摘要:

Small Al2O3additions (∼ 4 vol%) made to Y‐TZP using an infiltration technique increased the fracture toughness and strength by ∼15% and the amount of transgranular fracture. Ionic conductivity measurements showed decreased grain‐boundary conductivity, confirming a change in the grain‐boundary composition. The predominant failure origins for both the unmodified Y‐TZP and the Al2O3—Y‐TZP were surface flaws related to agglomerates in the original powder. Finishing reduced the severity of these flaws and substantially increased the strength of both materials (>50%). The infiltration approach introduced a new flaw population in some specimens; however, this problem was overcome by a simple processi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08967.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Internal friction, torsional creep, and shear modulus relaxation experiments were conducted on a model Si3N4polycrystalline material, which contained a continuous amorphous film of pure SiO2at the grain boundary. Internal friction experiments were performed in the frequency range between 3 and 13 Hz, in 5 Pa of nitrogen atmosphere. Very high temperatures (up to 2000°C) could be applied for the first time by using a newly developed torsional pendulum apparatus. This apparatus was also capable of precise torsional strain measurements under static‐load conditions. The internal friction curves at various frequencies were generally found to consist of a grain‐boundary peak super‐imposed on an exponential‐like background. The peak, of anelastic diffusive origin, was centered in the temperature range of 1612–1710°C depending on the frequency of the measurement, namely within an interval of about 100°C below the nominal melting point of the pure SiO2phase (i.e., ∼ 1730°C). The background was instead found to be of viscoelastic nature. A common micromechanical origin between the creep plastic strain and the internal friction background curves was identified and the data could be fitted by the same Arrhenius plot. Structural and chemical characterization of internal grain boundaries was performed by high‐resolution electron microscopy (HREM) in addition to electron energy‐loss spectroscopy (EELS). A small amount of nitrogen was detected within the amorphous residue along grain boundaries. According to the above set of microstructural/chemical and mechanical data, the viscosity properties of the intergranular phase were assessed and the sliding mechanism between adjacent Si3N

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08968.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The fictive temperature dependence of the relative abundances of three‐ and four‐coordinated boron was investigated in several sodium borate and borosilicate glasses using high‐resolution11B nuclear magnetic resonance (NMR). In the compositions with low sodium/boron ratios, no effect was observed, but in the borosilicates, the fraction of the tetrahedral species decreased significantly as the fictive temperature increased because of the higher content of nonbridging oxygens.In situ, high‐temperature magic‐angle spinning NMR demonstrated the exchange of the two species at a rate comparable to the shear relaxation rate, indicating a close link between B‐O bond breaking and v

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08969.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Viscous deformation and the adhesion force at the contact point between amorphous silica particles under ultrahigh isostatic pressure (up to 1 GPa) are important in the densification of powder compacts. The amount of viscous deformation and the strength of adhesion force have been changed in the present study by altering the calcination temperature and particle diameter, and the new values have been determined successfully using a diametral compression test. The diameter of spherical and monosized alkoxide‐derived silica powders has been controlled within the range of 10–400 nm. Close‐packed granules of these powders have been produced by spray drying. Because of viscous deformation, as‐spray‐dried ultrafine silica powders without calcination could be consolidated into highly dense compacts (>74% of theoretical density) by applying ultrahigh isostatic pressure (1 GPa). Relatively high temperature in the calcined particles (400°C) causes viscous deformation at the contact point to disappear almost completely and clearly increases the adhesion force, because of neck growth that has resulted from viscous sintering. At temperatures>200°C, the green density of the calcined powders decreases to 65% of theoretical density, even under 1 Gpa pressure. The relationship between green density and viscous deformation in silica particles at the point of contact has been analyzed quantitatively by the Hertz and Rumpf model. On the other hand, if relatively low isostatic pressure (Pc<100 MPa) is applied, the green density and intergranular pore volume depend on the strength of the spray‐dried granules. The relationship between granule strength and neck growth at the contact point with calcination has been estimated q

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08970.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Reaction‐bonded aluminum niobate‐based (AlNbO4‐based) composites with tailorable microstructures were fabricated through controlled nucleation and growth of needlelike AlNbO4grains. Equiaxed to highly elongated grains were obtained by different heat treatments at 1340° and 1400°C in air. Scanning electron microscopy observations revealed that increasing the heating rate promoted the development of large, homogeneously distributed needlelike grains. Because of the elongated nature of the AlNbO4grains and the high residual stresses at the interface, enhanced crack wake bridging occurred. This bridging accounted for the observed increase of fracture to

