Materials Science Forum Vols. 730-732

Abstract: For the growing interest in developing of more attractive multiferroic and magnetoelectric ceramics an alternative is a double-doped Pb(Zr1-_xTi_x)O₃- material with trivalent lanthanum and iron ions. Such material has a structure based on ABO₃ perovskite and properties depending on the chemical and structural composition. In the present work the results of the preparation and characterization of Fe³⁺- modified 9/65/35 PLZT ceramics have been reported. Two samples of Pb_0,91(La_1-zFe_z)_0,09(Zr_0,65Ti_0,35)_0,9775O₃ ceramics, for different Fe ions concentration (z = 0,0 and 0,5 at.%), were prepared from the MOM derived powders and sintered by the hot uniaxial pressing methods. To analyze the obtained ceramics properties the XRD, SEM and also application oriented dielectric and magnetic measurements were performed. The Fe³⁺ substitution at the La-site of PLZT has shown many interesting and unusual dielectric, piezoelectric and magnetic properties. The obtained material was proposed to implement into Piezoelectric Transformer structure to change gain characteristics proportionally to magnetic field intensity.

Abstract: Due to its non-crystalline, amorphous structure, glass is particularly susceptible to radiation-induced coloration/discoloration. Oxide glasses reveal a variety of colors depending upon their composition when exposed to high energy radiations such as gamma and X-rays, and the colors induced have been explained in terms of the formation of color centers. These effects can be reversed by heating or upon exposure to light at wavelengths corresponding to the absorption region of the color centers, a process known as discoloration. Laser can be an efficient process for accomplish this in a localized manner. The aim of this work was to study local discoloration of gamma radiation exposed silicate glasses by application of a nanosecond pulses infrared laser beam. Experimental results validated a numerical model and proved the viability of local laser discoloration of gamma ray irradiated silicate glasses. Although there has been much work focusing the creation and destruction of color centers in glasses, to the best of our knowledge, the application of infrared laser radiation in the local annealing of gamma irradiated glasses was for the first time explored.

Abstract: PZT ceramic system with presence of La contents, have been proposed and prepared using sol gel sintering method for practical application of photostriction, which is the superposition of photovoltaic and piezoelectric effects. Such a ceramics produced by conventional mixing oxide method does not exhibit photostrictive properties due to the defects and inhomogeneous distribution of grains and pores. In this study, an investigated lanthanium(III) doped PZT ceramics were obtained by sol-gel technique from the organometallic precursors. It was found that fabricated material were effective in the enhancement of photovoltaic and photostrictive properties. Consequently, lanthanium influence deviation of piezoelectric parameters were studied as a function UV light illumination. For the determination lighting dependancy of the transformation parameters the resonant and antiresonant method was implemented. The improved Piezoelectric Transformer structure successfully changed gain characteristics proportionally to light intensity. The authors invention of a light driven output gain adjustment in Piezoelectric Transformers (PT) yields a novel “smart” multifunctional wireless device. This new created application area can be utilized in self-adopting shutters in photo cameras due to improved sensitivity to surrounding illumination conditions.

Abstract: Metakaolin (MK) is nowadays a well-known pozzolanic material used in cement-based materials. Its benefits are related to its great pozzolanic reactivity when compared with some others pozzolanic materials, like fly ash or ground-granulated blast furnace slag. When MK reacts with calcium hydroxide, cementitious products are formed. It is known that the main phases produced during its pozzolanic reaction at ambient temperature are calcium silicate hydrates (CSH), stratlingite (C₂ASH₈) and calcium aluminate hydrate (C₄AH₁₃). However, in literature there are several discrepancies regarding these phases stability, namely the transformation of stratlingite and C₄AH₁₃ into hydrogarnet (C₃AH₆) at long term. The consequences of that instability are a reduction in the porosity and a loss of microstructural strength that can induce a complete material degradation. The MK is a material with great potential in Portugal, since there is a large abundance of kaolinitic clays, however its development is not yet achieved. In order to answer this demand a research project was initiated with the aim of optimizing the production of MK in Portugal and enhancing the durability of aerial calcitic lime mortars, to be mainly used in conservation and restoration of historical buildings. With the objective of studying the compounds formed in lime/MK and their stability during time, we have prepared different lime/MK ratio pastes (molar ratios). These lime/MK pastes were stored at saturated conditions (RH > 95%) and 23 ± 2 °C temperature being analysed at several ages by TGA-DTA, XRD and SEM-EDS. In this paper the results obtained are presented and discussed.

