Non-conducting Solids Research Articles

Mechanistic origins of methyl-driven Overhauser DNP.

The Overhauser effect in the dynamic nuclear polarization (DNP) of non-conducting solids has drawn much attention due to the potential for efficient high-field DNP as well as a general interest in the underlying principles that enable the Overhauser effect in small molecules. We recently reported the observation of 1H and 2H Overhauser effects in H3C- or D3C-functionalized Blatter radical analogs, which we presumed to be caused by methyl rotation. In this work, we look at the mechanism for methyl-driven Overhauser DNP in greater detail, considering methyl librations and tunneling in addition to classical rotation. We predict the temperature dependence of these mechanisms using density functional theory and spin dynamics simulations. Comparisons with results from ultralow-temperature magic angle spinning-DNP experiments revealed that cross-relaxation at temperatures above 60K originates from both libration and rotation, while librations dominate at lower temperatures. Due to the zero-point vibrational nature of these motions, they are not quenched by very low temperatures, and methyl-driven Overhauser DNP is expected to increase in efficiency down to 0K, predominantly due to increases in nuclear relaxation times.

Overhauser effects in non-conducting solids at 1.2 K

Recently, it was observed that protons in non-conducting solids doped with 1,3-bisdiphenylene-2-phenylallyl (BDPA) or its sulfonated derivative (SA-BDPA) can be polarized through Overhauser effects via resonant microwave irradiation. These effects were present under magic angle spinning conditions in magnetic fields between 5 and 18.8 T and at temperatures near 100 K. This communication reports similar effects in static samples at 6.7 T and, more importantly, at temperatures as low as 1.2 K, in a different dynamic regime than in the previous study. Our results provide new information towards understanding the mechanism of the Overhauser effect in non-conducting solids. We discuss possible origins of the fluctuations that can give rise to an Overhauser effect at such low temperatures.

Pulsed radio-frequency discharge inductively coupled plasma mass spectrometry for oxide analysis

A direct solid sampling technique has been developed based on a pulsed radio-frequency discharge (RFD) in mixture of N2 and Ar environment at atmospheric pressure. With an averaged input power of 65W, a crater with the diameter of 80μm and depth of 50μm can be formed on sample surface after discharge for 1min, suggesting the feasibility of the pulsed RFD for sampling nonconductive solids. Combined with inductively coupled plasma mass spectrometry (ICPMS), this technique allows to measure elemental composition of solids directly with relative standard deviation (RSD) of ~20%. Capability of quantitative analysis was demonstrated by the use of soil standards and artificial standards. Good calibration linearity and limits of detection (LODs) in range of 10−8–10−9g/g were achieved for most elements.

Determination of the Thermal Diffusivity of Electrically Non-Conductive Solids in the Temperature Range from 80 K to 300 K by Laser-Flash Measurement

The adoption of the popular laser-flash method at temperatures far below 300 K is restricted by the weak signal-to-noise ratio and the limited spectral bandwidth of the commonly used mercury cadmium tellurite (MCT) infrared (IR) detector used as a non-contacting temperature probe. In this work, a different approach to measure the temperature rise in pulse heating experiments is described and evaluated. This method utilizes the change of the temperature-dependent electrical resistance of a thin strip of sputtered gold for the detection of a temperature rise as it was proposed by Kogure et al. The main advantage of this method at lower temperatures is the significantly higher signal-to-noise ratio compared to the commonly used IR detectors. A newly developed laser-flash apparatus using this detection method for the determination of the thermal diffusivity in the temperature range from 80 K to 300 K is presented. To test the accuracy of the new detection method, the thermal diffusivity of a borosilicate crown glass (BK7) specimen at 300 K was determined and compared to results derived with a MCT detector. Good agreement of the derived thermal diffusivity values within 3 % was found. The thermal diffusivity of BK7 and polycrystalline aluminum nitride (AlN) was measured at temperatures between 80 K and 300 K by a laser-flash method to test the functionality of the apparatus. Finally, the thermal conductivity was calculated using values for the specific heat capacity determined by temperature modulated differential scanning calorimetry (MDSC). Comparisons with literature data confirm the reliability of the experimental setup.

