Phosphoric Acid Bath Research Articles

Synthesis and Properties of s-PBI/2OH-PBI Random Copolymer for High Temperature PEM Fuel Cells

High temperature polymer electrolyte membrane fuel cell (HT PEMFC), which has the advantages of high tolerance to fuel impurities and simplified system according to no humidification, is currently being studied extensively. The phosphoric acid (PA) with proton conductivity at high temperature (150℃) is used as an electrolyte in HT PEMFC. Therefore, the affinity between membrane material and phosphoric acid and membrane durability are very important. At present, membrane material for HT PEMFC is mainly used the polymer based on polybenzimidazole (PBI). However, the PBI has the problems such as low molecular weight, low PA doping level, PA leakage during operation, and membrane durability. According to some studies, sulfonic acid groups and hydroxyl groups have high affinity for phosphoric acid. Therefore, PBI-based polymer electrolyte prepared from each monomer showed improved doping level, proton conductivity and performance. In this study, we tried to fabricate electrolyte membrane for HT PEMFC with improved properties and cell performance by simultaneously introducing these two functional groups. For the synthesis of s-PBI/2OH-PBI random copolymer, monosodium salt of 2-sulphoterephthalic acid (STA), 2,5-dihydroxyterephthalic acid (2OH-TA) and 3,3'- diaminobenzidine were prepared with ratio of 1:0:1 (10:0), 0.5:0.5:1 (5:5), 0.4:0.6:1 (4:6). The condensation reaction was used for synthesis of copolymer. The polymer membranes were fabricated by direct casting method. The polymer membrane of the fabricated film type was placed in the humidity chamber at each hydrolysis condition. The degree of polymerization of s-PBI/2OH-PBI random copolymer was confirmed by measuring the torque of the polymer solution in real time. The characteristics of the prepared copolymer were analyzed by FT-IR and doping level. Finally, single cell performance according to monomer ratio and hydrolysis condition was evaluated by IV curve measurement. The IV curves of s-PBI/2OH-PBI random copolymer were affected by monomer ratio and hydrolysis condition. First, the IV curves showed better performance as the monomer ratio of 2OH-TA increased in hydrolysis conditions of 50℃ and RH 50%. Despite the lowest doping level in the 4:6 monomer ratio, the highest cell performance in 4:6 was attributed to the improvement of the mechanical properties with the increase of polymerization degree. On the other hand, the IV curve at 4: 6 monomer ratio showed the best performance in RH 80% and 70% phosphoric acid bath condition. This confirmed that the polyphosphoric acid was only partially hydrolyzed in the RH 50% condition. Figure 1

