Differential AC-chip calorimeter for glass transition measurements in ultrathin films

A differential AC-chip calorimeter capable of measuring the step in heat capacity at the glass transition in nanometer-thin films is described. Because of the differential setup, pJ/K sensitivity is achieved. Heat capacity can be measured for sample masses below 1 ng in broad temperature range as needed for the study of the glass transition in nanometer-thin polymeric films. Relative accuracy is sufficient to investigate the changes in heat capacity as the step at the glass transition of polystyrene. The step is about 25% of the total heat capacity of polystyrene. The calorimeter allows for the frequency dependent measurement of complex heat capacity in the frequency range from 1 Hz to 1 kHz. The glass transition in thin polystyrene films (50–4 nm) was determined at well-defined experimental time scales. No thickness dependency of the glass transition temperature was observed within the error limits (±3 K)—neither at constant frequency (40 Hz) nor for the trace in the activation diagram (1 Hz–1 kHz). © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2996–3005, 2006     

Thermochemical transformations of polycarbosilane precursors into a ceramic matrix

The possibility of preparing polymeric ceramic-forming precursors from polycarbosilane and oligosilazane was examined. Oligosilazane as a component of polycarbosilane formulations acts as a curing agent, ensuring curing at 150–300°C in the absence of oxygen and modifier. Pyrolysis of the cross-linked copolymer in argon yields an inorganic matrix whose yield is 1.3 times higher than that of pyrolyzates of the starting components. The major phase of the ceramic products obtained, according to the results of X-ray diffraction and chemical analyses, is X-ray amorphous silicon carbide. The ceramic obtained is highly resistant to thermal oxidation (up to 1400°C). The composition of the ceramic and the heat resistance and service characteristics of ceramic-matrix composite materials prepared on its basis using the “polymer technology” can be controlled by varying the ratio of the initial oligomers.     

An investigation of the effect of Rolivsan on the process of synthesis of ceramic matrix from polycarbosilane precursor

Effect of Rolivsan MV-1 on the process of curing and pyrolysis of polycarbosilane PCS-M in argon and on the properties of the ceramic matrix being formed. Use of Rolivsan in polycarbosilane formulations provides effective curing of the latter to give network block-copolymers. Pyrolysis of the resulting precursors in the atmosphere of argon yields ceramic matrices, with the yield of these being 1.3–1.4 times that of the pyrolyzate of the starting components. A study of the elemental and phase composition demonstrated that the matrices belong to the class of amorphous ceramics of nonstoichiometric composition, which contain microcrystalline silicon carbide SiC. The ceramic obtained has a high thermo-oxidative stability (up to 1400°C) and changing the relative amounts of the starting oligomers enables a purposeful variation of its composition, temperature stability, and service characteristics of ceramic-matrix composite materials obtained on its basis by the “polymer technology.”     

New method for measuring the IR surface impedance of metals

A method for measuring the IR optical properties of metals, which implies the analysis of the angular distribution of the photons emitted from the edge of a metal plate due to the conversion of thermal surface plasmons, has been theoretically and experimentally shown. In this work an increased accuracy of the proposed method in the IR region is demonstrated in comparison with more conventional methods based on the Fresnel formulas. This conclusion is supported experimentally with data obtained from the surface impedance measurement of copper in the range from 8 to 9 μm at 185°C.     

Superheating in linear polymers studied by ultrafast nanocalorimetry

To study phase transition kinetics on submillisecond time scale a sensitive ultrafast nanocalorimeter was constructed. Controlled ultrafast cooling, as well as heating, up to 10⁶K/s was attained. The method was applied for the measurements of the superheating phenomenon in a set of linear polymers: iPS, PBT, PET, and iPP. A power law relation between the superheating and the heating rate holds in the heating rate range 10^-2-10⁴K/s. A limiting superheating of about 10% of the melting temperature was observed at rates above 10⁴-10⁵K/s. This limit depends on annealing conditions before sample melting. The observed superheating limit, as well as the power law, can be accounted for the internal stresses near the crystalline amorphous interface in semicrystalline polymers induced by heating, which are related to the thermal expansion gradients inherent in a semicrystalline material.     

Low-temperature anomalies in the specific heat and thermal conductivity of MgB<Subscript>2</Subscript>

The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB₂ were measured at low temperatures and in the neighborhood of T _c . In addition to the well-known superconducting transition at T _c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB₂ was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB₂ of a second group of carriers and its transition to the superconducting state at T_c2≈10?12 K.     

Optical properties of porous silicon processed in tetraethyl orthosilicate

We investigate the change in the composition and optical properties of porous silicon (por-Si) obtained by electrochemical etching of a palate made of n-type (111) silicon single crystal under high-temperature annealing and processing in tetraethyl orthosilicate (TEOS). It is shown that TEOS processing and annealing prevent contamination of a sample stored for a long time in atmosphere. The processing of por-Si in TEOS does not change the position of the photoluminescence (PL) peak and suppresses PL to a smaller extent as compared to annealing of por-Si. This increases the reliability of optoelectronic devices based on por-Si.     

Application of new numerical algorithm for solving the Navier—Stokes equations for modelling the work of a viscometer of the physical pendulum type

A model is proposed, which describes the work of the viscometer sensor of the physical pendulum type. The model enables the obtaining of data on fluid viscosity directly from the measurement of the settling frequency of sensor oscillations or the amplitude of these oscillations. To describe the sensor operation a numerical computational algorithm is developed. This method enables the solution of a wide class of three-dimensional laminar fluid flow problems involving moving solids of arbitrary geometry. The results of testing the proposed numerical technique are presented.     

Non-adiabatic thin-film (chip) nanocalorimetry

To study the kinetics of processes on a millisecond time scale a thin-film nanocalorimeter based on a commercially available microchip (thermal conductivity vacuum gauge, TCG 3880, from Xensor Integration, NL) was constructed. The gauge consists of a submicron silicon nitride membrane with a film-thermopile and a film-heater, which are located at the 100 μm × 100 μm central part of the membrane. Controlled fast cooling is possible in addition to fast heating at essentially non-adiabatic conditions. To allow fast cooling the measurements are performed in an ambient gas atmosphere. It is proved that the maximum rate of the controlled cooling can be achieved with a gas cooling agent, rather than in a system with a solid heat-sink. The advantage of the gauge TCG 3880 is that its central heated region is small enough to be considered as a point source of the heat-flow into the gas, which essentially simplifies the calorimeter calibration. The maximum cooling rate is inversely proportional to the radius of the heated region. The gauge is placed in a thermostat with controlled gas pressure and temperature to be utilized as a device for fast scanning calorimetry of sub microgram samples with sensitivity 1 nJ/K and time resolution ca. 5 ms.