Study of Flexoelectric Phenomenon from Direct and from Inverse Flexoelectric Behavior of PMNT Ceramic

Flexoelectric phenomenon describes creation of electric polarization in non-conducting solids by applying of gradient of deformation. The inverse flexoelectric phenomenon describes the generation of a mechanical deformation in an insulating solid by an applied gradient of the electric field. Lead magnesium niobate-lead titanate (PMNT) ceramic permits study of the flexoelectric behavior because of its large flexoelectric coefficient. Special conical samples were prepared for the generation of a high electric field gradient and cylindrical samples with uniform field. Theoretical study of flexoelectric phenomenon seems to be very interesting in the last time because it was confirmed from direct (polarization caused by gradient of deformation) and from inverse behavior (deformation developed from gradient electric field) outgoing from thermodynamic theory.

Determination of the electron beam irradiated area by using a new procedure deriving from the electron beam lithography technique

The study of the charge trapping properties in non-conductive solids by means of a Scanning Electron Microscope requires the knowledge of both the injected charge and the current densities over the irradiated sample area. The aim of this paper is to set-up an experimental technique leading to a more accurate determination of this area. After investigation of the parameters which influence the determination of the irradiated area size by using the electron beam lithography technique (electron beam energy, resist thickness and nature of the substrate), we describe a new experimental procedure derived from this technique. In this procedure, we replace the usual resist develop process by a direct SEM resist surface imaging immediately after the irradiation step. Thus, we find that the irradiation area on insulating glass substrate is considerably wider than that on conducting copper substrate. Moreover, a SEM contrast inversion is observed. These different behaviours are interpreted by considering the charging ability of glass and the conductivity nature of copper.

Multireference configuration interaction treatment of excited-state electron correlation in periodic systems: the band structure of trans-polyacetylene

A systematic method to account for electron correlation in periodic systems which can predict quantitatively correct band structures of non-conducting solids from first principles is presented. Using localized Hartree–Fock orbitals (both occupied and virtual ones), an effective Hamiltonian is built up whose matrix elements can easily be transferred from finite to infinite systems. To describe the correlation effects wave-function-based multireference configuration interaction (MRCI) calculations with singly and doubly excited configurations are performed. This way it is possible to generate, both, valence and conduction bands with all correlation effects taken into account. trans-polyacetylene is chosen as a test system.

Sliding-spark spectrometry of sediment samples.

Direct analysis of non-conducting solids and dielectric surface layers for elements at trace levels has been shown to be feasible by the new so-called sliding-spark spectroscopy, in which a discharge plasma is bound by, and propagates on, the surface of a dielectric sample matrix. By using a river sediment as a model matrix, bound in PVC to improve elemental atomization and excitation efficiency, as a result of the formation of volatile chlorides, the potential of the technique for environmental applications has been investigated by studying the characteristic UV-visible radiation emitted in the wavelength range 210-510 nm for the elements Fe, Cr, Ni, Cu, Pb, Cd, Zn, Co, V, Ti, and Mn. The radiation was detected by means of a CCD spectrometer and analysed for spectral line identification, selection, and the linearity of the spectral response, to determine whether this was suitable for establishing a calibration strategy for quantitative analysis using the sliding-spark source.

The ac-Heated Strip Technique for the Measurement of Thermal Properties of Thin, Solid Nonconducting Layers

The differential equation describing the temperature oscillations generated by a metallic strip of thickness 2L1, coated with a thin nonconducting layer of thickness δ and heated by an ac current, was solved with and without reference to the heat transfer by radiation from the outer surface of the coating layer. The solution obtained shows that for L1/δ>5, the measurement of the amplitude (and phase) of the temperature oscillations of the strip allows the following thermal properties to be measured: thermal diffusivity (a), volumetric heat capacity (ρCp), and thermal conductivity (λ) coefficients of the thin nonconducting layers investigated when the layer thickness is less than the thickness (Λ) at which the temperature oscillations are practically damped in the material of the coating layer (δ<Λ). For thin films (δ≪Λ), a simple formula is obtained which allows the volumetric heat capacity of the film material to be determined alone. On the other hand, when the coating layer is very thick (δ≫Λ), only the thermal activity coefficient (λ/\(\sqrt a \)) can be determined. To test this technique, thin layers of polyethylene and polystyrene were investigated. Bulk specimens were investigated by the ac-heated wire technique. Both sets of results obtained are in good agreement with each other and with published data. The setup described was then used to measure the thermal properties of thin layers of Synthesite AC-43 (440 to 80°C) and Krylon (40 to 160°C). The results are reported. It is worth mentioning that this technique can be used for the estimation of the thickness of opaque thin layers of nonconducting solids, provided that their thermal properties are known. The thermal properties mentioned can be measured applying the ac-heated wire technique, but for bulk material.

