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| 3. |
- Melhus, Åsa, et al.
(författare)
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Amoxicillin treatment of experimental acute otitis media caused by Haemophilus influenzae with non-beta-lactamase-mediated resistance to beta-lactams : aspects of virulence and treatment
- 1997
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Ingår i: Antimicrobial Agents and Chemotherapy. - 0066-4804 .- 1098-6596. ; 41:9, s. 1979-1984
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Tidskriftsartikel (refereegranskat)abstract
- Through alterations primarily in the penicillin-binding proteins, a non-beta-lactamase-mediated resistance to beta-lactams has evolved in Haemophilus influenzae. The virulence of these chromosomally changed strains has been questioned. To ascertain whether these alterations involve a reduction in virulence of H. influenzae and whether they could be advantageous for the bacterium during amoxicillin treatment of acute otitis media, a total of 70 Sprague-Dawley rats were challenged with a susceptible recipient strain or a genetically similar resistant transformant strain. Antibiotic therapy was started on day 3 after inoculation, and the animals were monitored by daily otomicroscopy and analysis of bacterial samples from middle ear effusions obtained on day 8, the last day of observation. The animals were also sacrificed on days 4 and 8 and after 2 months for morphological examination. Compared with the susceptible recipient strain, recovery from infections caused by the resistant transformant strain was delayed, and the late structural changes were more severe in the animals challenged with the latter strain. The results of the study indicate that chromosomal alterations mediating a relatively low level of resistance to beta-lactams may be advantageous for H. influenzae during antibiotic treatment of a local infection in the rat, and the alterations may occur without any significant loss of virulence.
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| 5. |
- Westman, Anna-Karin, et al.
(författare)
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Chemical reactions in the system Si3N4-SiO2-B2O3
- 1998
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 18:6, s. 633-640
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Tidskriftsartikel (refereegranskat)abstract
- Chemical interactions in the system of silicon nitride with borosilicate glass have been studied as part of an evaluation of glass encapsulated HIP. Theoretical calculations have been performed to predict the thermodynamically stable phases under conditions reflecting different stages in a HIP-cycle. Experimental studies were made on heat treated mixtures of the silicon nitride and the silicate glass. These samples were evaluated with X-ray diffraction. At temperatures commonly used for densification, the system reacted to BN and Si2N2O in agreement with the theoretical calculations. At typical temperatures for pressure application no chemical reactions could be detected but the theoretical calculations showed that BN and, for larger amount of silicon nitride, also silicon oxynitride were stable. Minor amounts of the phases may have formed or non-equilibrium conditions could be explanations for the absence of the expected phases
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| 6. |
- Westman, Anna-Karin
(författare)
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Hot isostatic pressing of ceramics and influence of glass encapsulation
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hot isostatic pressing (HIP) is one of the industrial techniques for production of structural ceramics. A high quality of the as-HIPed surface is highly desirable, as it may make it possible to minimise or eliminate costly finishing machining. When a porous ceramic is HIPed, an encapsulation of glass can be applied on the component as a gas tight envelope. Silicon nitride ceramics work well with a borosilicate encapsulation glass but oxides, such as alumina and zirconia, react severely. A protective barrier between the oxides and the borosilicate glass has been developed earlier. With such a system of protective layers it was possible to demonstrate HIP of some oxides, with good mechanical as well as biocompatibility properties as a result. However, there is a difference between the bulk and the surface properties of these materials. A model system of silicon nitride and borosilicate glass was chosen for a more thorough study on interactions between the encapsulation glass and the porous ceramic green body during HIPing. Mechanical, chemical and compositional gradient from the surface of the ceramic into the bulk were studied. Theoretical calculations on chemical reactions in the system Si3N4 - B2O3 were done accompanied by practical experiments. Special attention has been given the part of the HIP-cycle when the pressure is applied and the glass is pushed against the still porous ceramic component. Possible viscous flow of the glass into the ceramic was analysed. TEM-studies of the densified silicon nitride revealed two different types of hexagonal boron nitride formed in the surface region and their role in the encapsulation is discussed. The increased knowledge can beused in work for improved surface quality and for development of intermediate protective layers.
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| 7. |
- Westman, Anna-Karin, et al.
(författare)
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Interaction of encapsulation glass and silicon nitride ceramic during HIPing
- 1999
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 19:16, s. 2739-2746
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Tidskriftsartikel (refereegranskat)abstract
- Interaction between ceramic compacts and the encapsulation glass during the HIP process has been studied in a model system of silicon nitride and borosilicate glass. Attention has been focused on what happens when the pressure is first applied in the HIP-cycle, i.e. between about 1200 and 1500°C. At this stage the pore system of the ceramic green body is still rather unaffected by sintering. The model system was characterised to evaluate a possible viscous flow of glass into the green body. Two glass compositions, one with high and one with low viscosity, were used, measurements being made of their viscosity and their contact angle on the nitride. Applying Darcy's law it was predicted that the encapsulation glass with the lowest viscosity should penetrate about 1200 microns into the still open pore structure at 1450°C, but this was not observed experimentally. In the calculations no chemical reactions were assumed to take place. However, increases in hardness of heat-treated mixture of glass and silicon nitride powder indicates that nitrogen dissolves in the glass. It is known that nitrogen increases the viscosity of the glass and this would result in a more limited glass intrusion. After HIP the surface region of the dense ceramic exhibited a phase composition gradient of silicon oxynitride, down to approximately 100-200 microns into the bulk
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| 8. |
- Westman, Anna-Karin, et al.
(författare)
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Preparation for TEM of layered samples with fragile microstructure and weak layer interface
- 1999
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Ingår i: Microscopy research and technique (Print). - 1059-910X .- 1097-0029. ; 45:3, s. 198-202
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Tidskriftsartikel (refereegranskat)abstract
- The objective of this work was to prepare for transmission electron microscopy (TEM) a layered structure of materials with fragile microstructure. The samples consisted of two layers of different materials, silicon nitride and borosilicate glass, loosely bonded together. The low strength of the sample resulted in fragmentation during more conventional preparation. However, it was possible to prepare the fragments by mounting them in a titanium specimen carrier with aluminium strips as support. After grinding and polishing, a technique of low-angle ion milling was used to obtain electron beam transparent areas at the nitride/glass interface.
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| 9. |
- Westman, Anna-Karin, et al.
(författare)
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Ultra-micro indentation technique used for examination of mechanical properties close to a HIPed surface of silicon nitride
- 1998
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 18:7, s. 879-890
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Tidskriftsartikel (refereegranskat)abstract
- Ultra-micro indentation using both pointed and spherical tipped indenters has been used to characterize mechanical properties of silicon nitride densified by glass encapsulated hot isostatic pressing (HIP). Young's modulus and hardness have been studied as a function of distance to the interface between silicon nitride and the encapsulation glass. The Young's modulus values are 10 to 20% lower in the close vicinity of the silicon nitride surface compared to bulk values. At distances of 75 to 150 microns from the glass-silicon nitride interface, bulk values are measured. The differences in hardness values between the region close to the surface and the bulk is less pronounced. A possible explanation for these gradients is formation of new phases at the surface of the silicon nitride. Routines for the calibration of both the pointed and spherical tipped indenters are presented
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