Atmospheric effects of friction,friction noise and wear with silicon and diamond. Part II. SEM tribometry of silicon in vacuum and hydrogen

Scanning electron microscope (SEM) tribometric data on polycrystalline silicon (poly-Si) vs. poly-Si, Si(100) vs. Si(100) and Si(111) vs. Si(111) interfaces, obtained in Torr and in 0.2 Torr partial pressure of hydrogen gas ( ) from room temperature to 850^°C, were performed under standard and much slower thermal ramping rates. The friction data were analyzed per the methodology described in part I of this paper series. The results indicate a highly beneficial friction- and wear-reducing regime within a relatively narrow thermal region. This desirable region coincides with some chemisorption of excited species of molecular hydrogen just before the mass thermal desorption of surface hydrides. These data represent the tribochemical equivalent of a method routinely used in electronics, whereby deep electron traps (dangling Si bonds) are passivated by baking in molecular hydrogen. The also exerts a moderating influence on the size of the friction noise at all test temperatures. However, the general level of friction beyond the beneficial thermal region is high. In parallel, the general wear rate of Si representative of the entire range of standard thermal ramping in both atmospheric environments is in the extremely high 10^-12m³/(N m) range. Operating strictly in the beneficial, low-friction thermal regime resulted in a several orders-of-magnitude reduction in the wear rate over those measured under standard thermal ramping conditions. Although the results confirm previous findings that Si is not a good material of construction for miniaturized moving mechanical assemblies (e.g., microbearings and gears), there seems to be some limited possibility of gas-phase lubrication of Si micromechanisms with rarefied hydrogen at surface temperatures between 100 and 300^°C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of aluminum sulfate and ferric chloride coagulant residuals on polyamide membrane performance

Pretreatment may constitute up to one-fourth of the total costs of a membrane desalting facility. By using preexisting conventional filtration plants for pretreatment, significant cost savings may be realized. However, coagulant residuals from the pretreatment process may negatively affect reverse osmosis (RO) membrane performance. Various RO membranes were tested at three different treatment plants in southern California, using either aluminum sulfate (alum) or ferric chloride coagulants and chloramines. Repeated testing using alum with multiple RO elements revealed rapid deterioration in specific flux (up to 60% over 100 h of operation), as well as progressive reductions in salt rejection (typically 3-4% over 500 h of operation). Microscopic analysis of the fouled membranes revealed that the foulants were primarily aluminum hydroxide and aluminum silicate materials. In contrast to the RO data for alum coagulation, which showed declining membrane flux, the specific flux data using ferric chloride and chloramines increased over time for all membranes. Salt rejection decreased significantly during testing of each membrane. These data suggest that the RO membranes were physically degrading over time. The RO membranes may have been degraded by residual iron catalyzing a chlorine-amide reaction on the membrane surface, despite the fact that chlorine was present as chloramines.     

Atmospheric effects of friction,friction noise and wear with silicon and diamond. Part III. SEM tribometry of polycrystalline diamond in vacuum and hydrogen

In this part III of a multi-part paper series, the results of additional SEM tribometric experiments are described, performed with polished, mostly C(100)-oriented polycrystalline CVD diamond film [PCD_C(100) vs. PCD_C(100)] counterfaces sliding in Torr and in 0.1–0.3 Torr partial pressures of pure hydrogen gas. These tests were completed under a 28 g (0.27 N) normal load, under standard and slow thermal ramping conditions at temperatures ranging from room temperature to 1000^°C. The friction data were examined per the computer logging and analysis techniques described in part I. The treatment of the data is similar to that of Si in part II: the maximum and the average coefficients of friction (MAX.COF and COF) and their ratios (the friction noise FN) are employed to measure possible lubricative interaction of the diamond surfaces with rarefied hydrogen. The results indicate that excited species of molecular hydrogen enter into tribothermally catalyzed reactions not only with Si but with PCD_C(100) surfaces as well. Similar to the behavior of Si, the most beneficial friction-reducing regime occurs in a temperature range just before the thermal desorption of adsorbates. The general magnitudes of MAX.COF, COF and the FN are significantly lower than those of the Si crystallinities, in both vacuum and . The wear rate of the PCD_C(100) film characteristic of the standard thermal ramping test procedure performed mostly in is around , in good agreement with the wear rate previously measured in vacuum for unpolished, fine-cauliflowered diamond films. The data indicate that smooth polycrystalline diamond is a significantly better bearing material for miniaturized moving mechanical assembly applications than Si. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pilot-scale testing of reverse osmosis using conventional treatment and microfiltration