Supramolecular ionics: electric charge partition within polymers and other non-conducting solids

Electrostatic phenomena in insulators have been known for the past four centuries, but many related questions are still unanswered, for instance: which are the charge-bearing species in an electrified organic polymer, how are the charges spatially distributed and which is the contribution of the electrically charged domains to the overall polymer properties? New scanning probe microscopies were recently introduced, and these are suitable for the mapping of electric potentials across a solid sample thus providing some answers for the previous questions. In this work, we report results obtained with two of these techniques: scanning electric potential (SEPM) and electric force microscopy (EFM). These results were associated to images acquired by using analytical electron microscopy (energy-loss spectroscopy imaging in the transmission electron microscope, ESI-TEM) for colloid polymer samples. Together, they show domains with excess electric charges (and potentials) extending up to hundreds of nanometers and formed by large clusters of cations or anions, reaching supramolecular dimensions. Domains with excess electric charge were also observed in thermoplastics as well as in silica, polyphosphate and titanium oxide particles. In the case of thermoplastics, the origin of the charges is tentatively assigned to their tribochemistry, oxidation followed by segregation or the Mawell-Wagner-Sillars and Costa Ribeiro effects.

ESR and NMR in Non-Conducting Solids

ESR and NMR in Non-Conducting Solids

Measurement of slurry solids content by electrical conductivity

Electrodes positioned within a sedimenting and flowing suspension have been used to measure the conductivity of a dispersion and the supporting liquid. Variation in the ratio between these conductivities with solid concentration has been investigated and shown to conform to known equations for isotropic solids. Anisotropic and conducting materials had to be correlated using an empirically determined parameter. This was used to deduce solid concentration during solid/liquid separations, in situ, under carefully controlled experimental conditions, and in a system where temperature and electrolyte concentration were allowed to vary at random. Using this correlation solid concentrations up to 56% by volume were measured, during ballotini sedimentation. Sedimentation of minerals other than ballotini did not exhibit simple zone settling alone. The particle size distribution of the suspended non-conducting solids did not influence the correlation.

Reheating of a Laser-Produced Plasma by a Second Pulse Laser

Focused radiation of a Nd:YAG laser was used for ablation and production of free sample atoms from electrically conducting and nonconducting solids for analytical purposes. The microplasmas were generated in an argon buffer gas atmosphere at reduced pressure. In order to increase the intensities of analyte lines the microplasmas were reheated by a second Nd:YAG laser after the process of atomization was completed. It is demonstrated for aluminum and manganese in glass and steel samples and for magnesium and manganese in glass, copper, and aluminum that by internal standardization a common calibration curve can be obtained.

Automated apparatus for the determination of the specific heat and the thermal diffusivity of nonconducting solids using ac techniques

A fully automated experimental setup is described that allows the determination of the heat capacity and thermal diffusivity on the same piece of a nonconducting thin solid sample. Both properties are measured using ac techniques. Measurements have been made on microscope glass cover slides for temperatures between 25 and 300 K. The values obtained for the specific heat are within 5% of those reported in the literature and the values of the thermal diffusivity are within 10% of reported values.

Characterization of a radio frequency glow discharge emission source

A radio frequency glow discharge atomic emission source is described. This source incorporates two optical windows, one positioned such that both the glow region of the discharge and the sample surface may be viewed simultaneously, and the other positioned such that only the glow may be viewed. Preliminary signal-to-noise ratios obtained with the two optical geometries are compared and found to be similar. However, optical monitoring of both the glow and sample surface results in significantly higher emission intensities, owing to the more complete viewing of the energetic regions of the discharge. As a result, this geometry may be considered to be analytically more advantageous. Additionally, data are presented to illustrate the dependences of the dc self-bias potential, excitation temperature, sputter rate, and emission intensities of sputtered species on discharge pressure and generator output power. Scanning electron micrographs of the surface of a machinable glass ceramic (MACOR) sample indicate that appreciable sputtering of electrically nonconductive solid samples occurs in the rf glow discharge. Comparison of data acquired with conductive and nonconductive samples is made to show the similar behavior of the discharge in the presence of the two sample types. Exemplary spectra are presented to illustrate the potential usefulness of the source for the emission spectrometric analysis of both electrically conductive and nonconductive solids.