Etching of Silicon Nitride in 3D NAND Structures

3D NAND structures present a unique difficulty in semiconductor device manufacturing. One method of production consists of cylindrical structures made of alternating layers of silicon dioxide (SiO2) and silicon nitride (SiN) with a core of SiO2. As part of the device manufacturing process the SiN is etched away, leaving the SiO2 core with disc-shaped SiO2 fins. Due to material selectivity requirements of etching only the SiN while leaving the SiO2 unchanged, the process of choice is often immersion in a high-concentration hot phosphoric acid (H3PO4) bath with a silica additive to inhibit SiO2 etching, which with proper tuning can have selectivity of greater than 100:1 for etching SiN:SiO2. Like all wet etch processes, uniformity of the process is a constant concern. One of our goals in this paper is to show how we use experimental measurements and simulations to better understand what parameters have the most effect on the process, and how we can use that knowledge to improve the process. Experimentally we have measured the etch rate of blanket SiN and SiO2 wafers. We fit the etch rate data to Arrhenius models to quantify the etch rate of SiN and SiO2 films. Simulations of the etch process on both blanket wafers and patterned wafers showed that the etch rate is the same and doesn’t depend on feature geometry. In this paper we propose a simple chemical reaction and diffusion mechanism that explains the experimental observations of the etched structures. The uniformity of the SiN etching between the top and the bottom of the 3D NAND stacks indicates that the reaction rate is much slower than the rate of diffusion of reactants and products into and out of the structure, so the etching is a reaction-controlled mechanism. This indicates within-wafer uniformity and wafer-to-wafer uniformity variations as being primarily due to differences of temperature within the bath. We also conducted simulations of the etching of the SiN in the 3D NAND structure and the liquid flow in the bath to model and explain this mechanism. In these simulations we varied the rate of the surface reaction of H3PO4 etching SiN, and showed that for a fast reaction rate that the structure would etch from the top to the bottom as the overall reaction rate become diffusion limited, while for a slow reaction rate the entire structure would etch uniformly, showing it to be a reaction limited process. Another issue with SiN etching in an H3PO4 bath is that the etch rate of SiO2 depends highly on the concentration of silica, in fact the bath is generally pre-seeded with silica in order to achieve a lower SiO2 etch rate, though if above a critical concentration deposition is known to occur. Based on a study of silica chemistry, it may be possible for silica deposition to occur even when below the critical concentration. This would be a film that would grow on a SiO2 surface but would later dissolve back into solution. We propose a mechanism that explains the formation and later disappearance of the growth film on the SiO2 surface as a result of the etching of the SiN and the formation of silica byproducts into a sol/gel network. Sol/gels have the unique property of their ability to form in an area of locally high flux, and then later dissolve over a much longer time scale. The difference in forward and backward reaction rates of the sol/gel film formation and disillusion, respectively, are such that the system is never in equilibrium. Thus, even though all the silica can dissolve in the phosphoric acid at equilibrium, the silica formed from the SiN etching forms into a sol/gel polymer network that only slowly dissolves after SiN etching has stopped. We discuss the implications of this sol/gel oxide film for further process improvement. With better theoretical understanding of the SiN etching and possible silica sol/gel formation and dissolution process, we now have tools to better optimize the phosphoric acid bath, improving both etching uniformity and necessary process time. Figure 1

Opaque White Anodic Oxide Film Formed on Aluminum

Anodic porous alumina film, which is formed by anodizing of aluminum in an appropriate acidic solution, has been studied on the protection or design of aluminum surfaces owing to commercial applications. The appearance and surface quality of aluminum products (e.g., architectural materials, electronic devices) are key points as with surface properties such as a corrosion resistance, hardness and wear resistance. Although dyeing, painting and electrolytic coloring are well known as industrial methods to provide color variation for aluminum surface, integral coloring, which produces color by anodizing step itself without use any pigment or metallic salt, is also applied in terms of the simplicity and production cost. In our previous work, it was reported that alumina films with irregular porous structure formed in Ematal bath or chromic acid showed opaque white appearance [1, 2]. Irregular porous structure in the direction of pore depth, which contained branching, curving and distortion of pores in anodic alumina, produced the roughening of the metal-oxide interface and made the appearance of the film more opaque [1, 2]. However, Ematal bath containing expensive potassium titanium oxalate deteriorates easily. In terms of reduction of environmental burdens, it is required to avoid using harmful chemical compound such as chromic acid. Under such background, we studied a low-cost alternative anodizing method to form opaque white oxide film. In addition, we also investigated in detail the relation between the appearance and porous structure of oxide film. Anodizing was conducted using high purity (99.99 %) aluminum sheets in various electrolytes (phosphoric acid, sulfuric acid, critic acid, and Ematal bath). Whiteness of the obtained samples was evaluated by spectrophotometry (KONICA MINOLTA, CM-5) based on standard of American Society for Testing Materials (ASTM E313-73). When anodizing was carried out in phosphoric acid at 60 °C at a constant current density of 200 Am−2, anodized specimen indicated whiteness value of 39. Surface appearance was gray in color. The aluminum specimen anodized in Ematal bath showed whitish appearance compared with that treated in phosphoric acid. Whiteness value of aluminum specimens anodized in Ematal bath increased from 35 to 47 with increasing final arrival voltage from 150 V to 350 V during constant current anodizing. On the other hand, whiteness value increased effectively using two-step anodizing; long first anodizing and short second anodizing were conducted in phosphoric acid and subsequently Ematal bath. Under optimized conditions, whiteness value increased to 58. The more increase of the thickness of total oxide film and barrier layer and the degree of branching, the more white appearance indicated. Thus, it is confirmed that the appearance of anodized specimen was strongly affected by these factors. [1] S. Ono, S. Chiaki, T. Sato; J. Met. Finish. Soc. Jpn., 26, (10), 456-460 (1975). [2] S. Ono, T. Sato; J. Met. Finish. Soc. Jpn., 31, (3), 134-139 (1980).

