Low-temperature synthesis/densification and properties of Si2N2O
prepared with Li2O additive
Q.
F. Tong, J. Y. Wang, Z. P. Li, Y. C. Zhou
J. Europ. Ceram. Soc.,
27 (2007) 4767–4772
Abstract
Dense Si2N2O was successfully synthesized using 2 mol% Li2O as an additive by a hot-pressing method at 1500
oC. Compared to other metal oxide additives, Li2O can significantly decrease the sintering temperature of Si2N2O, which is ascribed to the lower melting point of Li2O–SiO2 and the formation of less viscous liquid phase. Increasing Li2O content has no apparent influence on the mechanical and dielectric properties of dense Si2N2O, which is due to the easy evaporation of Li2O at sintering temperature. The mechanical properties of Si2N2O with Li2O additive are comparable to those of Si2N2O synthesized with other additives. The as-prepared bulk Si2N2O with 2 mol% Li2O additive exhibits both low dielectric constant (6.17 at 1 MHz) and loss tangent (0.0008 at 1 MHz) and combines good mechanical performance, indicating it is a potential high-temperature structural/functional material.
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Trend in crystal structure of layered ternary T-Al-C carbidesc(T =
Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, W, and Ta)
Jingyang
Wang, Yanchun Zhou, Ting Liao and Zhijun Lin
J. Mater. Res.,
Vol. 22, No. 10, Oct 2007:2685-2690
Abstract
Layered ternary T-Al-C ceramics containing early
transition metal Sc, Zr, and Hf, crystallize with the TnAl3Cn+2
formula, while others containing neighbor elements Ti,
V, Cr, Nb, Mo, W, and Ta yield the Tn+1AlCn
formula. Ternary TnAl3Cn+2
ceramics are structurally characterized by NaCl-type TC
slabs being separated by Al4C3-type
AlC layers. In the present study, we suggest that the
ability of forming the TnAl3Cn+2
carbide could be traced back to the structure mismatches
between the TC, Al4C3 and TnAl3Cn+2
compounds. Ternary carbides following the TnAl3Cn+2
formula experience small lattice mismatches and strain
energies. Moreover, the discrepancy between crystal
structures of TnAl3Cn+2
and Tn+1AlCn is interpreted by
lattice mismatch and the produced strain energy for the
ternary T-Al-C ceramics. We also present close
relationships between the atomic radii of transition
metal and lattice mismatch, as well as the strain
energy. The proposed method is not only helpful to
explain the trend in crystal structure of T-Al-C based
ceramics, but may be also general to predict the crystal
structure of layered compounds constructed by
alternatively stacked structural units.
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Synthesis,
Microstructure, and Property of Cr2AlC
Review
Zhijun LIN, Yanchun ZHOU and Meishuan LI
J. Mater. Sci.
Technol.,
Vol.23 No.6, 2007:751-746
Abstract
Cr2AlC is an unusual layered ternary ceramic
that combines the merits of both metals and ceramics.
The
salient properties of Cr2AlC are strongly
related to its bonding characteristics and
microstructures. Synthesis,
microstructure, and property of Cr2AlC are
reviewed in this paper. First, theoretical calculations
and physical
properties are introduced. Then, the processing of Cr2AlC
ceramic in both bulk form and thin films and their basic
mechanical properties are summarized. Atomic-scale
characterizations of Cr2AlC, as well as the
microstructural relationships among Cr2AlC,
Al8Cr5, and AlCr2 were
achieved using a series of transmission
electron microscopy (TEM) techniques. Moreover,
high-temperature oxidation and hot corrosion behaviors
of Cr2AlC were investigated by means of
thermogravimetric analysis, X-ray diffraction, Raman
spectroscopy,
scanning electron microscopy, and TEM. Mechanism of the
excellent high-temperature corrosion resistance of Cr2AlC
is discussed based on systematic microstructural
analyses. Finally, concise conclusions are drawn.
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High-temperature oxidation and hot corrosion of Cr2AlC
Z.J.
Lin, M.S. Li, J.Y. Wang, Y.C. Zhou
Acta Materialia
55 (2007)
6182–6191
Abstract
High-temperature oxidation and hot corrosion behaviors
of Cr2AlC were investigated at 800–1300
oC in air. Thermogravimetric–differential scanning
calorimetric test revealed that the starting oxidation
temperature for Cr2AlC is about 800 C, which is 400
oC higher than other ternary transition metal
aluminum carbides. Thermogravimetric analyses
demonstrated that Cr2AlC displayed excellent
hightemperature oxidation resistance with parabolic rate
constants of 1.08×10 -12 and 2.96×10 -9
kg2 m -4 s -1 at 800
and 1300 oC, respectively. Moreover, Cr2AlC
exhibited exceptionally good hot corrosion resistance
against molten Na2SO4 salt. The mechanism of
the excellent high-temperature corrosion resistance for
Cr2AlC can be attributed to the formation of
a protective Al2O3-rich scale
during both the high-temperature oxidation and hot
corrosion processes.