In order to meet the growing needs of its consumers, the Metropolitan Water District of Southern California is looking for ways to desalt water from the relatively hard Colorado River. This study evaluated conventional treatment as the pretreatment step for reverse osmosis (RO) desalting. Other pretreatment options studied include microfiltration (MF) and conventional treatment with ozone and biologically active filters. Each of three pretreatment scenarios produced effluent waters generally considered appropriate for use with RO [median turbidity of less than 0.1 Nephelometric turbidity unit (NTU) and median silt density index of less than 3]. Both microfiltered and ozonated/ biofiltered waters gave steady RO performance over 3 months of testing. However, conventional treatment left the RO system vulnerable to organic and biological fouling. Despite maintaining a 2-2.5 mg/L chloramine residual, pretreatment using conventional treatment required more frequent cleanings than either MF or ozonation/biofiltration (O₃/BF). The good performance for biofiltered water may have resulted from the stabilization of the natural organic matter through the O₃/BF process. Microfiltration, with its superior particle and bacteriological removal characteristics, provided the best RO pretreatment technology.     

The role of dissolved aluminum in silica chemistry for membrane processes

The use of aluminum sulfate (alum) coagulation prior to reverse osmosis (RO) treatment has been shown to be problematic. Membrane fouling was theorized to occur through soluble aluminum (Al³⁺) reacting with ambient silica (H₄SiO₂) to form kaolinite (Al₂Si₂O₅(OH)₄) within the RO unit. Chelating agents (citrate at 34 mg/L and ethylenediaminetetraacetic acid [EDTA] at 16 mg/L) were tested for their efficacy in controlling aluminum silicate fouling. The results of bench-scale testing demonstrated that both citrate and EDTA did control aluminum silicate formation, citrate more so than EDTA. Additional aluminum-based fouling was encountered when a commercial, phosphonate-based antiscalant — used to control barium sulfate scaling — reacted with the excess aluminum despite the presence of either citrate or EDTA.     

The effect of naturally occurring biopolymers on polyamide membrane fouling during surface water treatment

Parallel experiments using a blend of surface waters were conducted to evaluate differential fouling rates among reverse osmosis (RO) membranes when operated under pilot- vs. full-scale conditions. Testing was conducted using a 230 L/min conventional (rapid mix/flocculation/sedimentation/filtration) package plant (CPP) and a 2,000 ML/d fullscale treatment plant (FTP) as pretreatment to separate RO membrane test units. Coagulation consisted of 10 mg/L alum (as Al₂(SO₄)₃-14H₂O) and 2.0 mg/L cationic polymer. A2.5-3.0 mg/L free-chlorine residual was maintained at the filter effluent and converted to chloramines through ammonium sulfate addition (3:1 chlorine-to-ammonia w/w ratio). Membrane performance was based on normalized flux and salt rejection data. Membrane surface analyses included scanning electron microscopy, energy-dispersive spectroscopy, and attenuated total reflectance Fourier transform infrared spectroscopy. Microbial activity and community analyses were conducted through (a) fluorescence staining with 4′,6′-diamidino-2-phenylindole, (b) polymerase-chain reaction amplification of isolated bacterial DNA, and (c) microscopic taxonomic identification. Results indicated that the RO membrane fed by the CPP fouled at least three times faster than the RO membrane fed by the FTP. The differential fouling between the two process streams was determined to be from lack of maintenance in the CPP influent piping that led to the establishment of biological communities consisting of algae, microbes, and, potentially, freshwater clams. These communities produced low levels of natural polymers, which when presented to the polyamide RO membrane surface, resulted in rapid fouling.     