Abstract: The (Ba1-xLa_x)Ti_1-x/4O₃ (BLT) ceramics for lanthanum content x in the range 0£x£0.005 were prepared by the sol-gel method. Barium acetate, lanthanum acetate and titanium (IV) propoxide were used as starting materials. Glacial acetic acid and n-propyl alcohol were used as solvents and acetyloacetone was added as stabilizer, followed by hydrolysis. Thermal evolution of the dried gel as well as ceramic powder was studied by simultaneous thermal analysis. The amorphous gel of BLT was calcined in the furnace and pressed into pellets. The compacts were next sintered at temperature T=1100°C. The crystalline structure of BLT ceramics was studied by X-ray diffraction method. Morphology of BLT ceramics was studied by scanning electron microscopy.

Abstract: This paper presents an overview of recent advances in the synthesis and preparation of solid oxide fuel cells (SOFCs) functional ceramic materials, focusing on low-/intermediary-temperature SOFCs. Novel synthesis processes for oxygen ion-conducting and mixed electronic and ionic conductors, fundamental to reduce the operating temperature of SOFCs were studied. Ni-Ce_0.9Gd_0.1O_1.95 (Ni-CGO) anodes were successfully synthesized by the so called “one step synthesis”. La_0.5Sr_0.5Co_0.8Fe_0.2O3 (LSCF), Ce_0.8Sm_0.2O1.9 (SDC) and their mixture were produced as a cobaltite-based composite cathode by mixing powders synthesized by microwave-assisted combustion and the modified polymeric precursor method, respectively. Preliminary electrochemical activity tests with the synthesized electrodes were performed in electrolyte-supported SOFCs using commercially available 200 µm thick yttria stabilized zirconia (8YSZ) as electrolyte. The maximum power density of 52 mW/cm² was reached at 850 °C. This result can be further improved replacing thick YSZ electrolytes by doped-ceria thin films, aiming at operation temperatures of 500–800 °C and power densities as high as 800 mW/cm². The assembling of anode-supported cells with the configuration Ni-CGO/CGO (10 µm thickness)/LSCF-SDC are for applications in 2 kW stacks are currently under way.

Abstract: Carbo-nitride synthesis was undertaken using a solar furnace at PSA in flowing N₂/Ar gas mixture under total pressure 1 atm and processing temperature T = 1600°C for some d-group transition elements (Ti; Zr, V, Nb, Mo, W) starting from 1.5G/M (graphite/metal powder mixture with mole ratio 1.5:1) compact to ensure co-presence of free carbon with the reaction product. Clear X-ray diffraction (XRD) evidence of formation of carbo-nitride was detected for Ti (IVa group metal) showing higher N content in the carbo-nitride synthesised in N₂ gas environment at partial pressure p(N₂) = 1 atm than that at p(N₂) = 0.5 atm. For M = V and Nb (Va group metals), formation of mono-carbide MC single-phase was detected in the N₂ environment showing no evidence of formation of carbo-nitride in spite of presence of N₂ in the environment. For M = Mo and W (VIa group metals), formation of higher carbide, among several options of carbide phases, appeared to be promoted in the N₂ gas environment although, like in cases with the Va group metals, no evidence of dissolution of N into the reaction product was detected. As such, at T = 1600°C in N₂ gas environment up to p(N₂) = 1 atm under concentrated solar beam, carbo-nitride formed from the 1.5G/M mixture only for IVa group metal (Ti) but not for Va and VIa group metals. Anyway, it seemed certain that N₂ gas affected somehow the reaction path between G and M to yield the carbide phase for M = V, Nb, Mo and W.