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Improved Cr2O3
adhesion by Ce ion implantation in the presence of interfacial
sulfur segregation
M.S. Li, P.Y. Hou Acta
Materialia
55 (2007) 443–453
Abstract
As-polished and preoxidized Ni–20Cr alloys were Ce-implanted
with a dosage of 1×10 17 ions/cm2,
then subsequently oxidized at 1050 oC in air.
The oxide adhesion and the extent of sulfur segregation
at the oxide–alloy interface were determined,
respectively, using tensile pull testing and scanning
Auger microscopy with an in situ scratch device. The
critical load for oxide failure was the lowest on the
unimplanted Ni–20Cr, and was slightly higher on those
with implantation made into a preformed oxide. Oxides
that formed directly on Ce-implanted Ni–20Cr never
failed under the pull test, which showed the strongest
scale adhesion; however, similar amounts of interfacial
sulfur, which segregated from the alloy during
oxidation, were found at all interfaces. Ce additions
were also found to reduce the oxidation rate and affect
the extent of voids at the scale–alloy interface. It is
suggested that the change in the oxide growth mechanism
reduces the number of interfacial voids and, unlike Al2O3,
these factors are more important for Cr2O3
scale adhesion than sulfur segregation to the
scale–alloy interface.
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Layered
stacking characteristics of ternary zirconium aluminum carbides
Z.J. Lin, L.F. He, M.S. Li, J.Y. Wang, Y.C. Zhou J.
Mater. Res.,
Vol. 22, No. 11, Nov 2007
Abstract
Layered stacking characteristics of ternary Zr–Al–C
carbides were investigated using scanning transmission
electron microscopy (STEM). Three previously unknown
compounds, i.e., Zr4Al3C6,
Zr5Al6C9, and Zr7Al6C11
were identified. The present study extends the
structural information of ternary Zr–Al–C ceramics. The
influence of the thickness of the NaCl-type Zr-C slab on
the elastic properties of ternary Zr–Al–C ceramics is
discussed based on first-principles calculations. In
addition, direct atomic-resolution observations
illustrate the process for forming the unique layered
crystal structures of ternary Zr–Al–C ceramics. These
results also provide insights into the formation
mechanism of layered ternary Zr–Al–C carbides.
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Synthesis and
Characterization of Bulk Zr2Al3C4
Ceramic
Lingfeng He, Zhijun Lin, Jingyang Wang,Yiwang Bao, Meishuan
Li,Yanchun Zhou
J.
Am. Ceram. Soc.,
90 [11] 3687–3689 (2007)
Abstract
Polycrystalline Zr2Al3C4
was fabricated by an in situ reactive hot-pressing
process using zirconium (zirconium hydrides), aluminum,
and graphite as starting materials. The investigation on
reaction path revealed that the liquid Al played an
important role in triggering the formation of ternary
zirconium aluminum
carbides. The mechanical properties of Zr2Al3C4
at room temperature were measured (Vickers hardness of
10.1 GPa, Young’s modulus of 362 GPa, flexural strength
of 405 MPa, and fracture toughness of 4.2 MPa .m1/2).
The electrical resistivity and thermal expansion
coefficient were determined as 1.10 μΩ.m and 8.1×10
-6 K -1, respectively.
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In Situ
Synthesis and Properties of Ti3AlC2/TiB2
Composites
Chao Li, Meishuan Li, Yanchun Zhou, Jie Zhang, Lingfeng He,
J. Am.
Ceram. Soc.,
90 [11] 3615–3620 (2007)
Abstract
In order to improve the mechanical properties of Ti3AlC2, nearfully dense Ti3AlC2/TiB2 composites were synthesized using Ti, Al, graphite, and B4C powders as the initial materials. Compared with monolithic Ti3AlC2, the composites exhibit a much higher strength (for the compressive strength, from initial 723 MPa to maximal 2205 MPa; for flexural strength, from initial 340 MPa to maximal 861 MPa), and the strengthening effect can be held at least up to 1100
oC. Moreover, besides the enhancement of the elastic modulus and hardness of Ti3AlC2, the introduction of a TiB2 phase makes a positive contribution to its electrical conductivity.
