La3+ and Ce3+ Doping of Hard-Magnetic Ferrites

Rare-earth-doped ferrites with the general formula M_{1 – x} R _x · nFe₂O₃ (M = Ba, Sr, Pb; R = La, Ce; x = 0–0.1; n = 4–6) are prepared by solid-state combustion synthesis. The effects of the doping procedure (before or after combustion synthesis), dopant content, and heat-treatment conditions on the magnetic and mechanical properties of the ferrites are examined. The results indicate that doped materials can be used to fabricate permanent magnets with enhanced density, remanence, and energy product. In addition, rare-earth doping improves the strength of the ferrite materials, eliminating their main drawback—inherent brittleness.     

Studies on the structure and properties of semiconductor ceramic cooling materials

Thermoelectric polycrystalline ceramic materials were prepared by a new ceramic technology. Samples of p-type 72% Sb₂Te₃ + 25% Bi₂Te₃ + 3% Sb₂Se₃ doped with tellurium and 90% Bi₂Te₃ + 5% Sb₂Te₃ + 5% Sb₂Se₃ doped with Sbl₃ or Agl were studied. The new ceramic cooling materials have an inhomogeneous structure, but higher mechanical strength and thermoelectric properties. The method is simpler than the Bridgman and powder metallurgy processes. Measurements of properties show that sintering temperature and time effect the thermoelectric properties of the samples. Scanning electron microscopy shows that the polycrystalline ceramic materials have an obvious layered structure. The doping of the materials was studied, including doping variety and doping concentration. The figure of merit for n-type doped with Sbl₃ is 2.9 × 10^–3 K^–1, while that for p-type doped with tellurium is 3.1 × 10^–1 K^–1.     

Electrodeposition of High-Purity Molybdenum Coatings on Nickel1

High-purity molybdenum coatings suitable as matrix materials for superconducting microwave systems were prepared on nickel substrates by electrodeposition from a CaCl₂–CaMoO₄–CaO melt. The contamination of molybdenum with nickel was found to be insignificant, so that, in producing matrices for microwave applications, the required coating thickness depends only on subsequent mechanical or other treatments.     

Research on innovative materials at the University “La Sapienza”

Research projects on innovative structural materials carried out at the University La Sapienza of Rome in the last years are described. Problems, objectives and future developments are reported on new Al-Li-Mg-Ce alloys, SiC reinforced titanium and aluminum composites, ceramic composites for fuel engines, structural polymer composites, modelling and simulation procedures with new experimental testing methods for composite components.Particularly, Al-Li (2–2.8%)–Mg(2%)–Ce(0.5–0.8%) alloys were studied for their very low density (less than 2.5 kg/d³), and good mechanical properties, especially fatigue resistance, and reasonable production cost.Plasma spray technique was used to produce SiC-fiber reinforced titanium alloys (Ti-6AI-4V), or pure aluminium, matrices. Squeeze-casting method was also used for SiC-particles reinforced aluminium alloys composites.As new synthetic ceramics, SiC-additivated Al₂O₃–MgO (ZrO₂, Y₂O₃) were prepared for fuel engines applications. For testing of this material a new high pressure fretting wear rig was developed.Curing agents for epoxy resin matrices were examined from the point of view of mechanical properties and durability of glass, or carbon, reinforced polymer composites. In testing of materials, especially structural composite components, new methods based on acoustic emission, image analysis, thermography (for polymer composites), ultasonic waves etc. were joined to classical tensile or impact tests to evaluate the damage intensity in real time. Al-electrocoated stainless steels and Superalloys were studied for high temperature oxidation and thermal cycling resistance.Beta-metastable titanium alloys were also examined.     

Calcium phosphate thin film processing by pulsed laser deposition and in situ assisted ultraviolet pulsed laser deposition

Calcium orthophosphates (CaP) and hydroxyapatite (HA) were intensively studied in order to design and develop a new generation of bioactive and osteoconductive bone prostheses. The main drawback now in the CaP and HA thin films processing persists in their poor mechanical characteristics, namely hardness, tensile and cohesive strength, and adherence to the metallic substrate. We report here a critical comparison between the microstructure and mechanical properties of HA and CaP thin films grown by two methods. The films were grown by KrF* pulsed laser deposition (PLD) or KrF* pulsed laser deposition assisted by in situ ultraviolet radiation emitted by a low pressure Hg lamp (UV-assisted PLD). The PLD films were deposited at room temperature, in vacuum on Ti–5Al–2.5Fe alloy substrate previously coated with a TiN buffer layer. After deposition the films were annealed in ambient air at 500–600 °C. The UV-assisted PLD films were grown in (10^–2–10^–1 Pa) oxygen directly on Ti–5Al–2.5Fe substrates heated at 500–600 °C. The films grown by classical PLD are crystalline and stoichiometric. The films grown by UV-assisted PLD were crystalline and exhibit the best mechanical characteristics with values of hardness and Young modulus of 6–7 and 150–170 GPa, respectively, which are unusually high for the calcium phosphate ceramics. To the difference of PLD films, in the case of UV-assisted PLD, the GIXRD spectra show the decomposition of HA in Ca₂P₂O₇, Ca₂P₂O₉ and CaO. The UV lamp radiation enhanced the gas reactivity and atoms mobility during processing, increasing the tensile strength of the film, while the HA structure was destroyed.     

