Ceramic Conversion Research Articles

Fundamental Issues in the Synthesis of Ferroelectric Na0.5K0.5NbO3 Thin Films by Sol−Gel Processing

In contrast to films fabricated by physical vapor deposition methods, thin films of Na0.5K0.5NbO3 (NKN) made by metal alkoxide sol−gel routes generally fail to exhibit polarization-electric field responses typical of a ferroelectric. This study sets out to investigate the reasons for the problems in producing sol−gel NKN films by examining the thermochemistry of the gel to ceramic conversion. The NKN precursor gels displayed multiple heating DTA crystallization exotherms in the temperature range 480−550 °C, which are attributed to compositional segregation of NKN components. At higher temperatures, 800−850 °C, a heating DTA endotherm and cooling DTA exotherm are indicative of melting and recrystallization of sodium/potassium carbonate secondary phases. Additionally, repeated thermal analyses, after storage of the gel decomposition product under ambient conditions, revealed a tendency for hydration and carbonation on exposure to air. Together, these are critically limiting reasons for the absence of strong...

Synthesis, Characterization and Ceramic Conversion Studies of Borosiloxane Oligomers from Phenyltrialkoxysilanes

Borosiloxane oligomers, which give ceramic residue in the range of 64–75% (at 900 °C in inert atmosphere), were synthesized through the condensation of boric acid with phenyltrimethoxysilane (PTMOS) and phenyltriethoxysilane in diglyme both in the presence and absence of hydrochloric acid as catalyst. The effect of concentration of the reactants and reaction time on the processability, yield and thermal stability of the product obtained was studied. The oligomers obtained are soluble in common organic solvents and were characterized by FT-IR and NMR spectroscopy, GPC and TGA. The ceramic conversion of one of the oligomers was carried out by heat treatment at 900, 1,500 and 1,650 °C in inert atmosphere. XRD studies reveal that the ceramic formed at 1,500 °C is amorphous and undergoes crystallization to β-SiC when heat treated at 1,650 °C.

Ceramic conversion treatment of zirconium alloys to combat corrosion wear

A series of ceramic conversion treatments have been conducted on Zr702 and Zr705 zirconium alloys by thermal oxidation at temperatures ranging from 400 to 700°C for 3–30 h. A wide range of material characterisation techniques were used to investigate the microstructure, composition, phase constituent, load bearing capacity and wear properties of the surface treated zirconium alloys. Experimental results show that after ceramic conversion treatments, the surfaces of Zr702 and Zr705 alloys can be converted to a ceramic layer consisting of α-ZrO2 and β-ZrO2; the optimal ceramic conversion treatment conditions for Zr702 and Zr705 alloys are 600°C/10 h and 600°C/3 h respectively; Zr705 alloy has a better response to ceramic conversion treatment than Zr702; and the wear resistance of the optimally treated zirconium alloys can be improved by more than 200 times. The wear mechanisms of both untreated and ceramic conversion treated zirconium alloys are discussed. The dominant wear mechanisms changed from severe adhesive wear for the untreated zirconium alloys to mild abrasive wear for the ceramic conversion treated materials.