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08971.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Compressive prestress effects on the electrical and mechanical properties of relaxor ferroelectric materials were studied as a function of temperature for several formulations of Pb(Mg1/3Nb2/3)O3‐PbTiO3‐BaTiO3(PMN‐PT‐BT) ceramics. Experimentally measured polarization and strain, induced by an ac electric field, decreased as compressive stress increased. Effective Young's moduli also were measured under constant dc electric fields. A significant decrease in modulus was observed with increasing field. The prestress and modulus experiments were modeled analytically using a proposed relaxor ferroelectric constitutive law. In general, excellent agreement between the model and experiments was obtained, indicating that the model accurately predicted the coupled behavior of this relaxor ferroelectric m

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08972.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Highly oriented Sr0.5Ba0.5Nb2O6(SBN50) thin films have been prepared using a sol‐gel method. A homogeneous and stable strontium barium niobate (Sr1‐xBaxNb2O6, SBN) precursor solution could be prepared via the reaction control of metal alkoxides. The SBN precursor was stabilized by the coordination of the 2‐ethoxyethoxy group to metals. SBN thin films on MgO(100) crystallized to a mixture of orthorhombic and tetragonal phase at 700°C and then transformed completely to the tetragonal phase of tungsten bronze at 1000°C. Two crystal lattice planes of SBN were intergrown at an orientation of 18.5° on MgO(100). SBN50 thin films on Pt(100)/MgO(100) substrates exhibited theP‐E

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08973.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Two processes for the crystallization of titanium‐doped LiNbO3(LNT) gel thin films were investigated in the present study. LNT gel thin films were prepared on platinized glass and platinized sapphire substrates, as the application of a platinum intermediate layer greatly affected the orientation of LNT films on these substrates. X‐ray pole figures revealed the well‐oriented growth for LiNbO3both on the platinum (111)/sapphire C substrate and bare sapphire C. Another approach to crystallizing LNT gel film was attempted by developing a new technique to fabricate a LiNbO3waveguide on the sapphire substrate using a Nd:YAG laser beam. The crystallization of LNT was achieved successfully with no cracking or peeling. The prepared crystalline LNT films also were characterized by Raman microspectroscopy. S. E. Trolier‐McKinstry—contribut

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08974.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

This paper evaluates the fracture process for fiber‐reinforced glasses under tensile loading. Two types of unidirectionally aligned Nicalon SiC‐fiber‐reinforced glass with different fiber coatings were examined. One channel acoustic emission (AE) measurement was employed during the tensile tests. Probabilistic fracture analysis as well as the replication technique were used to investigate the relation between the AE signals and fracture processes. The AE technique proved to be an effective method for observing fracture processes of the material systems studied. The fracture process could be distinguished in terms of the AE amplitude. AE signals with high amplitudes corresponded to fiber breaking; AE signals with low amplitudes corresponded to matrix cracking, interfacial debonding, and fiber pullout. In the well‐toughened material studied the reinforcing fibers would break extensively over 75% load of the ultimate s

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08975.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The effect of stable crack extension on fracture toughness test results was determined using single‐edge precracked beam specimens. Crack growth stability was examined theoretically for bars loaded in three‐point bending under displacement control. The calculations took into account the stiffness of both the specimen and the loading system. The results indicated that the stiffness of the testing system played a major role in crack growth stability. Accordingly, a test system and specimen dimensions were selected which would result in unstable or stable crack extension during the fracture toughness test, depending on the exact test conditions. Hot‐pressed silicon nitride bend bars (NC132) were prepared with precracks of different lengths, resulting in specimens with different stiffnesses. The specimens with the shorter precracks and thus higher stiffness broke without stable crack extension, while those with longer cracks, and lower stiffness, broke after some stable crack extension. The fracture toughness values from the unstable tests were 10% higher than those from the stable tests. This difference, albeit small, is systematic and is not considered to be due to material or specimen‐to‐specimen variation. It is concluded that instability due to the stiffness of test system and specimen must be minimized to ensure some stable crack extension in a fracture toughness test of brittle materials in order to avoid inflated fracture toughne

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08976.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY