Three‐Dimensional Stereolithography of Alumina Photonic Crystals for Terahertz Wave Localization

Using stereolithography as an additive manufacturing (AM) process, photonic crystals, with a diamond‐like structure composed of alumina microlattices, were fabricated and their electromagnetic band gap profiles, in terahertz frequency ranges, were investigated. Acrylic resins with dispersed alumina particles were fabricated by stereolithography with micrometer‐order accuracy. After dewaxing and sintering processes, alumina lattices were obtained with high relative densities that reflected the terahertz waves, through Bragg diffraction, perfectly in all directions. Twinned crystal structures with mirror symmetric diamond lattices were designed to introduce defect interfaces. Double‐cavity defects consisting of unit cells hollowed from the diamond lattices formed the coupled resonation modes.     

Fabrication of Photonic Crystal with a Diamond Structure Having an Air Cavity Defect and its Microwave Properties

Three-dimensional photonic crystals with a diamond lattice structure consisting of 5 × 5 × 5 unit cells with the unit cell dimension of 15 mm were fabricated using TiO₂-based ceramic particles dispersed epoxy by stereolithography. The diamond lattice showed a perfect band gap between 14.3 and 15.8 GHz. An air cavity defect with a rectangular shape (15 mm × 45 mm × 15 mm) was introduced at the center of the crystal by extracting 3 unit cells in order to investigate the shape effect of the defect on the formation of localized defect modes of electromagnetic wave. When microwaves were radiated normal to the wide sides (45 mm × 15 mm) of the rectangular shape defect, a sharp localized mode appeared at the middle of the band gap. However, no localized mode was observed for incident waves normal to the smaller side (15 mm × 15 mm) because of the symmetry mismatching between internal eigenmodes in the defect cavity and incident plane waves. The mode analysis using a simple cavity model showed the penetration of the electric field of resonant modes about 2.4 mm into the host lattice.     

Super-cell calculation of the nitrogen defect in diamond and cubic silicon carbide

The nitrogen point defect in diamond and cubic SiC was studied using an ab-initio plane wave pseudopotential approach that highlights the differences in the conduction bands of the two materials. Each N defect energy level is located relative to the various energy bands. The N defect level in diamond shows very little dispersion through the bands, which is indicative of its localized character. On the contrary, in cubic SiC the defect is far less localized, with the level essentially depending on the occupancy of either the C or Si site. Both sites may give n-type behavior, but N on the C site is preferred, with a defect level substantially pinned to the conduction band.     

Molecular Dynamics Computer Simulations of the Interface Structure of Calcium–Alumino–Silicate Intergranular Films between Combined Basal and Prism Planes of α-Al2O3

The interface structures of calcium–alumino–silicate (CAS) glassy intergranular films (IGFs) formed between the combined basal and prism orientations of α-Al₂O₃ crystals were studied using molecular dynamics simulations. Preferential adsorption of specific ions from the IGFs to the contacting surfaces of the alumina crystals was observed. This segregation of specific ions to the interface enables formation of localized, ordered structures between the IGF and the crystal. However, the segregation behavior of the ions is anisotropic, depending on the orientation of the α-Al₂O₃ crystals. The results show that the enrichment of Ca atoms at the basal interface inhibits growth in the 〈0001〉 direction. However, at the (11 2 0) prism plane, Ca ions have little effect on the epitaxial adsorption of Al and O ions from the IGF onto the (11 2 0) surface. Increasing alumina concentration in the glassy IGF enhances adsorption of Al ions onto the prism surface, with little effect on the basal surface, indicating the tendency of growth in the 〈11 2 0〉 direction on the prism plane, but limited growth on the basal plane. These results are consistent with the experimental data regarding anisotropic grain growth in alumina sintered in the presence of CAS IGFs.     

Shock-induced phase transition of oriented pyrolytic graphite to diamond at pressures up to 15 GPa

In studies of shock-induced phase transition of ordered pyrolytic graphite to a diamond-like phase, the lowest transition onset pressure was observed at 19.6 GPa. The phase transition in that case was considered to be martensitic. In the present study ordered pyrolytic graphite with voids between particles was loaded at pressures up to 15 GPa using a planar shock wave propagating along the basal plane of the graphitic crystal structure. As a result, both diamond-like carbon and diamond were observed in the postshock sample. The phase transition of graphite to diamond was assumed to occur by the release of distortional energy stored in the graphite particles, that is, diffusional-controlled reconstructive mechanism, on the basis of the data by high resolution electron microscopy together with electron energy loss spectroscopy.     

Creation of Titania Artificial Interfaces with Geometric Patterns by Using Microstereolithography and Aqueous Solution Techniques

Titania micropatterns with periodic arrangements were successfully formed on glass substrates for use with electromagnetic wave energy resonance and localizations in terahertz frequency ranges. Geometric arrangements of acrylic polygonal tablets with titania particle dispersions were fabricated by using micropatterning stereolithography. Moreover, periodically arranged full anatase‐phase titania tablets were created homogeneously through liquid‐phase crystal depositions of water solvent processes, using microtemplates fabricated by using the stereolithography system. The terahertz wave properties were measured and calculated by using a time‐domain spectroscopic system and finite‐difference time‐domain method.     

Evolution and properties of adherent diamond films with ultra high nucleation density deposited onto alumina

In this work, we report on adherent diamond films with thickness of up to 4.5 μm grown on polycrystalline alumina substrates. Prior to deposition, alumina substrates were ultrasonically abraded with mixed poly-disperse slurry that allows high nucleation density of values up to ∼5×10¹⁰ particles/cm². It was estimated that the minimal film thickness achieved for continuous films was ∼320 nm, obtained after a deposition time of 15 min with diamond particles density (DPD) of ∼4×10⁹ particles/cm². Continuous adherent diamond films with high DPD (∼10⁹ particles/cm²) were obtained also on sapphire surface after abrasion with mixed slurry and 15 min of deposition. However, after longer deposition time, diamond films peeled off from the substrates during cooling.The poor adhesion between the diamond and sapphire is attributed to the weak interface interaction between the film and the substrate and to difference in coefficient of thermal expansion. On the other hand, it is suggested that the reason for good adhesion between diamond film and alumina substrate is that high carbon diffusivity onto alumina grain boundaries allows strong touch-points at the grooves of alumina grains, and this prevents the delamination of diamond film. This adhesion mechanism, promoted by sub-micron diamond grain-size, is allowed by initial high nucleation density.The surface properties, phase composition and microstructure of the diamond films deposited onto alumina were examined by electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and high-resolution scanning electron microscopy (HR-SEM). The residual stress in the diamond films was evaluated by diamond Raman peak position and compared to a theoretical model with good agreement. Due to the sub-micron grain-size, the intrinsic tensile stress is high enough to partially compensate the thermal compressive stress, especially in diamond films with thickness lower than 1 μm.     

Structure optimization of diamond personal dosimeters based on Monte Carlo simulations

We have studied the usefulness of diamond personal dosimeters for X-rays and gamma-rays by using a Monte Carlo simulation method. On one hand, the energy responses (ERs) calculated for both diamond and silicon with various thicknesses as a function of incident photon energy indicate that diamond yields much more suitable ERs for personal dosimeters in the energy range from 30 keV to 1.25 MeV than silicon when used in the current measuring mode. This is because the atomic number of diamond is close to the effective atomic number of the human soft tissue. On the other hand, we have found that diamond dosimeters with allowable energy-independent responses in the pulse counting mode can be fabricated for the photon energies ranging from 60 keV to 1.5 MeV by optimizing the layered structures of various materials such as epoxy resin, polyimide, Al and Cu located on the diamond layer sensing the incident photons. The advantage of diamond in terms of dosimeter performance is discussed.     