Fabrication and properties of alumino-borosilicate glass-ceramic systems

In current microelectronics packaging applications, low-temperature fired substrates with low dielectric constant are required. Formulations of SiO₂, B₂O₃, Al₂O₃, and CaO have been used as substrate materials which can be sintered as low as 1000C in air. The electrical behaviour, thermal expansion coefficient, and mechanical property of the fabricated substrate materials are evaluated. The as-sintered substrates possess the following characteristics: low dielectric constant of 4–5 at 1 MHz; a loss factor smaller than 0.2% at 1 MHz; and a thermal expansion of 3.57 × 10^–6C^–1 which is very close to that of silicon (3.5 × 10^–6C^–1).     

Electrical properties and microstructure of glass—ceramic materials from CaO-MgO-Al2O3-SiO2 system

The electrical (volume conductivity) and dielectric (loss factor and dielectric constant) properties of glass-ceramics belonging to the CaO-MgO-Al₂O₃-SiO₂ system have been studied, as a function of microstructure, in their glassy and ceramized forms on samples obtained as bulk materials or sintered powders. A possible application of these materials as substrates for electronic devices can be envisaged, on account of their low conductivities (<10^–14S cm^–1 up to 250°C), loss factor and permittivity values.     

High Strain Rate Response of Sandwich Composites with Nanophased Cores

Polyurethane foam materials have been used as core materials in a sandwich construction with S2-Glass/SC-15 facings. The foam material has been manufactured from liquid polymer precursors of polyurethane. The precursors are made of two components; part-A (diphenylmethane diisocyanate) and part-B (polyol). In one set of experiments, part-A was mixed with part-B to manufacture the foam. In another set, TiO₂ nanoparticles have been dispersed in part-A through ultrasonic cavitation technique. The loading of nanoparticles was 3% by weight of the total polymer precursor. The TiO₂ nanoparticles were spherical in shape, and were about 29 nm in diameter. Sonic cavitation was carried out with a vibrasound liquid processor at 20 kHz frequency with a power intensity of about 100 kW/m². The two categories of foams manufactured in this manner were termed as neat and nanophased. Sandwich composites were then fabricated using these two categories of core materials using a co-injection resin transfer molding (CIRTM) technique. Test samples extracted from the panel were subjected to quasi-static as well as high strain rate loadings. Rate of loading varied from 0.002 s^–1 to around 1300 s^–1. It has been observed that infusion of nanoparticles had a direct correlation with the cell geometry. The cell dimensions increased by about 46% with particle infusion suggesting that nanoparticles might have worked as catalysts during the foaming process. Correspondingly, enhancement in thermal properties was also noticed especially in the TGA experiments. There was also a significant improvement in mechanical properties due to nanoparticle infusion. Average increase in sandwich strength and energy absorption with nanophased cores was between 40–60% over their neat counterparts. Details of manufacturing and analyses of thermal and mechanical tests are presented in this paper.     

Synthesis and Properties of Phases in the System BaCO3–PbO–Nb2O5

The phase formation kinetics and phase relations in the BaCO₃–PbO–Nb₂O₅ system are studied. Barium lead niobates are prepared by solid-state reactions, and the composition ranges of solid solutions with different structures are determined. All of the materials are examined by x-ray diffraction, and their stability in acid media is assessed. The electrical conductivity of the Pb-containing niobates is measured. The results are used to analyze property–structure–composition relations. Based on the phase-equilibrium data and the chemical stability and electrical properties of the niobates, the structure types and compositions of niobates are identified which are the most attractive for producing ion-selective materials.     

Dielectric and Piezoelectric Properties of Pb(Mg1/3Nb2/3)O3–PbTiO3–Pb(Mg1/2W1/2)O3 Ceramics

Data are presented on the phase composition, crystal structure, microstructure, and dielectric and piezoelectric properties of (1 – y)[(1 – x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃]–yPb(Mg_1/2W_1/2)O₃ (x = 0.30–0.36; y = 0, 0.05, 0.10) ceramics. It is shown that the use of fine-particle magnesia as a starting reagent ensures the formation of single-phase materials. The ceramics with a rhombohedral structure are found to exhibit relaxor behavior. Increasing the content of the Pb(Mg_1/2W_1/2)O₃ perovskite leads to ordering of the domain structure of poled ceramics and increases their piezoelectric charge coefficient d ₃₁ and the width of their phase transitions.     