SYNTHESIS OF POLYCARBOSILANE FROM LIQUID POLYSILANE WITH BENZOYL PEROXIDE AS AN INITIATOR

The intermediate product of the pyrolysis process of polydimethylsilane(PDMS),liquid polysilane(LPS),is a mixture of low molecular weight silanes with different structure and reaction activity.The diversity of the reaction activity among the components leads to the different reaction degree of rearrangement and polycondensation,and it results in a low synthesis yield of polycarbosilanes(PCSs).In this work,PCSs were synthesized from LPS,which was obtained by thermal decomposition of PDMS,with an reaction additive——benzoyl peroxide(BPO) as the initiator.By using this method the yield of PCS from LPS has an increase of 20%～25%.The reaction process was traced by FTIR and GPC.The structure,composition and properties of PCS were investigated by EA,()~1H-NMR,()~(13)C-NMR,TG-DTA and XRD.The reaction mechanism of LPS with BPO was studied in detail.The results showed that,with raising the mass ratio of BPO to LPS and the reaction time,the weight average molecular weight(M_w) and softening point of PCSs increased.The Si—H bond content of PCSs increased with temperature in the range below 440℃ and decreased over 440℃,while the Si—CH_2 bond content of PCSs increased with temperature rising.The conversion process included initially the formation of PCS by thermal decomposition of LPS and also then the increase in molecular weight by condensation among the PCS molecules.And BPO can effectively catalyze the thermal decomposition and rearrangement of Si—Si bonds in LPS when heated,and also the formation of Si—CH_2—Si from Si radical and Si—CH_2 radicals.The molecular chain of PCS grows up,and the yield of PCS increases accordingly.Some phenyl and benzoyloxy groups are introduced into PCS molecules;it results in a little increase of carbon and oxygen contents of PCS.However the excessive BPO leads to excess crosslinking of PCS and makes the reaction uncontrollable.The proper softening point of the achieved PCS can be tailored by controlling the mass ration of BPO to LPS about 0.02,the reacting temperature(about 450℃) and reacting time(3～4 h),and a high synthesis yield of about 67% can be obtained,which is much higher than that of PCS synthesized by existed method.In addition,the carbon content of PCS increases by about 4%,and the oxygen content of PCS increases by about 0.4%.However they have little effect on the PCS's ceramic yield and pyrolysis conversion properties.When the softening points of PCS is in the range of 210～220℃,the ceramic yield is over 70%.It is proved by the TG-DTA and XRD analyses of the pyrolsis products of PCS.

Thermal behaviour of a series of poly[ B-(methylamino)borazine] for the preparation of boron nitride fibers

The thermal behaviour of a series of poly[ B-(methylamino)borazine] prepared at various temperatures ranging from 140 to 200 °C is studied in the present paper as potential BN fiber precursors. It was shown that the softening capability of poly[ B-(methylamino)borazine] can be tailored by controlling the temperature at which polymers were prepared to achieve melt-spinning and produce high quality green fibers. Thus as-spun fibers could be next converted into boron nitride fibers using ammonia (25–1000 °C) and nitrogen (25–1800 °C) atmospheres. The quality of boron nitride fibers was shown to depend on the first part of the pyrolysis step (25 and 1000 °C; ammonia atmosphere) in which the great majority of the weight loss necessary for boron nitride production occurs. Ideal poly[ B-(methylamino)borazine] as BN fiber precursors are those prepared between 170 and 180 °C. They display appropriate melt-spinnability and ceramic conversion capability.

Polymer–ceramic conversion of a highly branched liquid polycarbosilane for SiC-based ceramics

Liquid polycarbosilane (LPCS) with a highly branched structure was characterized by fourier-transform infrared spectrometry (FT-IR) and 1H, 13C, 29Si nuclear magnetic resonance spectrometry (NMR). The LPCS was then cured and pyrolysized up to 1,600 °C under flowing argon. The structural evolution process was studied by thermogravimetric analysis and differential scanning calorimetry (TG-DSC), FT-IR, and X-ray diffraction (XRD). Hydrosilylation, dehydrocoupling, and polymerization cross-linking reactions between Si–H and C=C groups occurred at low temperatures, which mainly accounted for the high ceramic yield (70%) up to 1,400 °C. The organic groups gradually decomposed and the structure rearranged at high temperatures. The FT-IR analysis revealed that Si–CH2–Si chains, the backbone of original polymer, can be retained up to 1,200 °C. At temperatures higher than 1,200 °C, the Si–CH2–Si chains broke down and crystalline SiC began to form. The final crystalline products were β-SiC and a small amount of carbon.

