Low-pressure,thermo-compressive lamination

Lamination of green ceramic tapes is one of the most important technological processes in multilayer ceramic technology. Lamination affects the quality of all 3D structures (e.g., channels, chambers, membranes, etc.). Novel chemical methods of lamination reduce the deformation of 3D structures. However, these methods are useless in the fabrication of thin membranes and structures with thick-film electronic components or electric vias. Therefore, thermo-compressive lamination is still the best solution for the lamination of green ceramic tapes. Low-pressure thermo-compressive lamination with an insert material is presented in this paper. The influence of pressure and Low Temperature Cofired Ceramics (LTCC) material on the compressibility and shrinkage of LTCC, as well as the influence of the insert material on deflection and distortion of the membranes are presented and discussed in this paper.     

Hot Embossing: An Alternative Method to Produce Cavities in Ceramic Multilayer

New applications of ceramic multilayers, for example, in biotechnology, sensor technology, and chemical micro-reaction technique, call for cavities with complex geometries. Hot embossing offers a promising, cost-effective way to generate these structures on the surfaces of green tapes or laminates. Cavities inside low-temperature co-fired ceramic multilayer were manufactured by a combination of hot embossing, lamination by a special adhesive technique, and zero shrinkage sintering. The edge and surface quality in the green state as well as the sintered multilayers with surface structures and cavities were extensively characterized by laser surface scanning, optical and ultrasound microscopy. Sintering shrinkage of hot-embossed laminates could be reduced in the x and y directions to less than 0.5%.     

Tape casting of ferroelectric,dielectric, piezoelectric and ferromagnetic materials

Tape casting is a feasible method for preparing ceramic tapes with different electrical and magnetic properties for multilayer ceramic devices. This paper describes the tape casting process for several different electroceramic materials (BST, PZT, NZF and ZSB) utilising similar organic additive and solvent systems. The properties of tapes with different ceramic compositions before and after sintering are investigated, including surface roughness, shrinkage and microstructures. The parameters affecting the casting, shrinkage, lamination, thickness and tensile strength of green tape are also presented. This enables process design for tape which can be used in devices with true integration of dielectric and piezoelectric, ferroelectric and ferromagnetic layers in 3-dimensional multilayer structures.     

Tape casting system for ULTCCs to fabricate multilayer and multimaterial 3D electronic packages with embedded electrodes

A 3D multilayer structure built by two ultra‐low temperature co‐fired ceramic (ULTCC) compositions with silver embedded electrodes are co‐fired at a temperature of 450°C. The 3D multilayer module is prepared by laminating the ULTCC green tapes with a new binder system, which organics can be completely burned out at temperature of 250°C before the sintering of the ULTCC 3D modulus. High‐density microstructures are achieved for the sintered module. In this study, the ULTCC feasible binder system is introduced. Also, ULTCC multilayers and multimaterial structures with surface and embedded silver electrodes are fabricated. This research opens up a new horizon for fabrication of electroceramic devices with embedded electrodes in multimaterial devices.     

Low-Sintering High-k Materials for an LTCC Application

High-k LTCC tapes with ultralow sintering temperatures were developed from lead-free perovskite powders. Lowering of the sintering temperature from 1250°C down to 900°C has been achieved by means of ultrafine ceramic powders in combination with suitable sintering aids. The tape-casting process has been optimized for ultrafine powders with an enhanced sintering activity. Low-sintering high-k tapes of a thickness down to 40 μm, suitable for LTCC processing, were obtained. The sintering behavior of these high-k tapes has been studied and compared with other LTCC materials. Dielectric properties of the high-k material have been investigated on a multilayer test structure consisting of up to 20 dielectric layers. After metallization with an Ag conductor, the green tapes were stacked and laminated. Sintering of these multilayer stacks at 900°C gives dense ceramic samples. Permittivities up to 2000 have been obtained, together with low dielectric losses. Material compatibility with several Ag/Au-thick-film-paste systems has been tested.     

3D打印成型陶瓷零件坯体及其致密化技术

3D打印技术在陶瓷零件成型方面具有较大应用潜力,被认为是近净尺寸成型高性能复杂结构陶瓷零件的一种新途径。本文比较了陶瓷零件或其坯体的激光选区熔化、薄材叠加制造、熔融沉积造型、光固化、三维打印和激光选区烧结等不同3D打印工艺及其致密化手段的优势和不足,认为较低的相对密度和强度是阻碍3D打印陶瓷零件实现产品应用的主要障碍。本团队近年来采用造粒混合法制备出具有良好流动性的3D打印复合陶瓷粉体,再通过激光选区烧结(SLS)和冷等静压(CIP)技术分别进行坯体成型及均匀致密化处理,制备出了高性能、复杂结构的Al_2O_3致密陶瓷零件。本文回顾了这些工作,并补充介绍了溶解沉淀和溶剂蒸发这两种制备复合陶瓷粉体的新方法,利用SLS/CIP复合工艺进一步制造了ZrO_2、SiC、高白土等其它材质的复杂陶瓷零件,为3D打印陶瓷用于航空航天、医疗、艺术等领域奠定了基础。     

