Revealing hidden pore structure in nanoporous thin films using positronium annihilation lifetime spectroscopy

The highly inhomogeneous pore morphology of a plasma-enhanced-chemical-vapor-deposited ultralow-kk dielectric film (k = 2.2)(k=2.2) has been revealed using depth-profiled positronium annihilation lifetime spectroscopy (PALS) combined with progressive etch back of the film surface. The film is found to have a dense surface layer, an intermediate layer of 1.8 nm1.8nm diameter mesopores, and a deep region of ∼ 3 nm∼3nm diameter mesopores. After successively etching of the sealing layer and the isolated 1.8 nm1.8nm pore region, PALS reveals that the underlying large pores are highly interconnected. This inhomogeneous pore structure is proposed to account for observed difficulties in film integration.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87843/2/121904_1.pd

Two-loop corrections to the decay rate of parapositronium

Order $\alpha^2$ corrections to the decay rate of parapositronium are calculated. A QED scattering calculation of the amplitude for electron-positron annihilation into two photons at threshold is combined with the technique of effective field theory to determine an NRQED Hamiltonian, which is then used in a bound state calculation to determine the decay rate. Our result for the two-loop correction is $5.1243(33)$ in units of $(\alpha/\pi)^2$ times the lowest order rate. This is consistent with but more precise than the result $5.1(3)$ of a previous calculation.Comment: 26 pages, 7 figure

DDS/DGEBA Epoxy Used as a Model for Studying Polymer Confinement in Polymer Nanocomposites

An interesting class of materials is polymer nanocomposites (PNC). Essentially, PNC’s are polymer matrices with nanoparticles dispersed within it. These nanoparticles provide a surface for attachment within the polymer, potentially resulting in alterations of macroscopic properties, such as conductivity, hardness, and flame-retardence. This change in attachment can also alter microscopic properties, such as voids in the polymer. Since it has been well studied, an ideal model system for studying polymer attachment to silica nanoparticles (15-20 nm in diameter for this study) is diglycidyl ether bisphenol A (DGEBA) with 4,4’-diaminodiphenyl sulfone (DDS) hardener. Using Positronium Annihilation Lifetime Spectroscopy (PALS), we can study these voids by analyzing the lifetime of positronium (the bound state of an electron and its antiparticle, a positron) which tends to localize in the voids of the polymer matrix. Preliminary results will be discussed, as will the PNC fabrication process

Generation of Diffraction Gratings Using Photographic Film

The properties of light is one of the most interesting and complex topics in physics. For instance, multiple sources of light can interact with each other to create what are known as diffraction patterns – a series of alternating bright and dark spots. These patterns can be produced from a single source of light by use of a diffraction grating. A diffraction grating splits a source of light into multiple sources that can interact with each other. Commercial diffraction gratings are generally made from etched glass. This project determines how well diffraction gratings can be made using an alternative method: photographic film. This would give scientists a way to create diffraction gratings onsite rather than ordering them from another company. Photographs of a diffraction grating pattern were taken and developed onto slide film. The slides were tested using a helium-neon laser and factors such as exposure (aperture size and time), focal length of the camera’s lens, and line spacing of the grating were tested for effectiveness. Effective gratings were ones producing a clear diffraction pattern

Matter‐Antimatter Interactions in Gases

Theoretical work on the heavier noble gases, xenon in particular, has suggested that the temperature dependence of the rate at which orthopositronium (o‐Ps) decays is non‐linear with increasing temperature [1]. However, there is little experimental data on the heavier noble gases to support the theory, and the data that does exist is inconclusive [2]. This non‐linearity is in opposition to both the theoretical and experimental work on the lighter noble gases like helium, neon and argon, which have shown a linear dependence with respect to temperature [3]. The goal of this experiment is to investigate the temperature dependence of the decay rate of o‐Ps in xenon gas. We will use a high‐pressure gas cell with a positron source inside as the basic setup of the experiment. So far, design and construction has been completed on the temperature control system, including the temperature controller electronics, the heaters, and the insulated housing for the gas cell. In addition, construction is nearly complete for the gas handling system. Pressure and temperature tests are underway on the seal of the gas cell. The first sets of data over our temperature range of 20 to 300 deg. C. will be collected in the near future

Deducing nanopore structure and growth mechanisms in porogen-templated silsesquioxane thin films

Adjusting the functional group of a porogen is found to have a tremendous effect on the pore structre of porous low dielectric constant films with silsesquioxane as the matrix precursor. The pore size and interconnection length measured by positronium annihilation lifetime spectroscopy can be used to deduce the pore shape and its evolution with porosity from templates of isolated porogen molecules through film percolation. Inert, self-linkable, and amphiphilic porogens are demonstrated to randomly aggregate three-dimensionally, linearly polymerize, and form micelles, respectively.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87828/2/161903_1.pd

Tunability of Free Volume and Viscoelastic Damping of Thiol–Ene Networks Deep in the Glassy State

Model thiol-click glassy networks with low shrinkage and sharp glass transition temperatures are studied down to cryogenic temperatures to relate viscoelastic damping to changes in microscopic hole volume. Networks synthesized by polymerization of divinyl sulfone with pentaerythritol tetrakis­(3-mercapto­propionate) (PETMP), trimethylol­propane tris­(3-mercapto­propionate) (TMPTMP), and pentaerythritol tetrakis­(2-mercaptoacetate) (PETT) were studied to ∼50 K using positron annihilation and thermal analysis methods. Across the glass transition temperature, overall volume expansivity is dominated by expansion of microscopic holes. Beneath <i>T</i>_g, holes contract upon cooling and then become static with negligible expansivity at temperatures well above 0 K. A trade-off between complete rigidity of static free volume and viscoelastic damping ability below <i>T</i>_g is discussed. All samples show moderate damping (tan δ > 0.05), and one sample exhibits a prominent β-transition in its viscoelastic loss spectra. These results indicate the potential for tuning the molecular design of low-temperature glassy networks to optimally incorporate damping with low shrinkage

Acidity Effects in Positron Annihilation Lifetime Spectroscopy of Zeolites

Positron annihilation lifetime spectroscopy (PALS) is a tool used to study pore networks in zeolites, as the diffusion of metastable <i>ortho</i>-positronium (<i>o</i>-Ps, an electron–positron bound pair) provides insights into connectivity that cannot be obtained by other techniques. The accurate assessment of porosity requires knowledge of the interaction of <i>o</i>-Ps with acid centers commonly present in these materials. Although previous studies have highlighted a potential effect, the specific impact of the nature and concentration of the acid sites remains unclear. By preparing a series of well-crystallized aluminum- and tin-containing MFI-type zeolites and with the study of commercial samples, we map the effects of Brønsted and Lewis acidity on the PALS response. The results reveal that both types of acid sites decrease the amount of <i>o</i>-Ps detected but through different mechanisms. Brønsted acid sites strongly affect the amount of <i>o</i>-Ps being annihilated in the micropore network but only nominally influence the amount out-diffusing from the crystal, which is attributed to an energy threshold for the interaction. Lewis acid sites originating from the incorporation of framework tin have a more substantial but uniform impact ascribed to the suppressed formation of <i>o</i>-Ps. A similar effect is observed due to the Lewis acid sites in sodium-exchanged aluminum-containing MFI, but the introduction of larger extraframework cations also hinders the diffusion of <i>o</i>-Ps. A preliminary model is put forward to describe the changes in the PALS response in the presence of acid centers