Heat dissipation in 3D printed cellular aluminum nitride structures

The improvement of heat dissipation in electronic and energy devices is a challenge that can be addressed through the use of highly porous materials. Presently, the additive manufacturing of 3D aluminum nitride is described, and different lattice patterns with porosities in the range 45–64 % are achieved by direct ink writing. All the structures are robust and the effective thermal conductivity (k_eff) for cuboid structures decreases by 50–75 % with the filament separation and shows anisotropic characteristics, since k_eff along the longitudinal axis of the scaffold is up to six times greater than for the transversal one. Heat transfer during free cooling experiments for cuboid and cylinder scaffolds, after rapid heating at temperatures above 1000 °C, takes place by radiation for temperatures >500 °C and by convection through the complete cooling process. The heat dissipation time constants of both processes decrease almost linearly with the designed scaffold parameters of porosity and rod separation.     

The quantitative investigation of the lattice oxygen and grain edge oxygen on the thermal conductivity of aluminum nitride ceramics

The good thermal conductivity of AlN is essential for insulation and high heat dissipation applications. However, the influence of oxygen impurities at various locations (the lattice oxygen and grain edge oxygen) on the thermal resistivity of AlN ceramics is unclear. In this study, AlN ceramics with various oxygen distributions are prepared by different methods, and the oxygen contents of different regions are distinguished. The results indicate that the lattice oxygen is the main factor affecting thermal resistivity. Meanwhile, high-temperature annealing and pre-sintering processes can lower the lattice oxygen content from 0.061 wt% to 0.038 wt% and 0.036 wt%, respectively. Additionally, when grain edge phase volume is less than 4 vol%, it does not contribute significantly to thermal resistivity. The main formation of thermal resistance changes from phonon-defect scattering to phonon-phonon scattering with increasing temperature. These results may be informative for the microstructure design of AlN ceramics with high thermal conductivity.     

The effect of graphene nanoplatelets on the thermal and electrical properties of aluminum nitride ceramics

The thermal conductivity (κ) of AlN (2.9 wt.% of Y₂O₃) is studied as a function of the addition of multilayer graphene (from 0 to 10 vol.%). The κ values of these composites, fabricated by spark plasma sintering (SPS), are independently analyzed for the two characteristic directions defined by the GNPs orientation within the ceramic matrix; that is to say, perpendicular and parallel to the SPS pressing axis. Conversely to other ceramic/graphene systems, AlN composites experience a reduction of κ with the graphene addition for both orientations; actually the decrease of κ for the in-plane graphene orientation results rather unusual. This behavior is conveniently reproduced when an interface thermal resistance is introduced in effective media thermal conductivity models. Also remarkable is the change in the electrical properties of AlN becoming an electrical conductor (200 S m⁻¹) for graphene contents above 5 vol.%.     

Directional properties and microstructures of spark plasma sintered aluminum nitride containing graphene platelets

Graphene platelets (GPLs) containing aluminum nitride (AlN) composites were produced by using both pressureless sintering and spark plasma sintering (SPS). Poor densifications were obtained when composites were pressureless sintered whereas highly dense composites were successfully produced by using SPS. In addition, the applied uniaxial load in the SPS resulted in the orientation of GPLs in the microstructure of composites, indicating that composites would have anisotropic properties. All the mechanical, thermal and electrical properties in the in-plane direction were better than the through-plane direction. Fracture toughness of composites with the addition of 1 wt% GPLs were increased more than 30% compared to AlN matrix. Increased anisotropic effect with increasing amount of GPLs led to even larger differences on the thermal conductivities in through-plane and in-plane directions. AlN also became an electrically conducting material after ∼1 wt% GPLs addition in both through-plane and in-plane directions.     