Towards industrialisation of microarc oxidation (MAO) for ceramic conversion surface treatment of magnesium and aluminium alloys

Microarc oxidation (MAO), as a novel surface treatment, has demonstrated significant achievements both in academic research and in industrial application. Here, the electrical energy consumption and the role of the electrolyte during the MAO process are discussed. The corrosion resistance of MAO coatings on a magnesium alloy has been evaluated by salt spray test and electrochemical analysis. The wear resistance of MAO coating on an aluminium alloy has been tested by an abrasion technique. Finally, application prospects are discussed.

Tribological characteristics of ceramic conversion treated NiTi shape memory alloy

NiTi shape memory alloys are very attractive for medical implants and devices (such as orthopaedic and orthodontic implants) and various actuators. However, wear is a major concern for such applications and a novel surface engineering process, ceramic conversion treatment, has recently been developed to address this problem. In this study, the tribological characteristics of ceramic conversion treated NiTi alloy have been systematically investigated under dry unidirectional wear, reciprocating-corrosion wear and fretting-corrosion wear condition. Based on the experimental results, the wear behaviour under different conditions is compared and wear mechanisms involved are discussed.

Effect of ceramic conversion treatments on the surface damage and nickel ion release of NiTi alloys under fretting corrosion conditions

Recent researches have demonstrated that surface modification can improve the fretting wear resistance of NiTi alloys in air or enhance their aqueous corrosion resistance without fretting. However, little is known about the behaviour of surface engineered NiTi under fretting corrosion conditions. This is important for such body implants as orthodontic arch wires and orthopedic bone fixation devices because they need to withstand the combined attack of corrosion from body fluid and mechanical fretting. In this study, a NiTi alloy was ceramic conversion (CC) treated at 400 and 650 degrees C. The effect of the surface treatment on the fretting corrosion behaviour of NiTi alloy was investigated using fretting corrosion tests in the Ringer's solution. The experimental results have shown that the CC treatment can convert the surface of NiTi into a TiO2 layer, which can effectively improve the fretting corrosion resistance of NiTi alloy and significantly reduce Ni ion release into the Ringer's solution. Detailed SEM observations revealed that the untreated samples were severely damaged by adhesion and delamination; the high temperature (HT) (650 degrees C/1 h) treated samples were damaged mainly by spallation and adhesion; and the low temperature (LT) (400 degrees C/50 h) treated samples were characterised by mild abrasion. Mild oxidation and corrosion were also observed for all three types of samples tested under fretting corrosion conditions.

Effect of ceramic conversion surface treatment on fatigue properties of Ti6Al4V alloy

Samples of Ti6Al4V alloy were ceramic conversion (CC) treated. The fatigue properties of untreated and CC treated samples were evaluated with a conventional Wöhler cantilever rotating-bending fatigue machine under both plain fatigue and fretting fatigue conditions. Experiment results showed that CC treatment had a detrimental effect on plain fatigue properties and reduced the plain fatigue limit of the investigated Ti6Al4V alloy by about 24%. However, the fatigue property loss could be fully recovered by an additional shot peening after CC treatment. More importantly, shot peening after CC treatment resulted in considerably longer fatigue life over untreated and CC treated materials in the finite-life region. In contrast to its detrimental effect on plain fatigue, CC treatment appeared to be beneficial to fretting fatigue property. It increased fretting fatigue strength (FFS at 10 7 cycles) of the Ti6Al4V alloy by approximately 10%. Shot peening after CC treatment further enhanced the fretting fatigue properties, with an improvement in FFS (10 7 cycles) of about 20% over the untreated material under the present treatment and testing conditions.