Colloidal Processing of Glass–Ceramics for Laminated Object Manufacturing

Green tapes of Li₂O–ZrO₂–SiO₂–Al₂O₃ (LZSA) parent glass were produced by aqueous tape casting as the starting material for the laminated object manufacturing (LOM) process. The rheological behavior of the powder suspensions in aqueous media, as well as the mechanical properties of the cast tapes, was evaluated. According to ξ potential measurements, the LZSA glass powder particles showed acid surface characteristics and an IEP of around 4 when in aqueous media. The critical volume fraction of solids was about 72 wt% (27 vol%), which hindered the processability of more concentrated slurries. The glass particles also showed an anisometric profile, which contributed to an increase in the interactions between particles during flow. Therefore, the suspensions could not be processed at high solids loadings. Aqueous-based glass suspensions were also characterized by shear thickening after the addition of dispersants. Three slurry compositions were formulated, suitable green tapes were cast, and tapes were successfully laminated by LOM to a gear wheel geometry. A higher tensile strength of the green tapes corresponded to a higher tensile strength of the laminates. Thermal treatment was then applied to the laminates: pyrolysis at 525°C, sintering at 700°C for 1 h, and crystallization at 850°C for 30 min. A 20% volumetric shrinkage was observed, but no surface flaws or inhomogeneous areas were detected. The sintered part maintained the curved edges and internal profile after heat treatment.     

Processing of 1-3 Piezoelectric Ceramic/Polymer Composites

Several methods of forming fine-scale 1–3 piezoelectric ceramic/polymer composites for possible transducer applications were demonstrated. These methods include tape casting, honeycomb dicing, and ceramic fiber weaving. In the tape casting technique, laminated structures were formed using thin PZT tapes. The tapes were stacked, with spacers separating the layers, and the stack embedded in polymer. Dicing the stack resulted in a composite with 1–3 connectivity. The thin tape technique can be used to develop composites with ceramic or polymer volume fraction gradients and multifunctional ceramics. Dicing of PZT honeycombs yields 1–3 composites with uniquely shaped rods. Shapes included +, T, and L. In the ceramic fiber weaving technique, green PZT fibers were woven through a PZT honeycomb support structure. The structure was fired to sinter the PZT fibers, and embedded with polymer to yield 1–3 composites. All 1–3 composites showed high and uniform piezoelectric coefficients across the electroded area.     

Influence of Low-Temperature Co-Fired Ceramics Green Tape Characteristics on Shrinkage Behavior

To establish a better understanding of the complex densification and shrinkage processes of low-temperature co-fired ceramics (LTCC) and to improve the dimensional control in the manufacture of LTCC multilayer devices, the influence of glass, composite, and microstructural green tape characteristics on the densification and shrinkage behavior of LTCC materials, with special focus on the development of anisotropy, was investigated. To study the influence of these factors, a commercial LTCC system was analyzed regarding chemical and microstructural composition as well as sintering behavior. The results of the analysis showed that the commercial LTCC system is composed of alumina as a ceramic filler and a CaO–SiO₂–B₂O₃–Al₂O₃ glass. Based on these results, a similar glass was produced. To understand the mechanisms of densification, its wetting behavior and viscosity as a function of temperature were investigated. As developed glass was mixed with an alumina powder and milled down to average grain sizes of 1, 2, and 3 μm, respectively. From these composite powders, slurries were prepared and tape cast. The sintering kinetics including onset temperature, development of viscous flow as well as phase development of both commercial and internally developed LTCC tapes LTCC tapes in relation to their modified composition and green tape structures were analyzed in situ by means of optical dilatometry, thermo-mechanical analysis (TMA), and high-temperature-X-ray diffraction. The viscous behavior of the glass-filler composites was determined by means of cyclic dilatometry in a TMA device.     

