Fabrication of porous poly(L-lactide) (PLLA) scaffolds for tissue engineering using liquid–liquid phase separation and freeze extraction

PLLA scaffolds were successfully fabricated using liquid–liquid phase separation with freeze extraction techniques. The effects of different processing conditions, such as method of cooling (direct quenching and pre-quenching), freezing temperature (−80°C and −196°C) and polymer concentration (3, 5 and 7 wt%) were investigated in relations to the scaffold morphology. SEM micrographs of scaffolds showed interconnected porous network with pore size ranging from 20 to 60 μm. The scaffolds had porosity values ranging from 80 to 90%. Changes to the interconnected network, porosity and pore size were observed when the method of cooling and polymer concentration was changed. Direct quenching to −80°C gave a more porous interconnected microstructure with uniform pore size compared to samples prepared using pre-quenching method. Larger pores were observed for samples quenched at −80°C compared to −196°C. Scaffolds prepared using direct quenching to −196°C had higher elastic modulus and compressive stress compared to those quenched to −80°C. The compressive elastic modulus ranged from 4 to 7 MPa and compressive stress at 10% strain was from 0.13 to 0.18 MPa.     

Determination of thermal properties of dilute LiBr-water solutions

We are developing an absorption air cooling system which can supply 2°C chilled water for air cooling by the usage of dilute solutions of LiBr in water with an evaporating temperature of −6°C as a nonfreezing refrigerant. However, there are few published data for the thermal properties of dilute LiBr water solutions (0 to 30%) below 10°C. In this paper, the freezing temperature and the saturated vapor pressure are reported. The results clearly show the possibility of developing a new type of LiBr absorption refrigerating machine to generate evaporating temperatures below 0°C. To obtain accurate data for the design of this new type of absorption refrigerating machine, an apparatus has been developed to measure the thermal properties of dilute LiBr water solutions below 10°C. The experimental arrangement consists of a cooling bath (340×240×190 mm) filled with fluorocarbon, a glass measuring bottle (ϕ120×100 mm), and an absolute pressure gauge (0–1.3 kPa). The accuracy of the temperature, pressure, and density are within ±0.1°C, 0.01 kPa, and ±0.005%, respectively. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Electromagnetic properties of liquid nitrogen nanolayers on the surface of various substances measured in microwave resonator

Parameters of a microwave resonator containing various substances have been measured on cooling to low temperatures in the frequency range of 12–17 GHz. It is established that the resonance frequency and maximum transmitted microwave power of the resonator sharply change at a temperature of −196°C. The observed effect is explained by the appearance of highly conducting layers at the boundary between the adsorbed liquid nitrogen film and solid substrate.     

Electrical properties of ion irradiated polypropylene films

The effect of high-energy (50 MeV) Li³⁺ ion beam irradiation on polypropylene (PP) film has been studied in the fluence range 2.4 × 10¹²−l.5 × 10¹⁴ ions/cm². The a.c. electrical properties of PP films were measured in the frequency range from 0.05– 100 kHz, and at temperature range between 30 and 140°C. This study indicates two peaks at 60°C and 120°C with comparatively high magnitudes. There is an exponential increase in conductivity with log of frequency and the effect is significant at higher fluences. The loss factor (tan δ) vs frequency plot suggests that PP film based capacitors may be useful below 10 kHz. The capacitance is constant over a wide temperature range up to 130°C. FTIR spectra of the PP films before and after irradiation indicate that intensity of C-H stretching vibration at 2900 cm⁻¹ is modified. The presence of many new peaks with the increase of fluence suggests the formation of alkanes and alkynes which might be responsible for the observed changes in the dielectric and electrical properties of PP films.     

X-ray diffraction and magnetic characterization of the retained austenite in a chromium alloyed high carbon steel

In the present work the amount of retained austenite present in quenched and tempered high carbon–chromium alloyed steel was quantified by X-ray diffraction and magnetization saturation measurements. The steel was forged and directly quenched. The retained austenite partially transformed into martensite on cooling down to −196 °C. The M_f temperature of about −150 °C was found by thermomagnetic analysis. Tempering at low temperatures (220 °C and 270 °C) promoted the stabilization effect of austenite. The intrinsic magnetization of the ferromagnetic martensite used in the phase quantification was 206.4 A² m/kg. The increase of the tempering temperature above 320 °C slightly decreases the m _s value of the martensite due to tempering reactions.     

