems near the MPB composition exhibit strong property fluctuations. We have measured several PZN-8.0%PT samples cut from different parts of the same crystal boule and found that although the fluctuation in PT composition of all samples were within ±2%, the measured value of d33 varied from 2000 to 4000 (pC/N) and the value of eT 33 varied from 5000 to 8000, which makes PZN-8.0%PT single crystal unsuitable for many practical device applications. On the other hand, it is very difficult to control the composition accurately because of the lead loss during growth. Therefore, we focused our attention to 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3 (PZN-7.0%PT) which has the PT composition slightly away from the morphotropic phase boundary and is in the rhombohedral phase at room temperature. A full set of matrix properties of PZN-7.0%PT was measured and is reported in this paper. The electromechanical coupling property fluctuation was also investigated as a reference for people who will use these material properties. After being poled along [001] of the cubic coordinates at room temperature, the multi-domain PZN-7.0%PT single crystal could be treated as pseudotetragonal 4 mm symmetry [1–10]. For materials with 4 mm symmetry, there are 11 independent material constants altogether: 6 elastic, 3 piezoelectric and 2 dielectric constants. In order to determine them unambiguously, an improved hybrid method combining the advantages of ultrasonic pulse-echo and resonance methods was used. The main ideas of this technique have been given in Refs. [8–11]. In previous measurements, usually e31 and e33 were derived from the measured cE 11, c E 12, c E 13, c E 33, d31 and d33 by using the following equations

Deposition efficiency, mechanical properties and coating roughness in cold-sprayed titanium

Abstract: In the cold-spray process, metal powder particles develop into a coating as a result of ballistic impingment on a substrate. In cold-spray, compressed gas (air, nitrogen or helium), at pressures ranging between 1.4– 3.4 MPa (200–500 psi), but typically around 1.7 MPa (250 psi), flows through a manifold system containing a gas heater and a powder feeder. The pressurized gas is heated electrically to around 100–600 ◦C then passed through a Laval-type converging/diverging nozzle until the gas velocities reach supersonic speeds. The powder particles are introduced into the gas stream just in front of the converging section of the nozzle and are accelerated by the expanding gas. The powder feedstock is delivered on the high-pressure side of the nozzle by the metering device, which is heated and maintained at the elevated pressure of the manifold. During the supersonic expansion through the Laval nozzle, there is a temperature reduction. Thus, the temperature of the gas stream is always below the melting point of the particulate material, providing coatings developed primarily from particles in the solid state with very little oxidation [1–5]. As cold-spray is a 100% solid-state process, the deposition “in air” of titanium coatings without significant oxidation represent an important technical achievement. Titanium and its alloys are employed in corrosive environments, aerospace and bio-implants [6]. Beyond the solid-state characteristic, a fundamental feature of the cold-spray method is the concept of critical velocity (V ∗). For each coating and substrate combination there is a V ∗. Above the V ∗ the particles will have enough kinetic energy to be incorporated into a coating. Below the V ∗, the particles will be either reflected from the surface (bounced-off) or cause erosion of the substrate and any coating buildup which had begun. For particle velocities V > V ∗, the coating process occurs and the deposition efficiency is seen to increase with increasing V [1, 4, 5]. The actual mechanisms by which the solid-state particles deform and bond has not been well characterized. It seems plausible, though it has not yet been demonstrated, that plastic deformation may disrupt thin surface films, such as oxides, and provide intimate conformal contact under high local pressure, thus per-

Empirical formula of isothermal bainite start temperature of steels

Abstract: Bainitic transformation has the upper limit of isothermal transformation temperature, so-called Bs temperature. There are only a few articles about the relationship between steel chemistry and Bs temperature, while many formulae for Ms temperature have been proposed. Steven and Haynes [1] first suggested a simple equation to predict the Bs temperature as a function of chemistry using British Steels data, which are isothermal transformation diagrams of the steels with two separate curves between pearlite and bainite and clear Bs temperature. Their formula is:

Organic-inorganic sol-gel coating for corrosion protection of stainless steel

Abstract: One of the most effective corrosion control techniques is the electrical isolation of the anode from the cathode [1, 2]. The chromium oxide (Cr2O3) passivation layer formed on the surface of stainless steel in oxidizing environments is one example. This is the main reason for the durability and corrosion resistance behavior of this particular metal [2, 3]. A more generic approach to enhance corrosion resistance is to apply protective films or coatings. Through the modification of chemical composition of the coatings, such protective coatings can also permit the introduction of other desired chemical and physical properties, such as mechanical strength and hydrophobicity. Various organic coatings have been studied for corrosion protection [4–6]. Specifically, various oxide coatings by sol-gel processing have been studied extensively for corrosion protection of stainless steel [9–13]. In spite of all the advantages of sol-gel processing, sol-gel oxide coatings suffer from several drawbacks. In general, sol-gel coatings are highly porous with low mechanical integrity; annealing or sintering at high temperatures (>800 ◦C) is required to achieve a dense microstructure [14–17]. Consequently, sintering at high temperatures might introduce cracks and/or delamination of sol-gel coatings due to a large mismatch of thermal expansion coefficients and possible chemical reactions at the interface. Sintering at high temperatures also limits application of sol-gel coatings on temperature sensitive substrates and devices. One viable approach to dense, sol-gel-derived coatings without post-deposition annealing at elevated temperatures is to synthesize organic-inorganic hybrid coatings. When appropriate chemical composition and processing conditions are applied, relatively dense organic-inorganic hybrid coatings can be developed for applications, including wear resistance [18, 19] and corrosion protection [20–22]. Messaddeq et al. [21] studied corrosion resistance of organic-inorganic hybrid coatings on stainless steel. The coatings were made by dispersing various amounts of polymethylmethacrylate (PMMA) into zirconia (ZrO2) sol and fired at 200 ◦C for 30 min. PMMA-ZrO2 coatings demonstrated promising corrosion resistance and increased the lifetime of the stainless steel by a factor 30 [21]. However, phase segregation, incomplete coverage, and delamination were observed when the coatings consisted of a high content of organic components. In this paper, we studied the corrosion resistance of sol-gel-derived, organic-inorganic hybrid single-layer coatings on two types of stainless steel. Sol-gel-derived coatings were made from tetraethylorthosilicate (TEOS) and 3-methacryloxypropyltrimethoxysilane (MPS) using a two-step acid catalysis process, and were annealed at 300 ◦C for 30 min. It was demonstrated that sol-gel derived hybrid coatings could significantly enhance the corrosion protection of both 304 and 316 stainless steel substrates. Furthermore, the corrosion resistance behavior of the hybrid coatings on both types of stainless steel was compared and possible mechanisms were discussed. The silica-based organic-inorganic hybrid sol was prepared with an acid-catalyzed, two-step hydrolysiscondensation process. The hybrid sol was prepared by admixing a silica precursor, tetraethylorthosilicate (TEOS, Si(OC2H5)4), and an organic component, 3-methacryloxypropyltrimethoxysilane (MPS, H2CC (CH3)CO2(CH2)3Si(OCH3)3), to control the flexibility and density of the sol-gel network. Silica (SiO2) sol containing 10 mol% MPS with a TEOS : MPS ratio of 90 : 10 was used for analysis. An initial stock solution was made by adding amounts of TEOS and MPS in a mixture of ethanol (C2H5OH), deionized water (DI H2O), and 1N hydrochloric acid (HCl), resulting in a TEOS : MPS : C2H5 : DI-H2O : HCl nominal molar ratio of 0.90 : 0.10 : 3.8 : 5 : 4.8 × 10−3. The mixture was vigorously stirred at a rate of 500 RPM for 90 min at a temperature of 60 ◦C, and further processing of the sol required an additional 3.6 mL 1N HCl and 1.2 mL DI H2O to 30 mL of the stock solution. The sol was stirred again at a rate of 500 RPM for 60 min at a temperature of 60 ◦C. Ethanol was added to dilute the sol in order to obtain a volume ratio of 2 : 1 ethanol to solution. The substrates (10 mm × 40 mm in dimension) used for the analysis of the sol-gel coatings were 304 and 316 stainless steel that had been electropolished. The exposure of the substrates to nitric acid (HNO3) decreased the iron content and increased the chromium content

Effect of grain size on the corrosion resistance of a high nitrogen-low nickel austenitic stainless steel

Abstract: Nitrogen alloyed austenitic stainless steels exhibit attractive properties such as high levels of strength and ductility, good corrosion resistance and reduced tendency of grain boundary sensitization [1]. The high austenitic potential of nitrogen allows the nickel content in steel to be reduced, offering additional advantages such as cost saving. The production of these low nickel steels is made possible by the addition of manganese that increases the N solubility in the melt and decreases the tendency of Cr2N formation [2]. Although there have been many studies on finely grained ferritic steels (e.g. [3]), only a few research reports are available on refined austenitic stainless steels. The grain size of ferritic steels can be easily refined by phase transformation, but in austenitic alloys, due to the absence of phase transformation, the grain diameter is usually controlled by recrystallization after cold working [4]. In the last case the behavior of the material is affected mainly by the working temperature, working ratio and recrystallization temperature. Recrystallization after hot rolling is reported to have the effect of grain refining [5] but this method seems to be limited. In previous papers [6, 7], we examined the effect of subzero working on the grain refining of austenitic stainless steels. In particular, ultrafine grained AISI 304 stainless steel with an average grain size below 1 μm was obtained by applying the reverse transformation of martensite to austenite, on subzero-worked steel, annealed at low temperatures. Furthermore, a great increase both in the mechanical [6] and in the localized corrosion [8] resistance was found. In order to further increase the strength, it is possible to combine the effects of nitrogen addition and grain refining. In previous works we analyzed the effect of grain size on the mechanical properties [9] and on the wear resistance [10] of a high nitrogen austenitic stainless steel. This paper deals with the corrosion behavior, in particular general corrosion (GC), intergranular corrosion (IGC) and pitting corrosion (PC) of ultrafine-grained high nitrogen austenitic stainless steel. Results are then compared to those of similar measurements on standard AISI 304 steel. The chemical composition of the steel (hereinafter HN) and of the AISI 304 steel under consideration is shown in Table I. After cold working down to 80% thickness reduction, the material was subjected to four different heat

Glass-ceramics from mixtures of coal ash and soda-lime glass by the petrurgic method

Abstract: The glasses used in this study were prepared from mixtures of coal ash and soda-lime cullet glass. The paper deals with the effect of the cooling rate on the crystallization behavior in mixtures with different soda-lime glass contents.