Influence of structural defects on the electrical conductivity of (Yb1 ? xScx)2Ti2O7 (x=0, 0.09, 0.3)

The ionic conductivity of Yb₂Ti₂O₇ and (Yb_0.91Sc_0.09)₂Ti₂O₇ is shown to correlate with the degree of antisite cation disordering in these materials. The highest ionic conductivity is exhibited by materials containing 3.53–4.5% Yb_Ti + Ti_Yb antistructure pairs. This degree of cation disordering can be achieved by heat treatment just below the melting point of the material, by mechanical activation with subsequent high-temperature firing, and by doping on the Yb or Ti site. The Yb_Ti + Ti_Yb antistructure pairs and the associated Frenkel defects in the oxygen sublattice are responsible for oxygen ionic conduction.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 479–484.Original Russian Text Copyright © 2005 by Shlyakhtina, Knotko, Boguslavskii, Stefanovich, Kolbanev, Peryshkov, Shcherbakova.     

Crystal-Chemical Approach to Predicting the Thermal Expansion of Compounds in the NZP Family

The general structural aspects of phosphates with {[L₂(PO₄)₃] ^p–}₃ frameworks (L = octahedral cation) are considered, and the possible isomorphous substitutions in NaZr₂(PO₄)₃ (NZP) phosphates are analyzed. The available data on the thermal expansion of NZP materials in the range 293–1273 K, together with crystal-chemical data on their structure, are used to identify the processes underlying the thermal expansion of these materials. The results provide basic guidelines in designing NZP-based materials with controlled (ultralow) thermal expansion and near-zero expansion anisotropy.     

Synthesis and Microwave Dielectric Properties of MgO–TiO2–SiO2 Ceramics

The dielectric properties of MgO–TiO₂–SiO₂ ceramics were studied. The results demonstrate that the presence of MgTi₂O₅ increases dielectric losses in the ceramics. The synthesized materials offer low microwave losses and temperature-stable permittivity in the range 10–20.     

Inorganic nanocomposites for the next generation photovoltaics

Photovoltaic is an attractive alternative of conventional energy source, but for the limitations of present materials and technology, we need to find out cost effective and environmentally stable new materials. We have synthesized a novel nanocomposite, named titania–germanium (TiO₂–Ge). TiO₂–Ge is a thermodynamically stable material. Ge nanodots are dispersed in the TiO₂ matrix of the nanocomposites. Bohr radius of Ge is relatively large, 24.3 nm, therefore, it is easy to vary the size of Ge nanodots, and consequently the properties (structural, optical and electrical) of TiO₂–Ge can be tailored in a wide range just by varying the size and density of Ge nanodots. TiO₂–Ge with size gradient of Ge nanodots is a promising active layer of the next generation solar cells.     

Hydrothermal Synthesis of K2ZrSi6O15 , K2ZrSi3O9 , K2ZrSi2O7, and ZrSiO4 in the System KOH–ZrO2–SiO2–H2O

The phase relations in the KOH–ZrSiO₄–H₂O, KOH–ZrO₂(cr)–SiO₂–H₂O, and KOH–ZrO₂(nanocr)–SiO₂–H₂O systems were studied at 400°C, 0.1 GPa, and KOH concentrations from 2 to 46 wt %. The crystallization fields of K₂ZrSi₆O₁₅, K₂ZrSi₃O₉, K₂ZrSi₂O₇, and ZrSiO₄ were located. The potassium zirconosilicates have open-framework structures made up of corner-shared ZrO₆ octahedra and SiO₄ tetrahedra. The composition of the crystallizing silicates is shown to depend on the nature of the starting Zr- and Si-containing materials.     

High-temperature stability of the mechanical and optical properties of Si–B–C–N films prepared by magnetron sputtering

Quaternary Si–B–C–N materials are becoming increasingly attractive due to their possible high-temperature and harsh-environment applications. In this work, amorphous Si–B–C–N films with two compositions (Si₃₄B₉C₄N₄₉ and Si₃₆B₁₃C₇N₄₀) and low contamination level (H + O + Ar < 4 at.%) were deposited on silicon substrates by reactive dc magnetron co-sputtering using two different targets and gas mixtures. Thermal stability of these films was investigated in terms of composition, bonding structure, as well as mechanical and optical properties after annealing in helium up to a 1300°C substrate limit. Films with a high nitrogen content (Si₃₄B₉C₄N₄₉, i.e. N/[Si + B + C]~ 1.0) were found to be stable up to 1300°C. After annealing, the hardness and elastic recovery of those films slightly increased up to 27 GPa and 84%, respectively, and the reduced Young's modulus remained practically constant (~ 170 GPa). The refractive index and the extinction coefficient at 550 nm were evaluated at 2.0 and 5 × 10^− 4, respectively, and the optical band gap was approximately 3.0 eV. In contrast, films with a lower nitrogen content (Si₃₆B₁₃C₇N₄₀, i.e. N/[Si + B + C]~ 0.7) were stable only up to 1200°C. Both Si–B–C–N materials studied here exhibited extremely high oxidation resistance in air up to the 1300°C substrate limit.     