Low-temperature cyclic crack resistance of steels of railroad wheels

We have established that the steels of high-and medium-strength railroad wheels in the presence of fatigue-crack-type defects are susceptible to low-temperature (up to −60°C) embrittlement only for high amplitudes of cyclic loading, when the growth rate of a fatigue crack exceeds 10⁻⁷ m/cycle. Here, the temperature of cyclic ductile-brittle transition for these steels constitutes −20 and −40°C, respectively. We have shown that the permissible and temperature-independent (within the range from 20 to −60°C ) size of a fatigue crack on the rim surface of the wheels under study does not exceed 1.9–2.5 mm for a hoop stress range of 400 MPa and 0.6–0.8 mm for 700 MPa. These results enable us to conclude that, for high-strength railroad wheels working at low climatic temperatures, it is necessary to carry out their flaw inspection with sensitivity to surface cracks of millimeter size. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 4, pp. 52–57, July–August, 2008.     

Synthesis,structure and characterization of ceramic Ca4Bi2Ti4Nb6O30

The polycrystalline samples of Ca₄Bi₂Ti₄Nb₆O₃₀ (herein designated CBTN) were synthesized by the conventional ceramic method. Preliminary X-ray structural study of the compound showed the formation of a single phase solid solution having orthorhombic structure in the paraelectric phase. Measurements of the dielectric constant (ε) and dielectric loss (tan δ) as a function of temperature (−180–200°C) at 1 kHz and 10 kHz and also as a function of frequency (10² Hz to 10⁴ Hz) at five different temperatures [−180°C, −40°C, − 10°C 26°C (room temperature) and 75°C] have shown a dielectric anomaly and a phase transition at − 13 ±1°C in CBTN.     

Design and Validation of a High-Temperature Comparative Thermal-Conductivity Measurement System

A measurement system has been designed and built for the specific application of measuring the effective thermal conductivity of a composite, nuclear-fuel compact (small cylinder) over a temperature range of 100 °C to 800 °C. Because of the composite nature of the sample as well as the need to measure samples pre- and post-irradiation, measurement must be performed on the whole compact non-destructively. No existing measurement system is capable of obtaining its thermal conductivity in a non-destructive manner. The designed apparatus is an adaptation of the guarded-comparative-longitudinal heat flow technique. The system uniquely demonstrates the use of a radiative heat sink to provide cooling which greatly simplifies the design and setup of such high-temperature systems. The design was aimed to measure thermal-conductivity values covering the expected range of effective thermal conductivity of the composite nuclear fuel from 10 W . m⁻¹ . K⁻¹ to 70 W . m⁻¹ . K⁻¹. Several materials having thermal conductivities covering this expected range have been measured for system validation, and results are presented. A comparison of the results has been made to data from existing literature. Additionally, an uncertainty analysis is presented finding an overall uncertainty in sample thermal conductivity to be 6 %, matching well with the results of the validation samples.     

A New Panel Test Facility for Effective Thermal Conductivity Measurements up to 1,650°C

A new steady-state panel test facility is presented which has been designed and constructed for effective thermal-conductivity measurements of insulations in the temperature range between 300 and 1,650°C following ASTM C201-93 and DIN V ENV-1094. Square-shaped samples (length of 400 mm) are used, heated from above and settled on a water-cooled calorimeter system to obtain a one-dimensional steady-state temperature field. The heat is supplied by electrical heating elements freely hanging inside a furnace which is completely constructed from ceramic components to withstand temperatures up to about 1,800°C. The calorimeter system consists of a square central measuring zone (length of 100 mm) surrounded by guard loops to avoid heat losses in all directions. The samples, e.g., a number of fiber mats, one on top of the other up to a maximum height of 110 mm, are open to ambient pressure and atmosphere (air). Measurements include the heat flow rate (taken in the central calorimeter), temperature differences across individual layers of the sample (measured by a series of thermocouples which regularly have to be calibrated), and the thickness of the respective layers (before and after the experiment). The thermal conductivities range from 0.025 to 2 W · m⁻¹ · K⁻¹, and both isotropic and non-isotropic materials can be investigated due to the one-dimensional characteristic of the temperature field. Measurements for alumina fiber mats are presented, and good agreement is found with respective results from other methods and test facilities. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5-8, 2005, Bratislava, Slovak Republic.     

