Highly sensitive miniature photonic crystal fiber refractive index sensor based on mode field excitation

A highly sensitive miniature photonic crystal fiber refractive index sensor based on field mode excitation is presented. The sensor is fabricated by melting one end of a photonic crystal fiber into a rounded tip and splicing and collapsing the other end with a single-mode fiber. The rounded tip is able to induce cladding mode excitation, which resulted in an additional phase delay. Linear response of 262.28 nm/refractive index unit in the refractive index range of 1.337 to 1.395 is obtained for the physical length of a 953 μm sensor. The sensor is also shown to be insensitive to environmental temperature.     

Reviving Pólya's “Look Back” in a Singapore school

This study is based on the stance that Pólya's “Look Back,” though understudied, remains relevant to Mathematics curricula that place emphasis on problem solving. Although the Singapore Mathematics curriculum adopts the goal of teaching Look Back, research about how it is carried out in actual classroom practice is rare. In our project, we focus on a redesign of a teacher development programme that is targeted to help teachers realize Pólya's original vision of Look Back in the classroom. We report the cases of two teachers who have participated in the teacher development programme: their thinking about Look Back (through interview records) and their interpretation of Look Back in their classroom practice (through video records). By bringing these two spheres of data together, we discuss their degree of ‘buy-in’ to Look Back in the overall problem solving enterprise.     

Interparticle forces arising from an adsorbed strong polyelectrolyte in colloidal dispersions: charged patch attraction

Adsorbed polystyrene sulphonate (PSS) shifts the pH of the zero zeta potential, pH_ς = 0, of ZrO₂ to a lower pH. The positive charge density of ZrO₂ at pH = pH_ς=0 determined from the amount of PSS adsorbed was in excellent agreement with that obtained from charge titration. Polystyrene sulphonic acid shifts pH_ς=0 to a greater degree compared with polyacrylic acid because it is a much stronger acid. A patch is likely to just consist of one adsorbed molecule. The patch is negative when the charges of the molecule exceed the underlying positive surface charge. Attraction between the negative patch and the bare positive surface of a second particle is responsible for increasing the yield stress of concentrated ZrO₂ dispersions at pH_ς=0. Its magnitude is only of the order of the van der Waals attraction. Increasing ionic strength and patch misalignment diminish the attraction. The upper limit of the patch area was estimated from the radius of gyration of the molecule in solution. With a known patch area, the patch charged density can be calculated. With the selection of an appropriate patch area, the yield stress due to charged patch attraction increases linearly with the product of the negative and positive patch densities. Received: 30 March 1998 Accepted: 18 September 1998     

Aqua(nitrilo­triacetato)­(1,10‐phenanthroline)iron(III) monohydrate: a seven‐coordinate iron(III) complex

Reaction of 1,10‐phenanthroline (phen) with iron trichloride in the presence of sodium nitrilo­tri­acetate (NTA) resulted in the formation of red crystals of the title complex, [Fe(C₆H₆NO₆)(C₁₂H₈N₂)(H₂O)]·H₂O. The Fe atom has a distorted capped trigonal prismatic coordination comprised of one tetradentate NTA, one bidentate phen molecule and a water mol­ecule. Intermolecular O—H?O hydrogen bonds link the mol­ecules into infinite chains. The chains are crosslinked by hydrogen bonds involving the solvent water mol­ecule, leading to an infinite ladder packing mode.     

Influence of hue origin on the hue-temperature calibration of thermochromic liquid crystals

Liquid crystal thermography incorporates a process by which the liquid crystal color, bearing temperature information, is analyzed using image processing techniques to quantify complex temperature distributions on heat transfer surfaces. Previous researches have found that hue can be used to represent the liquid crystal color since there exists a monotonic relation between the hue and temperature of liquid crystals. It is found in this study that the white-light point, which is usually adopted as the hue reference point, does not generally lie at the center of the liquid crystal colors defined on a chromaticity diagram. This results that hue does not have uniform resolution to all liquid crystal colors and a biased error is produced subsequently in the temperature measurement. The non-uniform hue can be corrected by shifting the hue origin to the center of the colors or by a color calibration scheme so that the liquid crystal colors distribute evenly around the white-light point. Received on 15 November 1996     

Development of an integrated optical system for warpage and hermeticity test of microdisplay

We have developed an integrated optical system to measure the warpage of a microdisplay product at post-packaging die-attach temperature and to test its hermeticity after manufacturing. This system is constructed based on the principle of Twyman/Green (T/G) interferometry and Newton interferometry. T/G interferometry is used to characterize warpage of the microdisplay's base-plate, while Newton interferometry is used to characterize the gap variance between face-plate and base-plate of the microdisplay. With these measurements, deformation and shape of the microdisplay can be comprehensively determined during its manufacturing process, which is critical for its quality test and reliability evaluation. Delicate mechanical system was designed to facilitate the optics integration and alignment. To evaluate the performance of the optical system itself, warpage of a sample was re-measured with Tailor–Hobson profilometer whose resolution is 10 nm, the results from these two testing systems agree well with each other. Some application examples of the integrated testing system were presented to verify its work efficiency.     

Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent-Silyliumylidene Cation

The trapping of a silicon(I) radical with N-heterocyclic carbenes is described. The reaction of the cyclic (alkyl)(amino) carbene [cAAC_Me] (cAAC_Me=:C(CMe₂)₂(CH₂)NAr, Ar=2,6-iPr₂C₆H₃) with H₂SiI₂ in a 3:1 molar ratio in DME afforded a mixture of the separated ion pair [(cAAC_Me)₂Si:^.]⁺I⁻ ( 1 ), which features a cationic cAAC–silicon(I) radical, and [cAAC_Me−H]⁺I⁻. In addition, the reaction of the NHC–iodosilicon(I) dimer [I_Ar(I)Si:]₂ (I_Ar=:C{N(Ar)CH}₂) with 4 equiv of I_Me (:C{N(Me)CMe}₂), which proceeded through the formation of a silicon(I) radical intermediate, afforded [(I_Me)₂SiH]⁺I⁻ ( 2 ) comprising the first NHC–parent-silyliumylidene cation. Its further reaction with fluorobenzene afforded the C_Ar−H bond activation product [1-F-2-I_Me-C₆H₄]⁺I⁻ ( 3 ). The isolation of 2 and 3 confirmed the reaction mechanism for the formation of 1 . Compounds 1 – 3 were analyzed by EPR and NMR spectroscopy, DFT calculations, and X-ray crystallography.     

Thermal Conductivity of Soils at Very Low Moisture Content and Moderate Temperatures

An analysis of soil thermal conductivity data shows that, at very low moisture content, this property first varies insignificantly and then begins to increase from a certain critical moisture content, whose value tends to depend on clay mass fraction. Two simple models evaluating the critical moisture content were developed; the first one is a fraction of the permanent wilting point; the second one is a simple linear function dependent on clay mass fraction. An insignificant variation of soil thermal conductivity is observed at 20°C, within a water pressure head ranging from 1×10³ to 1×10³m, while for higher temperatures (45–50°C) from 5000 to 100000m. Three extensions of the enhanced thermal conductivity model by Sundberg, namely SUN-1, SUN-2 and SUN-3, were proposed and tested. They produce an average root mean square error of 27%, 24% and 30%, respectively, with respect to experimental data. SUN-1and SUN-2 predictions could be further improved if better estimates of thermal conductivity at the dry state and the permanent wilting point were provided. SUN-3 is a simple model which does not require information about the PWP and critical moisture content. All SUN models have a potential for implementing the latent heat transfer component.     

Synthesis, characterization and structures of copper(II) complexes containing carboxylate and sulfonate groups

Four copper(II) complexes containing Schiff base and reduced Schiff base ligands derived from pyridine-2-aldehyde and amino acid containing carboxylate and sulfonate functional groups (N-(2-pyridylmethylene)-amino acid and N-(2-pyridylmethyl)-amino acid, (amino acids = ??-alanine and aminoethanesulfonic acid) namely, [Cu(Pbals)(H₂O)₂]ClO₄·H₂O 1, [Cu(Pbal)(ClO₄)(H₂O)] 2, [Cu₂(Paes)₂(ClO₄)₂]·2H₂O 3, and [Cu(Pae)(H₂O)]·ClO₄·H₂O 4 have been synthesized and characterized. The structural features of carboxylate and sulfonate donor groups have been elucidated. These copper(II) complexes demonstrate different coordination behaviour of the carboxylate and sulfonate groups. Carboxylate groups in complexes 1 and 2 bridge the metal centers and facilitate the formation of 1D helical coordination polymeric structures. In compound 3, the sulfonate groups bridge the metal centers to form a discrete dinuclear complex. In 4, the sulfonate groups link the neighbouring metal centers to form a 1D coordination polymeric structure.     

Enantioselective palladium-catalyzed carbozincation of cyclopropenes

A highly enantioselective palladium-catalyzed carbozincation of cyclopropenes has been developed. The intermediate cyclopropylzinc species, after transmetalation with copper, were trapped with various electrophiles. This one-pot procedure furnished functionalizied cyclopropenes with excellent diastereo- and enantioselectivity.