Evaluation of Two Ultrasonic Systems for Analysis of Porosity in Ceramic

The Ultrasound Laboratory of the Nuclear Engineering Institute (LABUS / IEN) has developed an ultrasonic technique to measure porosity in nuclear fuel pellets (UO2). By difficulties related to the handling of UO2 pellets, Alumina (Al2O3) pellets have been used in preliminary tests, until a methodology for tests with pellets of UO2 could be defined. In a previous work, in which a contact ultrasonic technique was used, good results were obtained to measure the porosity of Alumina pellets. In the current studies, it was found that the frequency spectrum of an ultrasonic pulse is very sensitive to the porosity of the medium in which it propagates. In order to define the most appropriate experimental apparatus for using immersion technique in future tests, two ultrasonic systems, available in LABUS, which permit to work with the ultrasonic pulse in the frequency domain were evaluated . One system was the Explorer II (Matec INSTRUMENTS) and the other the ultrasonic pulse generator Epoch 4 Plus (Panametrics) coupled with an oscilloscope TDS 3032B (Tektronix). For this evaluation, several frequency spectra were obtained with the two equipment, by the passage of the ultrasonic wave in the same pellet of Alumina. This procedure was performed on four different days, on each day 12 ultrasonic signals were acquired, one signal every 10 minutes, with each apparatus. The results were compared and analyzed as regard the repeatability of the frequency spectra obtained.

Importance of bearing porosity in engineering and natural lubrication

The multilamellar structure of phospholipids, i.e. the surface amorphous layer (SAL) that covers the natural surface of articular cartilage, and hexagonal boron nitride (h-BN) on the surface of metal porous bearings are two prominent examples of the family of layered materials that possess the ability to deliver lamellar lubrication. This chapter presents the friction study that was conducted on the surfaces of cartilage and the metal porous bearing impregnated with oil (first generation) and with oil + h-BN (second generation). The porosity of cartilage is around 75% and those of metal porous bearings were 15–28 wt%. It is concluded that porosity is a critical factor in facilitating the excellent tribological properties of both articular cartilage and the porous metal bearings studied.

Reduction of porosity in RE1Ba2Cu3Oy pseudo crystals

The distribution of porosity in Y1Ba2Cu3Oy (Y-123) pseudo-crystals prepared by hot seeded melt growth of precursor pellet samples pre-sintered at elevated temperatures in air or vacuum has been investigated. The results demonstrate that the porosity within the Y-123 pseudo-crystal matrix could be reduced dramatically by pre-sintering in vacuum, compared with that for precursor pellets processed in air.

Crack Patterns in Ceramic Plates after Quenching

The crack patterns generated in a real ceramic plate and in a plate stacked by ceramic slabs under quenching are experimentally studied. The results here reveal that there are some distinct differences between the two crack patterns. The reasons that caused the differences are the size and boundary effects of the slabs. These crack patterns are very useful to understand the failure mechanisms of ceramic materials in thermal shock.

Reliability of a method for evaluating porosity in denture base resins

Background: The method of porosity analysis by water absorption has been carried out by the storage of the specimens in pure water, but it does not exclude the potential plasticising effect of the water generating unreal values of porosity. Objective: The present study evaluated the reliability of this method of porosity analysis in polymethylmethacrylate denture base resins by the determination of the most satisfactory solution for storage (S), where the plasticising effect was excluded. Materials and methods: Two specimen shapes (rectangular and maxillary denture base) and two denture base resins, water bath-polymerised (Classico) and microwave-polymerised (Acron MC) were used. Saturated anhydrous calcium chloride solutions (25%, 50%, 75%) and distilled water were used for specimen storage. Sorption isotherms were used to determine S. Porosity factor (PF) and diffusion coefficient (D) were calculated within S and for the groups stored in distilled water. anova and Tukey tests were performed to identify significant differences in PF results and Kruskal-Wallis test and Dunn multiple comparison post hoc test, for D results (alpha = 0.05). Results: For Acron MC denture base shape, FP results were 0.24% (S 50%) and 1.37% (distilled water); for rectangular shape FP was 0.35% (S 75%) and 0.19% (distilled water). For Classico denture base shape, FP results were 0.54% (S 75%) and 1.21% (distilled water); for rectangular shape FP was 0.7% (S 50%) and 1.32% (distilled water). FP results were similar in S and distilled water only for Acron MC rectangular shape (p > 0.05). D results in distilled water were statistically higher than S for all groups. Conclusions: The results of the study suggest that an adequate solution for storing specimens must be used to measure porosity by water absorption, based on excluding the plasticising effect.