Thermal expansion and theoretical density of 2,2′,4,4′,6,6′-hexanitrostilbene

The linear coefficient of thermal expansion (CTE) and the theoretical density are important for energetic materials. To obtain the CTE and theoretical density of 2,2′,4,4′,6,6′-hexanitrostilbene (HNS), X-ray powder diffraction (XRD) together with Rietveld refinement was employed to estimate the dimension and density change at a crystal lattice level, in the range of temperature 30–240 °C. The CTE of a-, b-, c-axis and volume were obtained as 7.6719 × 10⁻⁵/°C, 6.8044 × 10⁻⁵/°C, 1.1192 × 10⁻⁵/°C and 16.725 × 10⁻⁵/°C, respectively. Also, the possible reasons for the expansion property of HNS have been discussed by comparing its structure with 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Based on the refined lattice parameters, the theoretical densities of HNS at various temperatures were obtained. By extrapolation of linear fitting the theoretical density of HNS at 20 °C was gotten as 1.7453 g/cm³. Furthermore, a good thermal resilience of HNS has also been observed when the temperature returned from 240 to 30 °C.     

Studies on tin oxide films prepared by electron beam evaporation and spray pyrolysis methods

Transparent conducting tin oxide thin films have been prepared by electron beam evaporation and spray pyrolysis methods. Structural, optical and electrical properties were studied under different preparation conditions like substrate temperature, solution flow rate and rate of deposition. Resistivity of undoped evaporated films varied from 2.65 × 10⁻² ω-cm to 3.57 × 10⁻³ ω-cm in the temperature range 150–200°C. For undoped spray pyrolyzed films, the resistivity was observed to be in the range 1.2 × 10⁻¹ to 1.69 × 10⁻² ω-cm in the temperature range 250–370° C. Hall effect measurements indicated that the mobility as well as carrier concentration of evaporated films were greater than that of spray deposited films. The lowest resistivity for antimony doped tin oxide film was found to be 7.74 × 10⁻⁴ ω-cm, which was deposited at 350°C with 0.26 g of SbCl₃ and 4 g of SnCl₄ (SbCl₃/SnCl₄ = 0.065). Evaporated films were found to be amorphous in the temperature range up to 200°C, whereas spray pyrolyzed films prepared at substrate temperature of 300– 370°C were poly crystalline. The morphology of tin oxide films was studied using SEM.     

Strain-controlled fatigue life and modeling of conduit polymers

Strain-controlled fatigue lives of conduit polymers, viz., nylon 6, polypropylene (PP) and calcium carbonate filled black colored polypropylene (PP-blk) were studied. Thermal and mechanical analyses were conducted before fatigue tests. Thermal characteristics, such as the degree of molecular degradation, glass transition temperatures, and melting points were determined. Tensile strength, elastic modulus, and Poisson’s ratio were obtained from tests under quasi-static loading. Fatigue lives were measured under different displacement ranges and temperature conditions. Four different temperatures were selected to represent low (−40 °C), room (25 °C), and high (65 and 125 °C) temperature conditions. Hysteretic heating was found to be significantly operative in PP specimens. By optimizing the previously developed unified strain model [1], strain fatigue lives were predicted based on the studied materials.     

Effect of replacement of MgO by CaO on sintering,crystallization and properties of MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system glass-ceramics

The effects of replacement of MgO by CaO on the sintering and crystallization behavior of MgO–Al₂O₃–SiO₂ system glass-ceramics were investigated. The results show that with increasing CaO content, the glass transition temperature firstly increased and then decreased, the melting temperature was lowered and the crystallization temperature of the glass-ceramics shifted clearly towards higher temperatures. With the replacement of MgO by less than 3 wt.% CaO, the predominant crystalline phase in the glass-ceramics fired at 900 °C was found to be α-cordierite and the secondary crystalline phase to be μ-cordierite. When the replacement was increased to 10 wt.%, the predominant crystalline phase was found to be anorthite and the secondary phase to be α-cordierite. Both thermal expansion coefficient (TCE) and dielectric constant of samples increases with the replacement of MgO by CaO. The dielectric loss of sample with 5 wt.% CaO fired at 900 °C has the lowest value of 0.08%. Only the sample containing 5 wt.% and10 wt.% CaO (abbreviated as sample C5 and C10) can be fully sintered before 900 °C. Therefore, a dense and low dielectric loss glass-ceramic with predominant crystal phase of α-cordierite and some amount of anorthite was achieved by using fine glass powders (D₅₀ = 3 μm) fired at 875–900 °C. The as-sintered density approaches 98% theoretical density. The flexural strength of sample C5 firstly increases and then decreases with sintering temperature, which closely corresponds to its relative density. The TCE of sample C5 increases with increasing temperature. The dielectric property of sample C5 sintered at different temperatures depends on not only its relative density but also its crystalline phases. The dense and crystallized glass-ceramic C5 exhibits a low sintering temperature (≤900 °C), a fairly low dielectric constant (5.2–5.3), a low dielectric loss (≤10⁻³) at 1 MHz, a low TCE (4.0–4.25 × 10⁻⁶ K⁻¹), very close to that of Si (∼3.5 × 10⁻⁶ K⁻¹), and a higher flexural strength (≥134 MPa), suggesting that it would be a promising material in the electronic packaging field.     