Effects of two-step sintering on thermal and mechanical properties of aluminum nitride ceramics by impedance spectroscopy analysis

The effects of two-step sintering on the microstructure, mechanical and thermal properties of aluminum nitride ceramics with Yb₂O₃ and YbF₃ additives were investigated. AlN samples prepared using different sintering methods achieved almost full density with the addition of Yb₂O₃–YbF₃. Compared with the one-step sintering, the grain sizes of AlN ceramics prepared by the two-step sintering were limited, and the higher flexural strength and the larger thermal conductivity were obtained. Moreover, the electrochemical impedance spectroscopy of AlN ceramic was associated with thermal conductivity by analyzing the defects and impurities in AlN ceramics. The fitting grain resistance and the activation energy for the grain revealed the lower concentrations of aluminum vacancy in the two-step sintered AlN ceramics, which resulted in the higher thermal conductivity. Thus, mechanical and thermal properties for AlN ceramics were improved with Yb₂O₃ and YbF₃ additives sintered using two-step regimes.     

铝-氮化铝结合刚玉质滑板的抗氧化性能

研究了铝-氮化铝结合滑板在氧化气氛下加热升温过程中质量的连续变化特征,以及氧化前后试样的外形尺寸和强度的变化,通过热力学计算和动力学分析得知,试样在氧化过程中表现出的质量变化特性,有助于其组织结构的致密,特别是金属铝的氧化使得其在高温下形成氧化隔离层,提高了滑板的抗侵蚀能力和抗渗透能力。     

Synthesis and characterization of aluminum Oxy nitride (AlON) from the nanosized gel precursor

The purpose of this research was the synthesis of aluminum Oxy-nitride (ALON) nanocrystals with through the alkoxide sol-gel processing method. For the synthesis of the nanosized precursor, sucrose (SU, as a source of reducing and gel agent), carbamide (CR, to adjust the pH and source of atomic nitrogen), aluminum buthylate (as the source of Al³⁺), and magnesium acetate (as the source of Mg²⁺ and the doping agent) were used. Various characterizations including X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), diffusion reflectance spectroscopy (DRS spectrum), energy dispersive X-ray spectroscopy (EDX) and field emission scanning electron microscopy (FESEM) were used to characterize the as-synthesized samples. Our results indicated that the precursor solution containing SU:CR with the molar ratio of 1:3 M and 5% magnesium acetate led to AlON phase. This phase could be obtained after a special three-step calculation schedule of nanosized precursor consisting of 1 h at 1000 °C, 1 h at 1300 °C and 3 h at 1700 °C.     

Aluminum nitride as an alternative ceramic for fabrication of microchannel heat exchangers: A numerical study

Advances in micro electro mechanical systems (MEMS) necessitate utilizing efficient types of materials which are capable of dissipating high heat transfer rates. Aluminum nitride (AlN) as a member of advanced ceramics family, offers remarkable thermal conductivity which makes it suitable candidate in manufacturing of special and high-tech heat exchangers. The present work aims to investigate the application of a micro-sized heat exchanger made of AlN. According to the performed numerical simulations using Comsol Multiphysics, AlN made heat exchanger showed remarkable heat transfer enhancement of 59%, compared to the Al₂O₃ made one. Such a considerable improvement can be attributed to the higher thermal conductivity of AlN in comparison with Al₂O₃. The effectiveness of the heat exchangers were calculated for both AlN and Al₂O₃ made heat exchangers, and a 26% improvement was observed using aluminum nitride.     

Synthesis and characterization of high-purity aluminum nitride nanopowder by RF induction thermal plasma

High-purity aluminum nitride nanopowder was synthesized using the RF induction thermal plasma technique. The nitrogen gas flow rate, plasma power and reactor pressure were controlled to increase the conversion rate of Al powder to AlN nanoparticles. The compositions of the obtained powders were investigated through XRD and EDS analysis. The synthesized aluminum nitride nanoparticles included polygonal and rod-shaped nanoparticles and ultra-fine particles below 10 nm. The particle sizes generally ranged from 20–60 nm in TEM analysis. The specific surface area, band structure and impurities of aluminum nitride nanoparticles were also evaluated by BET, FTIR and ICP-OES analysis.     

Mechanism for the hydrolysis resistance of aluminum nitride powder modified by boric acid

Aluminum nitride (AlN) ceramics are becoming cutting-edge materials for electronic information and communication. However, raw AlN hydrolyzed rapidly, and the high storage costs of this material prevent widespread application. In this study, raw AlN was modified by boric acid (H₃BO₃) at 30 °C to enhance hydrolysis resistance. Transmission electron microscope (TEM), X-ray diffraction (XRD), the magic angle spinning nuclear magnetic resonance (²⁷Al-MAS-NMR and ¹¹B-MAS-NMR), and the fourier transform infrared spectrometer (FTIR) were used to characterize the powder before and after treatment, and the mechanism of hydrolysis resistance was determined. Modification with 0.1 M boric acid did not change the crystal phase of the AlN particles. The modified powder did not hydrolyse at 90% humidity and 70° Celsius. In the presence of boric acid, a network structure of B–O–B linkages ([BO_n], n = 3 or 4) formed that was connected to the AlN core via chemical bonds of B–N–Al and B–O–Al. The protective 6 – 10 nm-thick layer that formed on the surface of the AlN crystal, prevented attack by water molecules and hindered the hydrolysis of aluminium nitride. This study provides an alternative means of preparing anti-hydrolysis AlN powders.     

Powder injection molding of complex-shaped aluminium nitride ceramic with high thermal conductivity

Aluminum nitride (AlN) is a promising material for heat sinks and microelectronic applications because of the advantages of high theoretical thermal conductivity, high mechanical strength, good electrical insulation, low dielectric constant and low thermal expansion coefficient. However, the difficulties in shaping complex-shaped parts with a high thermal conductivity have retarded the wide applications of AlN ceramic. Herein, we design a new binder system containing resin components and adopt the powder injection molding technology to fabricate complex-shaped AlN parts. After the debinding process, the special binder system would produce residual carbon, which could react with Al₂O₃ and result in decreasing oxygen impurity and forming the yttrium-rich aluminates. The yttrium-rich aluminates can accelerate the densification of AlN ceramic and fasten the oxygen on the triangular grain boundary, leaving the clean grain boundary beneficial for high thermal conductivity. The as-prepared AlN parts with complex shape possess a high thermal conductivity of 248 W m⁻¹ K⁻¹.     

Fabrication and thermal conductivity of AlN/BN ceramics by spark plasma sintering

Aluminum nitride/boron nitride (AlN/BN) ceramics with 15–30 vol.% BN as secondary phase were fabricated by spark plasma sintering (SPS), using Yttrium oxide (Y₂O₃) as sintering aid. Effects of Y₂O₃ content and the SPS temperature on the density, phase composition, microstructure and thermal conductivity of the ceramics were investigated. The results revealed that with increasing the amount of starting Y₂O₃ in AlN/BN, Yttrium-contained compounds were significantly removed after SPS process, which caused decreasing of the residual grain boundary phase in the sintered samples. As a result, thermal conductivity of AlN/BN ceramics was remarkably improved. By addition of Y₂O₃ content from 3 wt.% to 8 wt.% into AlN/15 vol.% BN ceramics, the thermal conductivity increased from 110 W/m K to 141 W/m K.     

Side wall anodization of aluminum thin film on silicon substrate

Anodization of aluminum with restricted surface areas is reported in this study. Particularly, the side wall of aluminum thin film is anodized for the purpose of obtaining the confined number of pores with high aspect ratio. It has been observed that side wall anodization does not occur uniformly since the anodization speed is not uniform at the both interfaces and in the middle of the film. For this reason, the resultant pore front profile shows a parabolic shape, which resembles the parabolic velocity profile of fluid flow through two slabs. During the anodization process, the pores tend to break apart and the structure becomes more complex. Side wall anodization is investigated at various applied voltages and the resultant pore structures are shown.     