Efficient CVD diamond film/alumina composite substrate for high density electronic packaging application

Due to its extreme hardness, chemical and mechanical stability, large band gap, low dielectric constant and highest thermal conductivity, diamond film is expected to be an excellent electronic packaging material for high frequency and high power devices. Under an alcohol concentration of 0.8% and a substrate temperature of 850 °C, high quality diamond films deposited on alumina are obtained by hot filament chemical vapor deposition (HFCVD) method using the optimum parameters determined by an infrared spectroscopic ellipsometer. Prior to the deposition of diamond film, carbon ions are implanted into alumina wafers to release the residual stress between interfaces. The measurement results indicate that dielectric properties and the thermal conductivity of diamond film/alumina composites are improved further with the increase of diamond coating. When the thickness of diamond coating is up to 100 μm, dielectric constant and dielectric loss of diamond film/alumina composite are 6.5 and 1.1 × 10^− 3, respectively. However, a thermal conductivity of 3.98 W/cm·K is obtained.     

Normal-Pressure Hot-Pressing of α-Alumina-Diamond Composites

α-Alumina-diamond composites have been developed by normal hot-pressing procedures using a conventional pressure of 32 MPa and 1250°C. Heretofore this type of composite has required a pressure of 6 GPa (60 kbar) to prevent the transformation of diamond to graphite. The mechanical properties, density, and thermal expansion coefficient of these composites have been characterized. The fracture toughness ( K _Ic) of alumina shows a considerable increase with the addition of diamond particles. Diamond additions tend to decrease the thermal expansion coefficient of these composites. The composite properties are dependent on the volume fraction of diamond particles.     

Piezoresistive,optical and electrical properties of diamond like carbon and carbon nitride films

In the present study diamond like carbon (DLC) and carbon nitride (a-CN_x:H) films were deposited by closed drift ion source from the acetylene and nitrogen gas mixture. The piezoresistive, electrical and optical properties of ion beam synthesized DLC films were investigated. Piezoresistive properties of the diamond like carbon and carbon nitride films were evaluated by four point bending test. The piezoresistors were fabricated on crystalline alumina substrates using Al-based interdigitated finger type electrodes. Effects of the nitrogen concentration on the piezoresistive gauge factor were investigated. The dependence of the resistance of the metal/a-CN_x:H/metal structures on temperature has been studied. Current–voltage (I–V) and capacitance–voltage characteristics were measured for a-CN_x:H/Si heterostructures. The main current transport mechanisms were analyzed. Optical parameters of the synthesized films such as optical bandgap and B parameter (slope of the linear part of the Tauc plot) were investigated to study possible correlation with the piezoresistive properties.     

Characterization of alumina modified with sulfate and phosphate

The acidity and surface properties of alumina modified with sulfate (A1-S) and phosphate (Al-P) were characterized by IR, NH₃TPD and 1-butene isomerization reaction. The Point of Zero Charge (PZC) of modified alumina and the amount of Mo anion adsorbed on modified alumina were measured. The PZC of modified alumina and the amount of Mo anion adsorbed on modified alumina decreased with increasing modifier content. Non-linear relationship between the PZC and Mo uptake in modified alumina was observed in Al-S and Al-P. The bonding mode of sulfate on alumina changed with increasing sulfate content. The Bronsted acid site appeared in alumina modified with sulfate of 10 wt%.     

Fabrication and Microwave Properties of Three-Dimensional Photonic Crystals With a Diamond Structure Composed of Ceramic Spheres in Resin

Three-dimensional photonic crystals with a diamond structure composed of YSZ (3 mol% Y₂O₃-stabilized ZrO₂) spheres in a resin matrix were fabricated by using stereolithography. The lattice constant was 12 mm and the diameter of the spheres was 5 mm. These photonic crystals made of ceramic spheres showed complete photonic band gaps at around 12 GHz between the eighth and ninth bands. The propagation characteristics of microwaves agreed well with the calculated results using the plane wave expansion method.     