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Microstructure and High-Temperature Corrosion Behavior of a Cr–Al–C
Composite
Zhijun Lin, Meishuan Li, Jingyang Wang, Yanchun Zhou
J.
Am. Ceram. Soc.,
90 [12] 3930–3937 (2007)
Abstract
A Cr–Al–C composite was successfully synthesized by a hot-pressing method using Cr, Al, and graphite as starting materials. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses revealed that the composite contained Cr2AlC, AlCr2, Al8Cr5, and Cr7C3. The orientation relationships and atomic-scale interfacial microstructures among Cr2AlC, AlCr2, and Al8Cr5 are presented. ?This composite displays both excellent high-temperature oxidation resistance in air and hot-corrosion resistance against molten Na2SO4 salt. The parabolic rate constants for the oxidation in air at 1000oC, 1100oC, and 1200oC are 3.0×10
-12, 6.2×10 -11, and 6.2×10 -10 kg2 (m4 . s) -1, respectively, while the linear weight gain rates for the hot corrosion of Na2SO4-coated samples at 900oC and 1000oC are, respectively, 1.210
-3 and 4.410 -3 mg (cm2 . h)
-1. The mechanism of the excellent high-temperature corrosion resistance can be attributed to the formation of a protectively alumina-rich scale.
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Transient
oxidation behavior of nanocrystalline CoCrAlY coating at 1050 ℃
LIU Guang-ming, LI Mei-shuan, MA Jing-hui,DU Nan, ZHOU Yan-chun
Trans. Nonferrous
Met. Soc.
China.,
17(2007):595-599
Abstract
Nanocrystalline CoCrAlY overlay coating was prepared on
M38G superalloy by magnetron sputtering deposition. To
investigate the oxidation behavior and phase
transformation of alumina during oxidation, the
oxidation experiments were conducted at 1 050 ℃ for
various time in the range of 5−180 min. The phase
compositions of the oxide scales were investigated by
using glancing angle X-ray diffraction(XRD). The
microstructure analysis of oxide scales was carried out
by means of scanning electron microscopy(SEM). The
growth process of metastable alumina at the grain
boundaries and transformation to stable alumina were
discussed. The results show that at the initial
oxidation stage the mixture of δ-Al2O3, γ-Al2O3
and α-Al2O3 is formed on the
sample surface rapidly. Especially, δ-Al2O3
and γ-Al2O3 prefer growing at the
grain boundaries of CoCrAlY coating. With increasing
oxidation time, δ-Al2O3 and γ-Al2O3
transform to θ-Al2O3, afterwards
θ-Al2O3 transforms to α-Al2O3
gradually. After 180 min oxidation, θ-Al2O3
transforms into α-Al2O3
completely.
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Effects of
CeO2 applied to preformed oxide scales on subsequent
oxidation of Fe–20Cr at 1000oC
M.
Zhu, M.S. Li, Y.C. Zhou
Corro. Engin, Sci & Technol.,
42(1)2007:73-79
Abstract
In order to explore the relationships between rare earth elements existing in oxides and growth rate and adhesion of oxide scales, CeO2 thin films were prepared on Fe–20Cr alloys after preoxidation at 1000
oC; subsequent isothermal and cyclic oxidation was carried out, and the oxide adhesion was determined by using the tensile pull test. The results demonstrated that similar to the effects of CeO2 on the as polished Fe–20Cr alloy, the application of CeO2 to the preformed oxides could decrease the subsequent oxidation rate, improve the cyclic oxidation resistance of the alloy and raise the adhesion strength of the oxide scales. These beneficial effects of the applied CeO2 decreased with increasing pre-oxidation time. The fact that virtually none of CeO2 applied on the preoxidised preformed layer reached the metal/oxide interface suggests that the ‘sulphur trapping effect’, through which CeO2 may act, is minimal in the present system.
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Thermal
stability of Ti3AlC2/Al2O3
composites in high vacuum
J.X. Chen, Y.C. Zhou, H.B. Zhang, D.T. Wan, M.Y. Liu
Mater. Chem. & Phys.,
104
(2007) 109–112
Abstract
The thermal stability of Ti3AlC2/Al2O3 composites in high vacuum was investigated. On the sample surface, Ti3AlC2 can decompose with the formation of TiC0.67 and gaseous Al at first, then Ti evaporation results in the irregular morphology of non-stoichiometric TiCx. At the same time, the amount of Al2O3 particles becomes less with increasing the soaking time. Finally, Al2O3 particles disappear and a layer of non-stoichiometric TiCx forms on the sample surface. But the bulk Ti3AlC2/Al2O3 composites always keep stable during the vacuum treatment.
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