(Invited) Inhomogeneity of Barrier Layer Inducing Irregularity of Porous Anodic Oxide Film on Aluminum

Porous anodic oxide films formed on various metals, especially anodic alumina, have attracted huge attention as a starting material in the fabrication of nano-devices because of the high controllability of porous structures in nanometer scale such as pore size, pore interval, as well as pore depth by changing electrolysis condition in addition to their high regularity of pore arrangement. In recent years, mechanism of self-ordering of anodic alumina films has been intensely studied. We proposed anodizing under high electric field strength as a key factor for self-ordering of pore arrangement (1). However, only a few reports are found on the mechanism of irregularity or defects formation of porous anodic oxide films. For example, an regular deposition of metals into all anodic pores are often difficult because of irregularity of the barrier layer. In the case of anodic films formed on aluminum in chromic acid solution, for example, sever pore blanching appears especially at the higher voltage. Anodic alumina films formed in phosphoric acid and Ematal bath also indicates numerous pore branching and caved pores in the cell wall (2, 3) where anodizing conditions such as high formation voltage and relatively low dissolution ability of electrolytes to anodic oxides are used. These irregularities were caused by defects/holes induced by electric breakdown between pore bottom and protrusion at triple points of cell boundary. Impurity elements in metals and alloys also bring defects and irregularity into porous structures. Furthermore, anodic films formed at some specific condition, especially on titanium, yielded tubular structure having interspaces at cell boundaries. All of these irregularities should be created only at the barrier layer grown under the anodic field by specific reactions other than stable anodic oxide growth. This inhomogeneity of barrier layer such as unevenness of the thickness and voids that induce irregularity in porous oxide seems to be playing an important role at an application for industry both in a positive and negative way. In this paper, we report the specific futures of irregularity appeared in porous anodic oxides films formed on aluminum and evaluated to get unified formation mechanism of such imperfections to be induced in the oxides grown on various substrates. [1] S. Ono, M. Saito, M. Ishiguro and H. Asoh, J.Electrochem. Soc., 151, B473 (2004). [2] S. Ono, H.Ichinose and N. Masuko, J.Electrochem.Soc., 138, 3705 (1991) [3] S. Ono and N. Masuko, J. Jpn Inst. Light Metal, 43, 453 (1993).

Examination of the Surface Phosphorus Content of Anodized Medical Grade Titanium Samples

In case of titanium dental implants, the main goal is to create a surface where the bone cells can attach well, therefore osseointegration can occur. The chemical composition of the surface has an important role, because the surface has a direct contact with the living tissue and induces different reactions for example peri implantitis or osseointegration. In our work titanium sample made from the most commonly used dental implant material (Ti Grade 5) were investigated. The samples were treated by chemical etching in hydrogen-chloride and in phosphoric acid to remove the cut generated burr. After that the samples were anodized in phosphoric acid solution at 10 V, 20 V, 30 V or 40 V. As a result of these treatments, titanium-dioxide layers were created on the surfaces. Phosphorus (originating from the phosphoric acid bath) may also be found on the surfaces. This may promote osseointegration. The surface compositions were investigated with the aid of Secondary Ion Mass Spectrometry (SIMS). Based on these results we can conclude that anodization in phosphoric acid solution increases the phosphorus content of the surface. Approximately to the middle of the titanium-dioxide layer the phosphorus content is constant but lower with one order of magnitude than on the surface. In the deeper layers the phosphorus content continues to decrease until the base material where it significantly reduce.

Characterization of aluminum anodized layers modified in sulfuric and phosphoric acid baths and their effect on conventional electrolytic coloring

Modification of anodic aluminum oxides induces light interference leading to various tunes of color after electrolytic coloring process. In this research, nano-morphology of an anodic aluminum oxide was modified via two procedures in sulfuric and phosphoric acid baths. The metallurgical changes after the modifications were evaluated and compared with unmodified conventional layer. Layer thickness, crystalline structure, surface topography and morphology, mechanical characteristics including elastic modulus and nanohardness, corrosion behavior and electronic properties of the oxide layer were evaluated. From the overall view, the modification had some destructive effects on corrosion performance and mechanical properties of the anodized layers. The aluminum oxide layer was characterized as an n-type semiconductor and the modification increased its conductivity. The effects of the oxide modification on creating interference colors using the electrolytic coloring method were evaluated. The electrolytic coloring generated brown color on the oxide layer modified in the sulfuric acid bath. Conversely, a green interference color was obtained for the oxide layer modified in the phosphoric acid bath.