Modeling of skutterudite CoSb3 with molecular dynamics method

Skutterudite materials have generated considerable interest as new thermoelectric materials over the past few years. For binary skutterudite compound CoSb₃ with complicated cubic crystal structure, Morse potential energy function is employed to describe atomic interactions to lay special emphasis on its mechanical properties. The parameters of Morse potential function between different species of atom pairs (Co–Co, Sb–Sb and Co–Sb) are individually determined based on known crystal structures and elastic properties from experiment or first principle calculation results. To test the accuracy of the obtained potential parameters, molecular dynamics simulation was first performed to inverse deduce and compare with preceding used values, and the results show excellent agreement. Moreover, the practicability and feasibility of the potential was verified. In terms of the obtained potential parameters, the virtual uniaxial tensile simulation was performed for single crystal CoSb₃ with bulk model using the conventional molecular dynamics algorithm. The mechanical response and deformation behavior are carefully analyzed. In contrast to conventional bulk CoSb₃, single crystal bulk CoSb₃ exhibits much better mechanical performances. It undergoes elastic deformation before 10% strain, and achieves ultimate stress 13.3 GPa at the strain of 10.2%, subsequently sudden fracture occurs near the middle of the model, demonstrating typical brittleness.     

Study of Mechanical Activation of Diopside in a CO2 Atmosphere

Interaction of natural diopside CaMgSi₂O₆ with CO₂ during mechanical activation in a CO₂ atmosphere has been studied. It has been shown that, depending on the kind of mechanical activator used, two types of CO₂ sorption by diopside are possible. If grinding is not accompanied by crystal structure disordering, the sorption of CO₂ on the diopside surface is similar to the sorption on metal oxides in the form of undistorted CO₃ ^2– groups, resulting in a nonsplit absorption band at 1430 cm^–1 in the infrared spectrum. If the mechanical activation of diopside leads to amorphization of the sample, it absorbs CO₂ in the form of distorted carbonate groups, which results in the appearance of a double carbonate band with maxima at 1433 and 1522 cm^–1 in the infrared spectrum of the ground sample. This band is similar to the one corresponding to CO₃ ^2– groups in the infrared spectra of carbonate-containing silicate glass. The carbonate content in the diopside sample reaches about 15% CO2 (or 35% CaCO₃) after mechanical activation in an AL-1000 activator in the CO₂ atmosphere for 580 min. X-ray diffraction, infrared spectroscopic data, carbonate content, and BET surface area measurements indicate that CO₂ molecules are likely to penetrate the structurally disordered diopside sample by a tribosorption mechanism. The results on the relaxation of the activated diopside sample during heating are presented.     

Al2O3 and Al2O3--TiN wear resistance in a simulated biological environment

Alumina is widely used as a biomaterial because of its high biocompatibility and its good mechanical properties except toughness. In this study, a composite material Al₂O₃–TiN is considered as an alternative, the addition of TiN improving the mechanical properties of alumina. The wear behaviour of Al₂O₃ and Al₂O₃–TiN in aqueous solutions simulating living environments has been thus compared using a pin on disc wear-testing machine. The results show that the mechanisms of material removal during wear are different. For alumina, a mechanical wear mechanism is observed, reduced by the lubricating action of the wet media, and alumina–TiN is worn by a combination of tribochemical and abrasive effects.     

Interactions between Ti and alumina-based ceramics

Reactive metal coatings have been frequently used on ceramic materials for various purposes. However, little work was done in the past to understand the interactions between coating and ceramic substrates and their effects on the mechanical properties of the ceramics. In this study, titanium coatings were applied to single-crystal (sapphire) and polycrystalline alumina to study the interface reactions. Also, the effect of the coating on the mechanical properties of the substrates was quantified in terms of modulus of rupture (MOR) in four-point bending strength. Reactions between the coating and the Al₂O₃-based substrates at 980°C caused the formation of a new phase, Ti₃Al[O], and a significant decrease (15%–65%) in the MOR strength of the ceramic materials. This study showed that in polycrystalline alumina, interactions between titanium and the glassy grain-boundary phase in the ceramic materials were responsible for reduction in the MOR strength, while the effect of thermal expansion mismatch between titanium and the ceramic substrate appeared to be dominant for singlecrystal alumina.