Effect of NiS addition on nitridation of alumina to aluminum nitride using polymeric carbon nitride as a nitridation reagent

We investigated the effect of adding nickel(II) sulfide (NiS) on nitridation of alumina (Al₂O₃) to aluminum nitride (AlN) using polymeric carbon nitride (PCN), which was synthesized by polymerization of dicyandiamide at 500 °C. The product powders obtained from nitridation of a mixture of δ-Al₂O₃ and NiS powders (mole ratio of 1:0.01) at various reaction temperatures were characterized by powder X-ray diffraction, ²⁷Al magic-angle spinning nuclear magnetic resonance, and Raman spectroscopy. δ-Al₂O₃ began to convert to AlN at 900 °C and completely converted to AlN at 1300 °C. The as-synthesized sample powders contained nitrogen-doped carbon microtubes (N-doped CMTs) with a length of several tens of mm and thickness of ca. 3 µm. The addition of NiS to δ-Al₂O₃ resulted in the enhancement of the amount of N-doped CMTs and nitridation rate, which might be due to the catalytic action of Ni particles on the thermal decomposition of vaporized PCN. The change in Raman spectra with reaction temperatures indicated that the crystallinity of N-doped CMTs was increased by calcining at higher reaction temperatures.     

Fabrication and properties of Acid Yellow 49 dye-intercalated layered double hydroxides film on an alumina-coated aluminum substrate

The Acid Yellow 49(4-[2-(5-amino-3-methyl-1-phenyl-1H-pyrazol-4-yl)diazenyl]-2,5-dichloro benzenesulfonic acid) (denoted as PPDB) anion intercalated layered double hydroxides (LDH) film was fabricated through an ion-exchange method using a ZnAl-NO₃-LDH/alumina/aluminum film as precursor. The prepared film was investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), Thermogravimetric-differential thermal analysis (TG-DTA), UV-visible spectroscopy and the CIE 1976 L*a*b* color difference method. XRD patterns and FT-IR spectra confirm the successful incorporation of PPDB anions into the interlayer galleries of ZnAl-LDH with an expansion of d-spacing from 0.88 nm to 2.51 nm and the disappearance of characteristic absorption band of NO₃⁻ anions at 1384 cm⁻¹. The SEM morphologies show that the LDH films are mainly oriented with c axis of the platelet crystallites parallel to the substrate surface. Additionally, the obtained results suggest that the intercalation of PPDB into ZnAl-LDH host markedly improve the thermal stability and light fastness of PPDB.     

Fabrication and characterization of polyaniline coated carbon nanofiber for supercapacitor

Polyaniline coated carbon nanofiber was fabricated using one-step vapor deposition polymerization technique. Fourier transform infrared (FT-IR) spectra and transmission electron microscope (TEM) images indicated that uniform and ultrathin conducting polymer layers were formed on the carbon nanofiber surfaces regardless of the coating thickness. It was also confirmed that the thickness of polyaniline layer could be conveniently tuned by the feeding amount of monomer. The coating thickness was dependent on initiator/monomer ratio, the vacuum pressure of reaction chamber and polymerization temperature. Among them, the vacuum pressure was a major factor to control the coating thickness of polyaniline onto the carbon nanofiber surface. In addition, the electrochemical analysis demonstrated that polyaniline coated carbon nanofiber showed an improved performance as supercapacitor. The specific capacitance of polyaniline coated carbon nanofiber exhibited a maximum value of 264 F/g when the thickness of polyaniline layer was ca. 20 nm.     

Controlled growth of nanocrystalline aluminum nitride films for full color range

Color films are widely used for visual effect as well as for their functional properties. To date, however, synthesizing thin films with desired color remains challenging. In this work, AlN color films are deposited on Si wafers by precise control of the deposition time for different thickness during reactive magnetron sputtering from an Al target in Ar/N₂ atmosphere. The thickness, morphology, structure, composition and color index are carefully examined by field emission scanning electron microscopy, atomic force microscopy, grazing incidence X-ray diffraction, X-ray photoelectron spectrometry and colorimeter, respectively. As the film thickness changes from 57 nm to 165 nm, the film exhibits purple, indigo, blue, green, yellow, orange and red in color. These colors repeat in the same order when the thickness goes over 165 nm. Once the thickness exceeds 467 nm, overlapping of colors takes place. The mechanisms are elucidated.     