ADHESION FORCES OF SPHERICAL ALUMINA PARTICLES ON CERAMIC SUBSTRATES

Adhesion forces of spherical alumina particles on ceramic substrates were studied. Results of direct force measurements using an atomic force microscope (AFM) were compared with theoretical results of a new rod model and with molecular dynamic computer simulation. Spherical alumina particles were produced by a flame process. The particles were glued to cantilevers, and interaction forces were measured by the AFM. A significant reduction of adhesion forces due to adsorbed layers was observed. The interaction volumes were determined by AFM scanning using a soft cantilever. The measured interaction forces were compared with calculated forces using the Hamaker concept including an adsorbed surface layer and the determined interaction volume (rod model). It turned out that calculated adhesion forces, neglecting deformation, are smaller than measured ones. This problem can be overcome if deformation according to Hertz is included in the rod model. Even for such a hard material as alumina, deformation occurs in the contact zone, which was also observed in a molecular dynamic computer simulation.     

Design of a new TM021 mode cavity type MPCVD reactor for diamond film deposition

A new TM₀₂₁ mode cavity type microwave plasma chemical vapor deposition (MPCVD) reactor for diamond film deposition was derived by analyzing the TM₀₂₁ resonant pattern of microwave electric field in an idealized TM₀₂₁ mode reactor. Important characteristics of the reactor, including microwave electric field, electron density, gas temperature as well as absorbed microwave power density were obtained by using a phenomenological model of hydrogen plasma. On the basis of the simulation, a new TM₀₂₁ mode cavity type MPCVD reactor was built and 2-inch diameter freestanding diamond films were synthesized at 6 kW input microwave power. Raman and optical transmission spectroscopy analyses indicate that the diamond films prepared by using the new TM₀₂₁ mode cavity type reactor are of high quality.     

Net shape forming of green alumina via CNC machining using diamond embedded tool

Diamond embedded tools were fabricated through nickel electroplating of mild steel conical shaped shank for machining of green alumina compacts using Computer Numerical Control (CNC) machine. Flat and pointed end conical tools embedded with different grain sizes viz. ～117 μm (120 mesh) and ～20 μm (625 mesh) of diamond particles were used for green machining of alumina. The hardness values of 625 and 120 mesh tools were measured to be 13.79±3 GPa and 11.84±6 GPa, respectively. Diamond embedded tools were successfully used for net shape fabrication of symmetrical and unsymmetrical objects such as cylinder, dental crown, 3D pattern via CNC machining with submicron range surface roughness. Analysis of mechanical properties and Weibull modulus of the green and sintered alumina samples after green state machining revealed that net shape forming via green machining of alumina using diamond embedded tool is viable.     

Accurate measurement of fracture toughness of free standing diamond films by three-point bending tests with sharp pre-cracked specimens

The three-point bending test was used for the first time for evaluating plane strain fracture toughness, K_IC, of free-standing pre-cracked diamond film specimens. A novel approach to introduce sharp cracks into diamond films was presented. This approach, with a success rate of approximately 80%, allowed the initiation and growth of sharp cracks at the top of the notch in diamond films being observed and controlled in situ under a scanning electron microscope (SEM). This made a more accurate and reliable measurement of K_IC in diamond films possible. Fracture toughness values obtained with the sharp pre-cracked specimens were compared with those of simply laser-notched ones. Results showed that with pre-cracked specimens, the measured fracture toughness would be approximately 15% lower than that of laser-notched specimens with notch widths of 0.1–0.12 mm. The method provided for accurate determination of plane strain fracture toughness which may help to clarify the discrepancies and to understand the mechanical properties of CVD diamond films.     