Properties of Polymer Electrolyte Membranes Based on Poly(Aryl Ether Benzimidazole) and Sulphonated Poly(Aryl Ether Benzimidazole) for High Temperature PEMFCs

AbstractMembranes based on poly[2,2′‐(p‐oxydiphenylene)‐5,5′‐bibenzimidazole] (OPBI) and their sulphonated derivatives (SOPBI) were prepared and first investigated for high temperature proton exchange membrane fuel cells. OPBI and SOPBI membranes with good chemical stability were prepared by casting from their dimethyl sulphoxide (DMSO) solution and then impregnated in phosphoric acid baths to obtain H3PO4 (PA)‐doped membranes. The properties of PA‐doped OPBI membranes, including phosphoric acid uptake, swelling ratio, mechanical properties and thermal properties, were fully investigated and compared with PA‐doped SOPBI membranes. The fuel cell performances of PA‐doped OPBI and SOPBI membranes at different temperatures were first evaluated. When doped in different concentrations of H3PO4 solution, OPBI membranes adsorb more acid than SOPBI membranes. SOPBI membranes exhibit obviously lower swelling ratio in thickness direction than that of OPBI membranes. With an acid uptake of ∼110 wt.‐%, corresponding doping level around 5, PA‐doped OPBI and SOPBI membranes keep reasonably high tensile strength. PA‐doped OPBI membranes exhibit peak power density of 0.78 W cm–2 at 170 °C and 0.25 MPa. With the same acid uptake, PA‐doped SOPBI membranes displayed similar fuel cell performance to that of PA‐doped OPBI membranes.

Development of Electrolyte Membranes for Fuel Cells Operating at Intermediate Temperatures (130-200°)

The objective of this work was the preparation and characterization of poly(2,5-benzimidazole) (ABPBI) as electrolyte membrane for fuel cells applications at intermediate temperatures (130-200°). The ABPBI membranes were prepared by casting and then doped in phosphoric acid baths of different concentrations. The characterization included thermogravimetric analyses, elemental analyses and size exclusion chromatography. ABPBI membrane-electrode assemblies (MEAs) were tested in H2/O2 fuel cells.

Maintaining a Stable Etch Selectivity between Silicon Nitride and Silicon Dioxide in a Hot Phosphoric Acid Bath

Phosphoric acid (H3PO4) - water (H2O) mixtures at high temperature have been used for many years to etch silicon nitride (Si3N4) selective to silicon dioxide (SiO2) layers. The production requires removing the Si3N4 completely while keeping SiO2 loss to a minimum. The challenge for batch wafer cleaning is how to maintain a high etching selectivity of Si3N4 to SiO2 for a longer bath life. Experiments showed that increasing water concentration in the bath results in higher selectivity: more Si3N4 etched and less SiO2 etched. With the installations of a water concentration monitor and a water spiking apparatus, the bath is able to control the water concentration. The partial replacement of the chemical bath after each product lot reduces dissolved silicon concentration. The addition of H2SO4 saves initial conditioning time. With all of the efforts, the bath starts at a high selectivity and maintain this selectivity for an extended bath life.

Sulfonated poly(2,5-benzimidazole) (SABPBI) impregnated with phosphoric acid as proton conducting membranes for polymer electrolyte fuel cells

Previously cast ABPBI membranes were sulfonated by doping with sulfuric acid followed by heat treatment at 450 °C for 5 min in air. Sulfonation degrees between 35 and 49% of the benzimidazole rings were achieved. The resulting SABPBI membranes were impregnated with phosphoric acid (H 3PO 4 85%/H 2O, 70:30 bath). For concentrated phosphoric acid baths (above 65%), the capacity of these membranes for phosphoric acid uptake (and consequently also their conductivity) increased with the degree of sulfonation. Sulfonated and acid doped SABPBI membranes were characterized in terms of degree of sulfonation, thermal stability (TGA), X-ray diffraction, FTIR spectroscopy and proton conductivity in the dry state, and compared with phosphoric acid impregnated ABPBI studied earlier. The maximum conductivity measured in dry conditions was 3.5×10 −2 S cm −2 at 185 °C for SABPBI·4.6H 3PO 4 (with a degree of sulfonation of 41%) which compares favorably with non-sulfonated ABPBI and makes feasible their application in PEM Fuel Cells working at temperatures of 150–200 °C.