Control of residual aluminum from conventional treatment to improve reverse osmosis performance

Soluble aluminum (Al³⁺) may react with both ambient silica and antiscalant components to form colloidal foulants during reverse osmosis (RO) treatment. Whereas conventional treatment (coagulation/filtration/sedimentation/dual-media filtration) was being used prior to RO, aluminum sulfate (alum) and polyaluminum chloride (PACl) coagulants were evaluated at ambient pH (pH 7.8–7.9) and suppressed pH (pH 6.7) in an effort to lower the total aluminum to below 50 μg/L — a level previously observed to prevent RO membrane fouling. Additional tests were conducted with 5 mg/L citric acid added to the RO influent to chelate the soluble aluminum fraction. All tests were conducted with 1.5–2.5 mg/L chloramines present. Testing of a RO process fed with optimized alum- or PACl-coagulated water showed that PACl outperformed alum regardless of pH. Alum coagulation at ambient pH resulted in 184–273 μg/L total aluminum passing through the filtration process. Only by lowering the mean influent water pH to 6.7 was the mean soluble aluminum residual (45 μg/L) for alum coagulation reduced to below the 50 μg/L aluminum goal. Regardless of pH, for alum-coagulated waters, the higher aluminum carryover resulted in severe RO membrane fouling within 500 h of operation. Only when a chelating agent (citric acid) was added to the RO feed was the loss in productivity and selectivity arrested. However, PACl consistently met the 50-μg/L goal for both total and soluble aluminum for all pH levels tested, which resulted in more stable membrane performance over time. Further research on the compatibility of PACl and polyamide membranes in the presence of chloramines is needed as data from this project suggest PACl coagulation may facilitate membrane oxidation.     

Atmospheric effects of friction,friction noise and wear with silicon and diamond. Part I. Test methodology

This multi-part paper series gives evidence of tribothermally catalyzed, lubricative interactions of low partial pressures of hydrogen, water vapor and oxygen with silicon and polycrystalline diamond employed as bearing materials in moving mechanical assemblies (e.g., miniaturized rotors, bearings and gears) of microelectromechanical systems. In part I a test methodology is described, whereby wide environmental range SEM-tribometric friction data are combined with friction noise analysis and applicable literature information to further assist in interpreting atomic-level interactions governing the macroscopic friction and wear behavior of Si and diamond. To further correlate the wear- and thermal desorption-induced generation, re(de)construction and adsorbate-passivated annihilation of dangling σ bonds with high and low adhesion and friction, previously generated average coefficient of friction (COF) values are complemented with the concept of the associated MAX.COF: the highest coefficient of kinetic friction gleaned from the raw computer-logged friction force data of each oscillatory cycle of an experiment. The MAX.COF/COF ratios are used as measures of the friction noise as a function of temperature and atmospheric environment. These quantities, sampled at the appropriate data logging rate to circumvent test machine-related vibrational disturbances, demonstrated signs of friction- and friction noise-reducing gas-phase interactions of dry hydrogen with silicon (part II) and diamond (part III). Future installments will deal with similar lubricative properties of low partial pressures of wet hydrogen and dry oxygen. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrosorption of inorganic salts from aqueous solution using carbon aerogels

Capacitive deionization (CDI) with carbon aerogels has been shown to remove various inorganic species from aqueous solutions, though no studies have shown the electrosorption behavior of multisolute systems in which ions compete for limited surface area. Several experiments were conducted to determine the ion removal capacity and selectivity of carbon aerogel electrodes, using both laboratory and natural waters. Although carbon aerogel electrodes have been treated as electrical double-layer capacitors, this study showed that ion sorption followed a Langmuir isotherm, indicating monolayer adsorption. The sorption capacity of carbon aerogel electrodes was approximately 1.0-2.0 x 10(-4) equiv/g aerogel, with ion selectivity being based on ionic hydrated radius. Monovalent ions (e.g., sodium) with smaller hydrated radii were preferentially removed from solution over multivalent ions (e.g., calcium) on a percent or molar basis. Because of the relatively small average pore size (4-9 nm) of the carbon aerogel material, only 14-42 m2/g aerogel surface area was available for ion sorption. Natural organic matter may foul the aerogel surface and limit CDI effectiveness in treating natural waters.