Optimization of non-aqueous tape casting of high solid loading slurry for aluminum nitride ceramic substrates

Aluminum nitride (AlN) ceramic substrates have been fabricated using non-aqueous tape casting and pressureless densification under flowing N₂ atmosphere. Considering the economic and environmental impact, a new strategy of solvent and dispersant system was adopted to prepare AlN slurries with high solid loading. According to the viscosity characteristics of AlN slurries, dispersant content was adjusted to be 0.5 wt% of AlN powder in order to optimize the rheological behavior of AlN slurries. The addition contents of binder and plasticizer were both optimized as 5 wt% of AlN powders by combining the viscosity of slurries and tensile strength of green tapes. Green AlN tapes were fabricated with an optimized tape casting process such as dry temperature. The exclusion process of organic additives was investigated by employing thermogravimetric analysis. Flat and dense AlN ceramic substrates with a relative bulk density over 99.75 % were achieved after being sintered under 1800°C for 6 hours, which had a maximum size of 110 × 110 mm. The thermal conductivity of the AlN substrate could reach 145 Wm⁻¹K⁻¹.     

Improving the composition and multifunctional properties of amorphous boron nitride films prepared by post-annealing assisted femtosecond pulsed laser deposition method

Amorphous boron nitride (aBN) materials have similar density to crystalline phases and retain many unique electronic properties, valuable chemical inertness and high thermal stability characteristics. However, the current research on aBN materials has mainly focused on the synthesis and electrical properties of ultrathin aBN films. In this study, we developed a post-annealing assisted femtosecond laser deposition route towards stoichiometric, continuous, and multifunctional aBN films with thickness values of ∼1 μm. A series of boron nitride films were deposited on silicon wafers using a 1030 nm, 300 fs laser with a pulse energy of ∼1 mJ and a high repetition rate of 2 kHz to ablate a hexagonal boron nitride target. The deposited films were then annealed at 900 °C in a nitrogen atmosphere. The structures and chemical compositions of these obtained films were analysed by X-ray diffraction and X-ray photoelectron spectroscopy. Fourier transform infrared and nano-scratch tests were performed to measure the infrared optical and frictional properties of the adhered films. An infrared thermal imager was used to investigate the heat-dissipation performance of these films. The results indicate the components of the aBN film are further purified, the number of large heterogeneous particles is effectively reduced, and the surface becomes smooth after post-annealing treatment. This improvement promotes the transfer of heat flux and increases the transmittance in the mid-infrared light band. The significant effect mechanisms of the post-annealing treatment on the enhancement of the composition and multifunctional properties of aBN films prepared by the femtosecond pulsed laser were provided. The uniform coverage of the aBN films on the substrates, as well as the mid-infrared optical transparency and the protective performance are highly valuable and practical for infrared window protection applications.     

Phase,microstructure and sintering of aluminum nitride powder by the carbothermal reduction-nitridation process with Y2O3 addition

Aluminum nitride powders were synthesized by carbothermal reduction-nitridation method using Al(OH)₃, carbon black and Y₂O₃ as raw materials. The change of phase, microstructure and densification during the AlN synthesis and sintering process were investigated and the effects of Y₂O₃ was discussed. The results showed that Y₂O₃ reacted with Al₂O₃ to form yttrium aluminates of YAlO₃ (orthorhombic and hexagonal phases), Y₄Al₂O₉ and Y₃Al₅O₁₂ at the low temperature of 1350 °C. YAlO₃ could firstly be transformed into Y₂O₃ and then completely into YN when the firing temperature and holding time increased. However, YN could be oxidized into Y₂O₃ again after the carbon removal at 700 °C in the air atmosphere. There were two ways generating AlN when adding Y₂O₃ and the possible mechanism was proposed. Y₂O₃ from YN oxidation favored the densification of AlN ceramics because the liquid had better flowability and distribution in the sintering process at 1800 °C.