Polymer Electrolyte Fuel Cells Based on Phosphoric Acid-Impregnated Poly(2,5-benzimidazole) Membranes

Preparation and characterization of membranes of poly(2,5-benzimidazole) (ABPBI) (as thin as 20 μm) for polymer electrolyte membrane fuel cells are reported. These membranes were prepared by solution casting and then impregnated in phosphoric acid baths. Their characterization included thermogravimetric analyses, conductivity measurements, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. These membranes have high thermal stability and good proton conductivity at temperatures up to 200°C at 150°C and 30% relative humidity), similar to the well-known commercial polybenzimidazole (PBI). ABPBI membrane-electrode assemblies (MEAs) using commercially available E-TEK Pt/C electrodes were tested in cells, obtaining power densities similar to those of PBI MEAs studied here as reference at temperatures of up to 180°C, humidifying the reactants at room temperature. © 2004 The Electrochemical Society. All rights reserved.

Etude de la reactivite de l'alliage Ti-Cu (2%) traite a differentes tensions dans H3PO4 5M

Abstract Study of the reactivity of a Ti-Cu(2%) alloy treated at different potentials in 5M phosphoric acid. A Ti-Cu(2%) alloy was anodically treated in a 5 M phosphoric acid bath at different potentials. XPS measurements showed that the passive film is mainly formed by an hydrated titanium oxide. Furthermore, the anodically treated samples present an excellent passivity in a potential domain extending from the corrosion potential to the treatment potential. Above the latter, the polarization curves present a current transition followed by a leveling off. This transition is not seen after performing a complete sweep. EDS measurements of the anodized samples revealed a decrease in oxygen content with the potentiodynamic polarization in H 3 PO 4 , despite the improvement in passivity.

The role of aluminum anodizing conditions on the effect of the additive light green

The influence of operating conditions such as temperature, concentration of electrolyte and applied voltage, on the effectiveness of the additive light green in a phosphoric acid bath, on film porosity and current density during anodizing of pure aluminum has been investigated using electrochemical methods. X-Ray fluorescence was used to determine the incorporated amount of phosphorous in the anodic oxide. The principal factor which affects the action of this additive seems to be the applied voltage, while the other parameters have a much smaller influence.

Atomic Force Microscopy Study of the Initial Stages of Anodic Oxidation of Aluminum in Phosphoric Acid Solution

Aluminum foils with two different surface topographic textures were anodically oxidized at constant current in a phosphoric acid bath. In situ atomic force microscopy (AFM) was used to follow the initial development of surface topography on a 1 μm scale, during the early stages of porous oxide film formation. Microscopic convex features such as ridges on both foils begin to increase in height and width when the anodic film thickness exceeds the initial feature height. Equations of a mathematical model are presented incorporating established interfacial reactions and oxide conduction behavior. The model indicates that the film‐solution interface recedes into the metal during anodizing, since the current efficiency for oxide formation is smaller than the oxygen ion transport number in the film. Ridge surfaces increase in height due to the higher local conduction resistance to the film‐solution interface, while film deposits rapidly at ridges because of the low local resistance to the metal‐film interface. In agreement with the AFM results, enhanced oxide growth at ridges should start when the potential field in the film becomes two‐dimensional, as a result of the film growing to a thickness larger than the ridge height. © 2000 The Electrochemical Society. All rights reserved.

Anodic oxidation of titanium in phosphoric acid baths: Phosphorus incorporation into the oxide

Titanium samples were anodically oxidized in H 3PO 4 baths at various concentrations and voltages. The oxidation kinetics and the yield of incorporation of phosphorus ions into the oxide were studied as a function of the electrochemical parameters. Analytical measurements were performed using nuclear reaction analysis (NRA). A growing mechanism is discussed and we examine how to increase the self-protection of oxide films against corrosion.

