Scaling Theory and Numerical Simulations of Aerogel Sintering

A simple scaling theory for the sintering of fractal aerogels is presented. The densification at small scales is described by an increase of the lower cut-off length $a$ accompanied by a decrease of the upper cut-off length $\xi$, in order to conserve the total mass of the system. Scaling laws are derived which predict how $a$, $\xi$ and the specific pore surface area $\Sigma$ should depend on the density $\rho$. Following the general ideas of the theory, numerical simulations of sintering are proposed starting from computer simulations of aerogel structure based on a diffusion-limited cluster-cluster aggregation gelling process. The numerical results for $a$, $\xi$ and $\Sigma$ as a function of $\rho$ are discussed according to the initial aerogel density. The scaling theory is only fully recovered in the limit of very low density where the original values of $a$ and $\xi$ are well separated. These numerical results are compared with experiments on partially densified aerogels.Comment: RevTex, 17 pages + 6 postscript figures appended using "uufiles". To appear in J. of Non-Cryst. Solid

Pengaruh Metode Pembelajaran Listening Team Terhadap Kemampuan Mengidentifikasi Unsur Intrinsik Drama Siswa Kelas VIII SMP Swasta Al-ulum Medan Tahun Pembelajaran 2013/2014

Tujuan penelitian ini adalah untuk mengetahui lebih jelas pengaruh penggunaan metode pembelajaran Listening Team dalam meningkatkan kemampuan mengidentifikasi unsur intrinsik drama. Populasi dalam penelitian ini adalah seluruh siswa kelas VIII SMP Swasta Al-Ulum Medan dengan jumlah 127 siswa. Sampel dalam penelitian ini sebanyak 31 siswa, Metode penelitian yang digunakan adalah metode eksperimen dengan model desain penelitian One Group Pre-test Post-test Design. Instrumen yang digunakan untuk menjaring data adalah penugasan dengan tes objektif. Metode yang digunakan dalam penelitian ini adalah metode eksperimen

Fluctuating Bond Aggregation: a Model for Chemical Gel Formation

The Diffusion-Limited Cluster-Cluster Aggregation (DLCA) model is modified by including cluster deformations using the {\it bond fluctuation} algorithm. From 3$d$ computer simulations, it is shown that, below a given threshold value $c_g$ of the volumic fraction $c$, the realization of all intra-aggregate bonding possibilities prevents the formation of a gelling network. For $c>c_g$, the sol-gel transition occurs at a time $t_g$ which, in contrast to DLCA, doesnot diverge with the box size. Several results are reported including small angle scattering curves and possible applications are discussed.Comment: RevTex, 9 pages + 3 postscript figures appended using "uufiles". To appear in Phys. Rev. Let

Small Angle Neutron Scattering of Aerogels: Simulations and Experiments

A numerical simulation of silica aerogels is performed using diffusion-limited cluster-cluster aggregation of spheres inside a cubic box (with periodic boundary conditions). The volume fraction $c$ is taken to be sufficiently large to get a gel structure at the end of the process. In the case of monodisperse spheres, the wavevector dependent scattered intensity $I(q)$ is calculated from the product of the form factor $P(q)$ of a sphere by the structure factor $S(q)$, which is related to the Fourier transform of $g(r)-1$, where $g(r)$ is the pair correlation function between sphere centers. The structure factor $S(q)$ exhibits large-$q$ damped oscillations characteristics of the short range (intra-aggregate) correlations between spheres. These oscillations influence the $I(q)$ curve in the $q$-region between the fractal regime and the Porod regime and quantitative comparisons are made with experiments on colloidal aerogels. Moreover, at small-$q$ values, $S(q)$ goes through a maximum characteristic of large range (inter-aggregate) correlations. Quantitative fits of the maximum in the experimental $I(q)$ curves of base-catalyzed aerogel are presented. In the case of polydisperse spheres, $I(q)$ is calculated directly from a single aggregate simulation. It is shown that increasing polydispersity shifts the location of the cross-over between the fractal and Porod regimes towards low $q$-value.Comment: RevTex, 9 pages + 11 postscript figures, compressed using "uufiles". Proceeding of the 4th International Simposium on Aerogels (To appear in J. of Non-Cryst. Solids

Gas Transport in Porous Media: Simulations and Experiments on Partially Densified Aerogels