Electrochemical Surface Analysis of Nonconducting Solids: Ferricyanide and Phenol as Electrochemical Probes of the Surfaces of Layered Double Hydroxide Anion‐Exchanging Clays

Cyclic voltammetry may be added to the limited repertoire of methods that may be used to examine solid/solution interphases for nonconductive solids, and to select nonconductive solids as heterogeneous electrocatalysts. Hexacyanoferrate(III) was used as an electroactive probe to determine the availability of Zn2+ and Al3+ ions at the surface of a layered double hydroxide (LDH, or hydrotalcite‐like clay, anion exchanging clay, or mixed metal hydroxide). The solid LDH, was applied to the surface of a glassy carbon electrode via its suspension in polystyrene/methylene chloride solution. Cyclic voltammetric experiments showed formation of a Prussian blue‐like film on the surface of the LDH, showing that the zinc and aluminum sites were available to form complexes with species in solution. No film formed on the Mg/Al/Cl analog, or on , , or . Electro‐oxidation of phenol was catalyzed by the Mg/ Al/Cl LDH, but not by the Zn/Al/Cl compound. These results were used to assign effective pH values of >11.2 and 8.3, respectively, for the surfaces of these solids. Electrochemical results are compared with those obtained using x‐ray photoelectron spectroscopy (XPS).

Applicability of a radiofrequency powered glow discharge for the direct solids analysis of non-conducting materials by atomic emission spectrometry

Radiofrequency (r.f.) powered glow discharge atomic emission spectrometry is demonstrated to be a potentially useful technique for the elemental analysis of electrically conductive and non-conductive solids. As opposed to the direct current glow discharge, the r.f. glow discharge enables the non-conductive sample to be atomised directly from the solid state without the need for prior mixing with a conductive host matrix material. Data are reported to illustrate the dependencies of emission intensity on gas pressure and discharge power, as well as the stability of the discharge. Representative atomic emission spectra of both an electrically conductive and non-conductive sample are presented to illustrate that the r.f. glow discharge provides good atomisation efficiency for both sample types.

Electronic collective modes and instabilities on semiconductor surfaces. I

A Green's-function theory of electronic collective modes is presented which leads to a practical scheme for a microscopic determination of surface elementary excitations in conducting as well as nonconducting solids. Particular emphasis is placed on semiconductor surfaces where the jellium approximation is not valid, due to the importance of density fluctuations on a microscopic scale (reflected in the local-field effects). Starting from the Bethe-Salpeter equation for the two-particle Green's function of the surface system, an equation of motion for the electron-hole pair is obtained. Its solutions determine the energy spectra, lifetimes, and amplitudes of the surface elementary excitations, i.e., surface plasmons, excitons, polaritons, and magnons. Exchange and correlation effects are taken into account through the random-phase and time-dependent Hartree-Fock (screened electron-hole attraction) approximations. The formalism is applied to the study of electronic (charge- and spin-density) instabilities at covalent semiconductor surfaces. Quantitative calculations for an eight-layer Si(111) slab display an instability of the ideal paramagnetic surface with respect to spin-density waves with wavelength nearly corresponding to (2\ifmmode\times\else\texttimes\fi{}1) and (7\ifmmode\times\else\texttimes\fi{}7) superstructures.

Neutral primary beam SIMS analysis of surface layers on glass

The limiting condition for the analysis of mobile species like sodium or hydrogen bearing structural units in non-conducting solids is obeyed for the NPB-SIMS method. This is demonstrated by results on concentration profiles of differently treated glass surface regions, on a model reaction on a biological glass, and on the interdiffusion of sodium and hydrogen in a binary silicate glass.

A comparison of results of an ESCA study of nonconducting solids using spectrometers of different constructions

The binding energies of 17 photoelectron lines in the spectra of six chemical compounds have been measured using nine spectrometers of seven different types of construction. The results are treated statistically and the reasons for data variability analysed. The most reliable of these data, which would be suitable for checking the calibration of spectrometers, are indicated. For 13 lines of three compounds the relative intensities were measured and compared with theoretical data. It is shown that, provided the kinetic energies of the photoelectrons are sufficiently high, the results concerning relative intensities obtained using one instrument can be applied, with appropriate corrections, for quantitative analysis using other spectrometers, with an accuracy of ∼10%. The spectral characteristics, intensities, contrasts and peak widths are compared for the different spectrometers and the need for further wide and objective comparisons of characteristics of serially manufactured instruments is pointed out.