Abstract: We report on a comparative study of thermoelectric power measurements (S(T)) in ferrimagnetic Gd₄(Co_1-xA_x)₃ compounds with A = Cu, Pt, in the temperature range 8 K – 300 K. Whereas in Gd₄Co₃ S(T) is always negative, for x > 0 the substitution of Co for Cu/Pt gives rise to the appearance of a low temperature positive maximum in S(T) at around 30 K. Based on our previous study of Gd₄(Co_1-xCu_x)₃ compounds, we argue that this maximum in S(T) originates from electron-magnon scattering and is sensitive to electron band structure changes resulting from the substitution of Co for Cu/Pt and the accompanying reduction in the ratio between the electron-magnon and the electron-phonon scattering strengths. The decreasing role of Co 3d electrons with the progressive substitution of Co for Cu/Pt, evidenced by a strong reduction in the spin disorder resistivity and the Co magnetic moment, is seen to be crucial for the existence of such low temperature maximum in S(T) for x > 0. It is seen that the substitution of Co for Pt leads to higher values of the amplitude and temperature of the positive maximum in S(T) than the substitution of Co for Cu.

Abstract: Brazing has been used to join structural ceramics to devices mostly manufactured using metal alloys. Direct brazing employs filler alloys containing an active metal, usually Ti, to wet the ceramic substrate. However, the cost of active filler alloys is usually around 10 times higher than that of active metal-free alloys. In addition, the concentration of the active metal is usually small, thus limiting the formation of a continuous reaction layer onto the entire ceramic surface. Alternatively, ceramic substrates can be previously metallized and fully coated with an active element to be wetted by conventional active metal free filler alloys. Ceramics can be metallized by different techniques, including mechanical metallization. It consists in frictioning an active metal bit, traditionally made of Ti, against the ceramic. Conventional tools can be used and the method is easily automated to large batches. Moreover, contrary to other techniques, mechanical metallization is carried out at room temperature and no hazardous fluxing agents are used. Although Ti is traditionally employed in mechanical metallization, the technique is not limited to it. Indeed, the exclusive use of Ti univocally determines the microstructure of the resulting ceramic/titanium and titanium/filler alloy interfaces. Although the formation of a reaction layer is beneficial to the mechanical strength and reliability of brazed components, precipitation zones and intermetallics embrittle the joints and affect their mechanical behavior. Therefore, the objective of the present study was to assess the potential use of alternative active metals in the mechanical metallization of structural oxide (alumina e zirconia) and non-oxide (silicon carbide and nitride) ceramics. Ceramic substrates were mechanically metallized using Ti, Ta, Nb and Zircaloy 2 (mainly 98.25 % Zr and 1.45% Sn). These metals are abundant in Brazil and therefore strategically important. The wettability of the metallized surfaces was evaluated using three commercially available active metal free filler alloys: VH 780 (Ag-28 Cu), VH 950 (Au – 18 Ni) and SCP 2 (Ag – 31.5 Cu – 10 Pd). The results showed that it was possible to mechanically metallize all ceramic surfaces with the active metals investigated. The wetting tests revealed limited potential for the use of Nb and Ta. On the other hand Zircaloy 2 was successfully employed as active metal for both oxide and non oxide ceramics.

Abstract: In this work, a meteorite sample recovered in Morocco is characterized by detailed petrographic observations in transmitted and reflected light optical microscopy and by using XRD and SEM and it is tentatively classified. VIS/NIR spectral analysis of the same meteorite in a previous study suggested that it is seemingly related to the HAMLET meteorite, which was classified as LL4 chondrite. From the obtained results in the refinement of the analysis of the present study, this meteorite is classified as L4 to L5 chondrite. Composition maps across selected chondrules and in the matrix are presented.