Effect of surface treatment by ceramic conversion on the fretting behavior of NiTi shape memory alloy

Three types of surface-treated NiTi samples, M-1 (700 °C/0.5 h), M-2 (650 °C/1 h) and M-3 (400 °C/50 h), were prepared by ceramic conversion treatment under different conditions. The effect of the surface treatment on the fretting behavior of NiTi alloy was investigated in the Ringer’s solution by using a horizontal servo-hydraulic fretting apparatus. The experimental results indicated that the surface layer of the low temperature (400 °C) treated samples M-3 was dominated by a single TiO2 layer, while the high temperature (650 and 700 °C) treated samples M-1 and M-2 consisted of surface TiO2 layer followed by a TiNi3 layer. These surface layers were found to have a strong effect on the fretting behavior of the NiTi alloy in terms of changes in the shape of the curves of the tangential force (F t) versus displacement (d), the fretting regimes and the damage mechanisms involved. The stress-induced reorientation of martensite bands in the NiTi alloy could decrease the slope of the F t–d curve and thus increase the elastic accommodation ability of the NiTi plate against 1Cr13 steel ball pair. However, since the surface-treated layers could suppress the martensite reorientation in the NiTi substrate and thus decrease the elastic accommodation ability of NiTi, the gross slip started at a smaller displacement amplitude for the surface-treated NiTi samples than for the untreated one. The main wear mechanism of the as-received NiTi alloy in slip regime was adhesion and delamination, while the major damage to the high temperature treated NiTi samples M-1 and M-2 was determined as the spallation of surface-treated layers. Due to the high bonding strength of the surface-treated layer with NiTi substrate, the low temperature treated NiTi samples M-3 showed the best fretting wear resistance in all samples tested.

Kinetic Modeling of the Polymer-Derived Ceramics Route: Investigation of the Thermal Decomposition Kinetics of Poly[B-(methylamino)borazine] Precursors into Boron Nitride

A complete kinetic modeling of the polymer-derived ceramics (PDCs) route is achieved for the first time through the investigation of the solid-state decomposition of a typical melt-spinnable poly[B-(methylamino)borazine] into boron nitride fibers at various heating rates. Through the use of the Lorentz fitting approach, it is shown that the two-step weight loss associated with the polymer-to-ceramic conversion is governed by a complex interplay of five diffusion-type transport mechanisms that are independent of the applied heating schedule. The application of the Friedman method to dynamic thermogravimetry data yields Ea and ln A values that are seen to increase with the extent of the ceramic conversion from region one (Ea = 38.73 kJ mol(-1)) to region five (Ea = 146.64 kJ mol(-1)). This fact indicates that both the mechanisms within those regions are parallel routes to the formation of the final solid-state material and their complexity increases with the conversion progress. The cross-linking process (first weight loss) of the polymer is governed by three dependent poorly energetic mechanisms. The first weight loss is activated by ammonolysis reactions that provide a modified polymer capable of undergoing condensation reactions in regions two and three to yield a highly cross-linked polymer. A large evolution of methylamine is identified during this process. Mineralization (region four) and ceramization (region five) steps are represented by two highly energetic multistep mechanisms. The mineralization step is associated with a large evolution of methylamine and occurs during the transition between the cross-linking and ceramization processes through the cleavage of the inter-ring B-N bonds. Ceramization represents the end of the polymer-to-ceramic conversion in which the planar consolidation of BN hexagons occurs through complex structural rearrangements of the borazine units (cleavage of the intraring B-N bonds) accompanied with an ammonia evolution. Finally, the simulation of the polymer-to-ceramic conversion was demonstrated through a simplified model that appropriately predicted experimental data.

Fabrication of Ceramic Microchannels with Tailored Pores

This paper reports fabrication of high temperature stable, chemically inert SiC and SiCN monolithic porous microchannels by micromolding in capillaries (MIMIC) method. These types of high surface area materials hold tremendous untapped potential in micro-total analysis systems, micro-reaction technology and clean energy systems. Owing to their excellent durability under harsh conditions, SiC and SiCN micro-components are imperative in fields like fuel cells, for hydrogen generation and as environmental sensors. The report also describes the use of template particles of different size and type with diameter of ranging from 0.5 to 1.5 µm to prepare porous, high temperature stable ceramic microchannels. In the present work, ceramic porous channels have been fabricated via polymer to ceramic conversion route by using commercially available polymer precursors.