Surface forces and their action in ceramic materials

A descriptive account is given of the surface forces acting between two solids. Different contributions to the force are outlined, with particular attention paid to the underlying mechanisms, and how they are affected by the nature of the medium between the surfaces. This is followed by a discussion of the areas of ceramic science and engineering in which surface forces play a role.

The occurrence of porosity in reline acrylic resins. Effect of microwave disinfection

Microwave energy has proved to be an effective method for disinfecting acrylic dentures. However, the effect of microwave heating on the porosity of autopolymerising denture reline resins has not been investigated.The purpose of the study was to determine the effect of microwave disinfection on the porosity of autopolymerised denture reline materials (Kooliner-K, New Truliner-NT, Tokuso Rebase Fast-TR and Ufi Gel Hard-UGH) and a conventional heat-polymerised denture base resin (Lucitone 550-L).Specimens (10 mm x 20 mm x 1 mm) were obtained from the impression surface of the palatal mucosa in a single person and divided into four groups (n = 5). The porosity was evaluated after polymerisation (C1), after two cycles of microwave disinfection (MW2), after seven cycles of microwave disinfection (MW7) and after 7 days storage in water at 37 degrees C (C2). Specimens from group MW7 were exposed to microwave disinfection daily being stored in water at 37 degrees C between exposures. All the replicas were sputter coated with gold and micrographs/digital images were taken of each replica using scanning electron microscopy at magnification x 100. The SEM micrographs were then examined using an image analyser to determine the number of pores. Comparison between materials and groups were made using Kruskal-Wallis tests.MW7 resulted in a significant increase in the number from the pores of material K, but decreased in number in reline material TR and UGH reline resin. The number of pores in materials NT and L remained unaffected following microwave disinfection.Differences in the porosity amongst the materials and for different experimental conditions were observed following microwave disinfection.

Reliability of a method for evaluating porosity in denture base resins

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Detection of cracks in scratching tests in ceramic materials through acoustic emission

The mechanisms of material removal and the interactions among scratches performed in ceramic materials were investigated using acoustic emission signals, and scanning electron microscopy, in scratching experiments. Several testing conditions were used to produce different types of removing mechanism on a glass as well as on a polycrystalline alumina sample composed by heterogeneous grain size. It is known that the material removing process on a polycrystalline ceramic involves intergranular microfracture and grain dislodgement, unlike the chipping produced by the extension of lateral cracks in non-granular materials, such as glass. Distinct settings for velocities, loads, and two types of diamond indenter were tested. The material removal was carried out by three different methods of scratching: single passes, repeated overlapping passes, and parallel scratches. As a general result, there was a clear relationship between the acoustic emission signals and the damage intensity occurred in the material removal. More specifically, there were differences in the acoustic emission signal levels in the scratches made on the alumina and on the glass owing to the material removal mechanisms associated with the structure of these materials. A gradual increase in the acoustic emission levels was observed when the number of repeated passes was increased as a result of the damage accumulation process followed by severe material removal. It was also noticed that the acoustic emission signals were capable of reflecting the interactions between two parallel scratches.

SAXS measurements of the porosity in Cu(II)-doped SnO2 xerogels during crystallization

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)