Spillover effect in the hydrogen absorption by the polycrystalline powder alloy FeNi1.75Cu1.5Mo0.5 and electric conductivity of the pressed powder

The process of hydrogen absorption by the FeNi_1.75Cu_1.5Mo_0.5 alloy in the polycrystalline form was investigated for both the pure and palladized forms (0.008 76% Pd) at temperatures from ambient to 600 °C in a hydrogen flow. Using differential scanning calorimetry, (DSC) thermogravimetry (TG) and X-ray diffraction, the influence of palladization on the hydrogen absorption was demonstrated. Kinetic analysis of the DSC thermograms, the kinetic and thermic parameters of hydrogen absorption were determined. The TG thermograms showed that on hydrogen absorption a weight change took place due to water formation and reduction of the oxide film at the surface of the powder particles. The activation energies of hydrogen absorption were 170 kJ mol⁻¹ for the original powder and 32 kJ mol⁻¹ for the palladized one. The enthalpies of the absorption ranged from ΔH = −5 to ΔH = −380 J g⁻¹ for the original and palladized powder, respectively. The rate constants of the absorption depended on the palladization and were found to be 0.03 and 1.20 s⁻¹, respectively, at 162 °C. The electric conductivity of the pressed powder (9 kbar) increases on heating in both air and a hydrogen atmosphere up to 600 °C, tending to a constant value. The changes of the parameters characteristic of the palladized form are ascribed to the mechanism of a hydrogen spillover effect due to the presence of palladium.     

Thermal Conductivity of Thermoplastics Reinforced with Natural Fibers

With restrictions for environmental protection being strengthened, thermoplastics reinforced with natural fibers such as jute, kenaf, flax, etc., have replaced automotive interior materials such as chemical plastics. In this study, the thermal conductivity of several kinds of thermoplastic composites in the form of board composed of 48.5 mass% polypropylene (PP) and 48.5 mass% natural fiber (NF), and reinforced with 3.0 mass% maleated polypropylene (MAPP) and 0.3 mass% silane as the coupling agents, were measured at temperatures of −10, 10, and 30°C, using a heat flow meter apparatus. The results show that the thermal conductivity is in the range of 0.05–0.07 W · m⁻¹ · K⁻¹, and the thermal conductivity increased about 10–15% by adding MAPP and about 10–25% by soaking in a silane aqueous solution. The tensile strength was also measured, and the result shows similar trends as the thermal conductivity.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Measurement of the partial molar volume and solubility of hydrogen in its dilute solutions in palladium-silver alloys

Linear expansion of thin-wall tubes of palladium-silver alloys containing 10, 20, 27, and 40 wt% silver was measured on absorption of small amounts of hydrogen (hydrogen-to-metal atom ratio, 0.005 to 0.25) at temperatures between 150°C (423 K) and 350°C (623 K) for equilibration with gaseous hydrogen at pressures up to around 1 bar. Hydrogen uptake by the test specimens was determined concurrently with the expansion measurements. Thermal expansion coefficients for the hydrogen-free materials were also obtained. Analysis of the data indicates that fractional length change is linearly dependent on hydrogen-to-metal atom ratio within the experimental range of hydrogen concentrations, independent of temperature, and varies only slightly with alloy composition. Partial molar volumes of hydrogen (with 95% confidence limits) vary from 1.77±0.08cm³·(mol H)⁻¹ for the 10% Ag alloy to 1.92±0.03cm³·(mol H)⁻¹ for the 40% Ag alloy. Hydrogen solubility results are correlated by a relation that facilitates linear extrapolation for determination of limiting values for equilibrium constants. Heats of solution of hydrogen at infinite dilution and standard entropy changes are essentially independent of temperature within the range of experimental conditions. Paper dedicated to Professor Edward A. Mason.     