Charging Behavior at the Alumina–Water Interface and Implications for Ceramic Processing

The interaction of water and the alumina surface is comprehensively reviewed. Water can be incorporated in the alumina crystal structure resulting in the formation of aluminum hydroxides such as gibbsite. Alumina dissolves into water to an extent that depends primarily upon the solution pH and temperature. The soluble Al (III)_aq species (hydrolysis products) likewise depend upon the solution pH, temperature, aluminum, and other salt concentrations. The development of charge on the surface of alumina is controlled by amphoteric surface ionization reactions. The charging behavior of both alumina powders and single crystal faces is compared. The differences can be explained by the reactivities of different types of surface hydroxyl groups. The substantial difference in surface charging behavior of single crystal sapphire and alumina powders indicates that experiments and modeling conducted on single crystals is of limited use in predicting suspension behavior. The atomic scale structure of the hydroxylated sapphire (0001) basal plane is nearly identical to the gibbsite (001) basal plane. The observed surface structures are consistent with the charging behavior of the surfaces. The role of surface charge on the adsorption of processing additives is briefly discussed. How surface charge and processing additives at the alumina aqueous solution interface influence surface forces between particles is reviewed. The influence of these forces on suspension properties such as rheological behavior is outlined. The importance of controlling these behaviors to improve colloidal ceramic powder processing is stressed.     

Grinding of alumina ceramic with microtextured brazed diamond end grinding wheels

Brazed monolayer diamond grinding wheels have advantages of a high abrasive bonding strength, high protrusion, and a large chip disposal space. However, it is difficult to prepare ordered and fine-grained brazed diamond grinding wheels. This study presents a new method for grain-arranged, brazed diamond grinding wheels with microtextures with similar performance to ordered and fine-grained brazed diamond grinding wheels. First, coarse diamond grains (18/20 mesh) were orderly brazed to fabricate the end grinding wheels. Next, a series of microtextures were ablated on the diamond grains using a pulsed laser, and two types of textured end grinding wheels—TG-G (ablated microgrooves only) and TG-GH (ablated microgrooves and microholes)—were prepared. Then, an experiment involving the grinding of alumina ceramics was performed, and the grinding characteristics and grinding mechanism were analyzed. The results indicated that compared with untextured diamond end grinding wheels (TG), the textured diamond grinding wheels (TG-G and TG-GH) significantly reduced the grinding force and the roughness of the machined surface. The local stress concentration at the microtextures promoted the formation of microcracks in the diamond grains of TG-G and TG-GH, and the self-sharpness of the grinding wheel was significantly improved. The brittle fracture mode of ceramic materials in grinding included intergranular fracture and transgranular fracture. Ironing pressure action was a key material-removal mechanism. It had an important influence on the cutting force and plasticity characteristics of the TG machined surface. For the surfaces processed by TG-G and TG-GH, the effect of ironing was weakened, while shearing played a more important role. The TG-GH grinding wheel ablated with microgrooves and microholes was superior to the TG-G grinding wheel ablated with only microgrooves, with regard to the grinding force, roughness, and self-sharpening.     

Comparative study of thermally conductive fillers in underfill for the electronic components

General underfill for the flip-chip package had a low thermal conductivity of about 0.2 W/mK. Thermal properties of underfill were measured with various fillers, such as silica, alumina, boron nitride, (BN) and diamond. Coefficient of thermal expansion (CTE) was changed by filler content and CTE of silica 60 wt.% was 28 ppm; BN 30 wt.%, 25 ppm; alumina 60 wt.%, 39 ppm; and diamond 60 wt.%, 24 ppm. The viscosity of underfill was measured with the cone and plate rheometer. Thermal diffusivity was measured with the laser flash method. Diamond filler loaded underfill showed the highest thermal conductivity 60 wt.%; 1.17 W/mK at 55 °C. Thermal conductivity of underfill was changed with a transition of heat capacity by the temperature increment in same filler content. In case of different filler content, thermal conductivity was changed with a transition of the thermal diffusivity.