Template Routes to Non-Oxide Ceramic Nano- and Micro-Structures

AbstractNon-oxide ceramic materials like boron carbide and silicon carbide are technologically relevant as advanced high temperature materials, while boron nitride is a thermally robust low-k insulating material with electronic applications. Efficient routes to boron carbide, boron nitride, and silicon carbide ceramic nanostructures have been developed which employ molecular and polymeric precursors, including the boron carbide precursor 6, 6'-bis(decaboranyl)hexane, the boron nitride precursor polyborazylene, and a commercially available silicon carbide precursor allylhydridopolycarbosilane(AHPCS), in conjunction with colloidal silica and biological silica “diatom” templates. Layered submicron-sized ordered void structures with three-dimensional periodicity and tunable length scales were fabricated by the melt infiltration of the precursors into ordered colloidal silica bead templates. Pyrolytic ceramic conversion followed by dissolution of the silica beads by chemical treatment with aqueous HF or NaOH generated highly uniform ceramic structures with thicknesses up to 50μm and ordered voids ranging in diameter from 50-150 nm. Following on earlier work by Sandhage who generated polymer and oxide ceramic structures, vacuum filtration of the ceramic precursor solutions through bioclastic silica diatom frustule templates generated polymer coated replicas of their 3-D micro- and nanostructures. Subsequent pyrolysis and dissolution of the frustules in 48% HF yielded free-standing ceramic structures with fine features on length scales of 60-200 nm. This technique therefore provides a large scale route to nano- and micro structured non-oxide ceramic materials. Structural control of the end products was achieved by changing the concentration of the precursor solution, pore size and/or the frustule template. Characterization by XRD, DRIFT, SEM, TEM and possible uses of these uniform nano- and micro-structured ceramics will be discussed.

Polymer–Ceramic Conversion of Liquid Polyaluminasilazanes for SiAlCN Ceramics

Liquid‐phased polyaluminasilazanes were synthesized by simply reacting a liquid polyureasilazane with aluminum isopropoxide. The reaction was studied by Fourier‐transform infrared spectrometry (FTIR) and mass spectroscopy. The resultant polyaluminasilazanes were then pyrolysized to form amorphous SiAlCN ceramics at 1000°C in flowing N2. The structural evolution during pyrolysis was studied using solid‐state NMR and FTIR. Spectral analysis revealed that the amorphous structures of the ceramics consisted of SiC2N2 and SiCN3 as major building units, small amounts of SiC3N and SiN4 units, and penta‐ or hexa‐coordinated aluminum with nitrogen. Such units were connected with each other through C−C/C=C bonds or by shearing N to form network structures. The results also revealed that the relative amount of these units strongly depended on the Al content.

Preparation and characterization of porous, biomorphic SiC ceramic with hybrid pore structure

Bio-carbon template (charcoal) was prepared by carbonizing pine wood at 1200 °C under vacuum, and was impregnated with phenolic resin/SiO 2 sol mixture by vacuum/pressure processing. Porous SiC ceramics with hybrid pore structure, a combination of tubular pores and network SiC struts in the tubular pores, were fabricated via sol–gel conversion, carbonization and carbothermal reduction reaction at elevated temperatures in Ar atmosphere. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) were employed to characterize the phase identification and microstructural changes during the C/SiO 2 composites-to-porous SiC ceramic conversion. Experimental results show that the density of C/SiO 2 composite increases with the number of impregnation procedure, and increases from 0.32 g cm −3 of pine-derived charcoal to 1.5 g cm −3 of C/SiO 2 composite after the sixth impregnation. The conversion degree of charcoal to porous SiC ceramic increases as reaction time is lengthened. The resulting SiC ceramic consists of β-SiC with a small amount of α-SiC. The conversion from pine charcoal to porous SiC ceramic with hybrid pore structure improves bending strength from 16.4 to 42.2 MPa, and decreases porosity from 76.1% to 48.3%.