Evaluation of a.c. conductivity behaviour of graphite filled polysulphide modified epoxy composites

Composites of epoxy resin having different amounts of graphite particles have been prepared by solution casting method. Temperature dependence of dielectric constant, tan δ and a.c. conductivity was measured in the frequency range, 1–20 kHz, temperature range, 40–180°C for 0.99, 1.96 and 2.91 wt% graphite filled and unfilled epoxy composites. It was observed that the dielectric constant, tanδ and a.c. conductivity increase with increasing temperature. Near the transition temperature the materials show anomalous behaviour for the observed properties. Peaks of dielectric constant, tan δ and a.c. conductivity were observed to shift towards lower temperature with increasing frequency. Clear relaxation (tan δ) peaks around 169°C were observed in epoxy resin, which shifted to lower temperature side on increasing the frequency. Addition of 2.91 wt% graphite shifted the tan δ peaks towards higher temperature side by creating hindrances to the rotation of polymer dipoles. Addition of 2–91 wt% graphite leads to an increased relaxation time τ of dipoles in polysulphide epoxy from 1.44 × 10⁻⁵− 3.92 × 10⁻⁵ (s) at 90°C by creating the hindrance to the rotation of dipoles.     

Synthesis and properties of LaNi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3−δ</Subscript> as cathode materials in SOFC

The synthesis of LaNi_{1− x} Fe _x O_3−δ (LNF) perovskites with x = 0.0–1.0, for use as cathode materials for an IT-SOFC, was investigated using four combustion methods, Water Citrate (WC), Modified Water Citrate (MWC), Nitric Citrate (NC), and Modified Nitric Citrate (MNC). The structures and homogeneities of the synthesized powders were examined using an XRD, and the particle sizes were examined using an SEM and a particle size analyzer. All four combustion methods gave the single phase perovskites with the same structure. The main difference was shown in a particle size that the smallest to the largest sizes were obtained from MNC, MWC, NC, and WC, respectively. In this LNF series, as x is 0–0.5, the crystal structure is cubic and rhombohedral at the calcination temperature of 700 and 900 °C, respectively. Further investigation indicated that the cubic structure changed to rhombohedral structure at 900 °C, and was stable up to 1200 °C. As x is 0.6–1.0, the crystal structure is in orthorhombic phase when calcined between 700 and 1000 °C. This orthorhombic phase decomposed above 1100 °C. From the XRD and SEM–EDX results, LaNi_0.6Fe_0.4O_3−δ (LNF64) has a good chemical compatibility with 8YSZ from room temperature up to 900 °C. In addition, its thermal expansion coefficient is 13.2 × 10⁻⁶ K⁻¹ close to that of 8 mol% Y₂O₃ (8YSZ). Therefore, LNF64 also has a good physical compatibility with 8YSZ.     

The New KRISS Low Frost-Point Humidity Generator

A new low frost-point humidity generator (LFPG) has been designed, and its performance has been tested, in order to extend the calibration capabilities to the low frost-point range at KRISS. The water vapor–gas mixture is generated by saturating air with water vapor over a surface of an ice-coated saturator under the conditions of constant temperature and pressure. This LFPG covers a range of frost point from  − 99 °C to  − 40 °C. The temperature of the saturator, which is controlled by thermoelectric devices and a two-stage mechanical refrigeration system, is stable within 5 mK, and the difference between the saturator temperature and the frost point generated at the saturator outlet is less than 20 mK. This stability is achieved by using oxygen-free high-conductivity copper materials as the saturator body, and applying a precision PID temperature control system. The performance of this new LFPG system is compared with the KRISS standard two-temperature generator in the frost-point range ( − 80 to  − 40) °C, and its performance is tested with a quartz crystal microbalance (QCM), which was built at KRISS, to  − 91 °C.     

Low-temperature fracture mechanisms in a spheroidised reactor pressure vessel steel

The micromechanisms of fracture of a spheroidised A533B reactor pressure vessel steel over the temperature range of −190°C to + 60°C were investigated by performing uniaxial tensile tests on double-notched cylindrical specimens. Failure was by quasi-cleavage at temperatures between −190°C and −145°C. Quasi-cleavage fracture surfaces are characterised by clusters of planar facets that are separated from other facets either by large voids or by clusters of microvoids. At temperatures between −145°C and −25°C failure was by mixed microvoid coalescence and cleavage while complete microvoid coalescence was observed at temperatures higher than −25°C. Over the whole temperature range studied, fracture nucleation was either from large single voids or localised regions of microvoids.