The experimental density dependence of gas (argon and nitrogen) permeability of partially densified silica aerogels in the Knudsen regime is quantitatively accounted for by a computer model. The model simulates both the structure of the sintered material and the random ballistic motion of a point particle inside its voids. The same model is also able to account for the densit y dependence of the specific pore surface as measured from nitrogen adsorption experiments.Comment: RevTex, 11 pages + 5 postscript figures appended using "uufiles". Published in Europhys. Lett. 29, p. 567 (1995

Geometry Effects at Atomic-Size Aluminium Contacts

We present electronic structure calculations for aluminium nanocontacts. Addressing the neck of the contact, we compare characteristic geometries to investigate the effects of the local aluminium coordination on the electronic states. We find that the Al 3pz states are very sensitive against modifications of the orbital overlap, which has serious consequences for the transport properties. Stretching of the contact shifts states towards the Fermi energy, leaving the system instable against ferromagnetic ordering. By spacial restriction, hybridization is locally suppressed at nanocontacts and the charge neutrality is violated. We discuss the influence of mechanical stress by means of quantitative results for the charge transfer.Comment: 10 pages, 4 figures, accepted by Chem. Phys. Let

Small Angle Scattering by Fractal Aggregates: A Numerical Investigation of the Crossover Between the Fractal Regime and the Porod Regime

Fractal aggregates are built on a computer using off-lattice cluster-cluster aggregation models. The aggregates are made of spherical particles of different sizes distributed according to a Gaussian-like distribution characterised by a mean $a_0$ and a standard deviation $\sigma$. The wave vector dependent scattered intensity $I(q)$ is computed in order to study the influence of the particle polydispersity on the crossover between the fractal regime and the Porod regime. It is shown that, given $a_0$, the location $q_c$ of the crossover decreases as $\sigma$ increases. The dependence of $q_c$ on $\sigma$ can be understood from the evolution of the shape of the center-to-center interparticle-distance distribution function.Comment: RevTex, 4 pages + 6 postscript figures, compressed using "uufiles", published in Phys. Rev. B 50, 1305 (1994

New universality class for the three-dimensional XY model with correlated impurities: Application to 4^4He in aerogels

Encouraged by experiments on $^4$He in aerogels, we confine planar spins in the pores of simulated aerogels (diffusion limited cluster-cluster aggregation) in order to study the effect of quenched disorder on the critical behavior of the three-dimensional XY model. Monte Carlo simulations and finite-size scaling are used to determine critical couplings $K_c$ and exponents. In agreement with experiments, clear evidence of change in the thermal critical exponents $\nu$ and $\alpha$ is found at nonzero volume fractions of impurities. These changes are explained in terms of {\it hidden} long-range correlations within disorder distributions.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let

PENENTUAN PERAN DALAM ROBOT SEPAK BOLA DENGAN METODE FUZZY SUGENO

Makalah ini menggambarkan startegi bekerjasama dalam robot sepak bola. Satu tim terdiri atas 4 pemain, yaitu kiper dan 3 pemain tengah. Setiap pemain mempunyai kemapuan dasar seperti kemampuan untuk bergerak mandiri atau autonomous, kemampuan vision yang berfungsi untuk melihat daerah sekitarnya, kemampuan pencarian posisi dirinya atau self-location, kemampuan melacak objek, dan semua pemain dapat berkomunikasi untuk berbagi informasi yang didapat dari masing-masing pemain. Selanjutnya akan dilakukan penentuan peran menggunakan Fuzzy interference sistem dengan metode sugeno. Fuzzy akan menghasilkan output yang berbeda antar satu robot dengan memanfaatkan dua buah variabel fuzzy yang sudah dihitung sebelumnya berupa jarak dan sudut, perbandingan output fuzzy kemudian dikirimkan menggunakan komunikasi antar robot untuk mendapatkan hasil akhir berupa 4 peran pemain dan posisi setiap pemain di lapangan. Dari sepuluh percobaan sistem mencapai keberhasilan sebesar 90%. Hasil tersebut menunjukkan bahwa penentuan strategi permainan bekerja dengan baik menggunakan metode fuzzy. Perangkat lunak akan dijalankan menggunakan bahasa c dengan include tambahan dari eyesim simulator. Kata kunci : Robot sepakbola, Fuzzy sugeno, Eyesi