アルミニウムの陽極酸化皮膜の多色化におよぼす電解時間と添加剤の効果

Anodized oxide coating obtained from phosphoric acid bath at the voltage lower than that employed in the usual method was successfully colored by varying the a. c. electrolytic duration in nickel sulfate bath alone. Addition of boric acid and well known complex, such as ammonium compounds, amines, and aqueous ammonia, was very effective for multicoloring. Boric acid was particularly the best additive. These additives seem to promote the deposition of nickel in the pores. Addition of tartaric acid, sulfulic acid, and aluminium sulfate delayed multi-coloring and resulted in single-coloring. These additives seem to restrain growth of the deposits in the pore of the coating. Color of the coatings can be controlled by varing the electrolytic duration, and kind and amount of the additives.

Determination par fluorescence x du phosphore incorpore dans les films anodiques d'aluminium et de niobium formes en milieu phosphorique

The incorporation of phosphorus impurities from phosphoric acid baths into the dielectric films formed on aluminium and niobium by anodic polarisation have been studied by X-ray fluorescence analysis. Results are discussed in relation to the metallic substrate and electrochemical parameters such as acid concentration, apparent current density and limiting voltage of film formation.

アルミニウムおよびアルミニウム合金の電解研摩

As mechanical polishing of aluminium and its alloys is very difficult for their mildness, chemical and electro polishing are widely adopted for these metals, and various compositions of electropolishing bath, acidic or alkaline, for amphoteric aluminium have been suggested. But these baths are effective only for pure aluminium but not for alloys. The author, therefore, investigated electropolishing of various aluminium alloys. In this paper, the result of electropolishing in phosphoric acid-chromic acid bath which he proposes for carbon steels and copper alloys is reported.(1) In phosphoric acid-chromic acid bath at comparatively low bath voltage-current density, electropolishing effects on aluminium alloys are good.(2) Addition of chromic acid to the bath is effective to make oxydation or passivation of anode uniform, to prevent selective dissolution of added alloying elements and different structure of alloys, and to get smooth surface with luster. Silmin is well polished, silicon contained in it being oxidized and dissolved as colloidal silica.(3) Electropolishing of cast material in the bath shows little effect but becomes a little effective after heat treatment such as quenching or annealing.

ステンレス鋼の電解研摩

Mechanical polishing of stainless steels is difficult and handling skill is required, for they are apt to be overheated locally due to their low heat conductivity. Electropolishing, therefore, has long been adopted for stainless steels. As it has especially good effect on 18-8 type stainless steel, various composition of electropolishing bath have been suggested.In this paper, the result of electropolishing of 18-8 type stainless steel and 13% chromium steel in phosphoric acid-chromic acid bath which the author proposes for carbon steels, copper alloys and aluminium alloys, is reported.(1) Phosphoric acid-chromic acid bath showed good effect on 18-8 type stainless steel and 13% chromium steel.(2) The addition of chromic acid to the electropolishing bath promoted the oxydation of anode and prevented selective dissolution of chromium contained in the steel.(3) High anode current density causes to produce streaks by oxygen bubbles which is generated on the surface of anode, while low anode current density and long time treating improves the polishing effects.(4) In spite of heat treatments, 18-8 type stainless steel always consists of austenite crystal stracture, but as its grain size becomes smaller and strain is removed more completely by heat treatment, the electropolishing effect is better. Polishing effect of 13% chromium steel is influenced by the heat treatment. It is best for that of quenched martensite structure and better for that of solbite structure.

鋳鉄の電解研摩

Electropolishing of cast iron is seldom applied, for its throwing power is very poor due to the intricate structures such as ferrite, cementite and graphite. Especially, electropolishing effects depend on the distribution of graphite on the surface of cast iron. The author, however, investigated the electropolishing of cast iron in phosphoric acid-chromic acid bath and caustic soda-sodium dichromate bath.The results are as follws:1) The same smooth surface as in the case of carbon steel is obtained if acid or alkaline bath is used according to the quantity of graphite contained in the cast iron.2) Malleable cast iron can well be electropolished in both acid and alkalline bath with oxydizing agent such as chromic acid or sodium dichromate.3) Cast iron as cast has not much graphite but its crystalline structure is coarse, so that the bright surface by electropolishing cannot be obtained, which would be much improved by heat treatment of the cast iron.