Boron-modified polysilazane as a novel single-source precursor for SiBCN ceramic fibers: synthesis, melt-spinning, curing and ceramic conversion

A novel boron-modified polysilazane functionalised with Si- and N-bonded methyl groups has been synthesised and characterised by means of FT-IR and NMR spectroscopies as well as elemental analysis. Both Si- and N-bonded methyl groups inhibited extensive cross-linking to yield a tractable polymer which was successfully processed into polymer green fibers by a melt-spinning process. After the shaping processing, the N-bonded methyl groups offered the capability of facile polymer cross-linking in an ammonia atmosphere at 200 °C to increase the ceramic yield of the polymer and avoid inter-fiber fusion. The as-obtained cured fibers were subsequently pyrolysed at 1400 °C in a nitrogen atmosphere to provide amorphous and stable Si3.0B1.0C5.0N2.4 ceramic fibers with ca. 1.3 GPa in tensile strength, ca. 170 GPa in Young's modulus and ca. 23 μm in diameter.

Olefin hydroboration of borazine with vinylsilanes as precursors of Si–B–C–N ceramics

A processible polymeric precursor for Si–B–C–N ceramics was prepared by a hydroboration reaction between borazine and dimethyldivinylsilane using no catalyst with a synthetic yield 85%. The synthesized liquid polymer with an approximately 40% ceramic yield changed to an insoluble solid via a cross-linking reaction of the unreacted vinyl groups to the B–C alkyl bridges between the borazine ring and silane as post-heated from 200 to 600 °C. The insoluble solid was then transformed to SiCN and BN amorphous structures when pyrolyzed to 1400 °C. The synthetic and ceramic conversion chemistry was thoroughly investigated by infrared, 1H, 11B, 13C and 29Si nuclear magnetic resonance spectroscopy, X-ray diffraction and thermal gravimetric analysis.

Electrostatic Spinning, Pyrolysis, and Characterization of Boron Carbide Nanofibers Prepared from Poly(norbornenyldecaborane) - a Polymeric Ceramic Precursor

ABSTRACTElectrostatic spinning is a well-developed technique for the fabrication of fibers in the nanoscale domain. Novel boron carbide nanofibers were generated by the electrostatic spinning and ceramic conversion of poly(norbornenyldecaborane) (PND) - a polymeric ceramic precursor. The prepyrolyzed fibers were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The ceramic fibers were characterized by SEM, X-ray diffraction (XRD), 11B magic-angle spinning nuclear magnetic resonance (MAS NMR) and DRIFT spectroscopy. SEM analysis showed retention of the nanostructure in the pre- and postpyrolyzed fibers.

Solid-state NMR investigations of the polymer route to SiBCN ceramics

Silicon based polymers obtained by ammonolysis of organochlorosilylboranes and their pyrolytic transformation into Si-B-C-N ceramics were studied by a detailed solid-state NMR investigation. Sol–gel polymerisation/pyrolysis routes were applied to form Si-B-C-N materials with exceptional high-temperature stability. The polymer to ceramic conversion was analyzed by 11B, 13C, 15N, and 29Si MAS NMR spectroscopy as well as by thermal analysis measurements coupled with mass spectroscopy (TGA–MS). The results showed that a significant change in the carbon-, silicon-, and boron-coordination environments occurs during pyrolysis. An evolution of cleavage of silcon–carbon–boron bridges and the formation of new BN3 sites was observed. The NMR data obtained suggest the presence of a rather homogeneous dispersion of the boron atoms in the as synthesized silicon carbonitride phase, supporting the high thermal stability with respect to decomposition found in these compounds.Key words: organosilicon polymers, polymer pyrolysis, SiBCN ceramics, solid-state NMR.