Cold Chemical Lamination—New Bonding Technique of LTCC Green Tapes

The low-temperature co-fired ceramic (LTCC) technology enables fabrication of sensors, actuators, microfludic devices² and fuel cells. The structures consist of screen-printed components, gas/liquid channels, reactive chambers and mixers. The lamination process determines the quality of such devices. Thermo-compression is the most popular bonding method. The LTCC green tapes are joined together at high temperature (up to 80°C) and high pressure (up to 30 MPa) for 2 to 15 minutes. The method allows good encapsulation of the LTCC structures, but the channels geometry is strongly affected by elevated temperature and pressure. Cold Chemical Lamination (CCL) is a new LTCC green tapes bonding technique, which allows for fabrication of 3D modules. A solvent-based method is used in the CCL lamination instead of the thermo-compression process. A special liquid agent is screen-printed on the green tape in the CCL method. The liquid melts the tape surface. Then the tapes are stacked and compressed at room temperature by a printing roll. The influence of the CCL and the thermo-compression methods on the chamber's geometry quality as well as basic electrical properties of screen-printed resistors (sheet resistance R_φ standard deviation of sheet resistance σ_R, variability coefficient of sheet resistance V_R, and long-term stability) are analyzed and compared in this paper. The bonding quality is examined by a scanning electron microscope (SEM).     

Cold chemical lamination of ceramic green tapes

The cold chemical lamination (CCL) is a new technique of bonding ceramic green tapes into one 3D structure. Instead of a standard thermo-compression method, new solvent-based lamination is presented. A film of a special chemical agent is put on the green tape surface. The solvent melts the surface. Then the tapes are stacked. The bonding of the green tapes is made at a room temperature. The new method is used for joining green tapes of the low temperature co-fired ceramics (LTCC). A quality of the bonding depends on the solvent type. The cold chemical lamination is examined on two types of the LTCC tapes: DuPont 943 and DuPont 951. Six types of the solvents are analyzed in the paper. The bonding quality and geometry of the test structures are examined. The lamination quality is investigated by the scanning electron microscope.     

The effect of laminating factors on the thermal properties of unfired multilayer glass-ceramic and alumina substrates

This paper presents experimental data on heat capacities and thermal conductivities of two ceramic tapes measured using standard ASTM methods. The two ceramic tapes were prepared using alumina and a glass ceramic of aluminum-magnesium silicate with poly(vinyl butyral) binder, and laminated at pressures of 208E5 to 620E5 Pa at two laminating temperatures and times. The measured properties showed that laminating pressures have a greater effect, and that thermal conductivities are affected more than heat capacities. Empirical models have been developed to fit the experimental data. A correlation is developed to express the density of laminated green sheets in terms of the laminating variables—time, temperature, and pressure.     

3D-customized thin-walled ceramic mouldings produced by deep drawing of green tapes

Thin-walled 3D-customized ceramic components can be used, e.g. for inert and thermal resistant housings as well as special surface structured kiln furniture. For manufacturing such components, deep drawing of ceramic green tapes has been used. The technology has been almost not applied for processing of ceramic green tapes. A new approach was developed to realize a homogenous ceramic particle packing and hence uniform green density within a deep drawn green tape. After debinding and sintering, defect-free model structures with a dense microstructure (density > 99%) were achieved. Within this study, the deep drawing of ceramic green tapes by using a new approach was investigated. The developed approach brings several beneficial properties together, e.g. a reproducible form deviation and more importantly a homogeneous particle distribution allowing homogeneous and dense structures. With this results, the transformation of non-complex and cheap ceramic green tapes to 3D-customized and near-net-shape thin-walled mouldings becomes possible.     

Permeability of Laminated Green Ceramic Tapes as a Function of Binder Loading

The gas permeability of laminated green ceramic tapes was determined versus organic loading for samples subjected to air oxidation. The dielectric in the tapes was barium titanate, and the binder consisted primarily of poly(vinyl butyral) and dioctyl phthalate. Both the normalized gas flux and the gas permeability were seen to approach constant values for five or more tapes laminated together. Because the characteristic pore size was 0.5–1 μm, Poiseuille flow was the dominant flow mechanism, and thus Darcy's law was valid. The permeability of five laminated tapes was a factor of five smaller than for unlaminated tapes. Values were also obtained for the permeability versus binder loading in terms of the microstructural features of specific surface, porosity, and a parameter to account for tortuosity and constrictions.     

Strength of Tape Cast and Laminated Ceramics

Monolithic A1₂O₃ ceramic laminates were fabricated via a tape casting process. The strength of single tapes was compared with that of laminates, using biaxial flexure tests. The fracture stress was similar. However, the laminates presented a lower Weibull modulus. The feasibility of eliminating or diminishing void-type flaws present in the green tapes was also assessed. To this end, tapes were first punctured, then laminated and sintered, and the effect of these known flaws in the final ceramic was assessed in four-point flexure tests. The thermocompression of green tapes during laminate fabrication was found to modify the flaws to a more forgiving morphology.     

Influence of Cold Low Pressure Lamination Parameters on the Joining of Low Temperature Co‐Fired Ceramics Green Tapes and Sintered Alumina Substrates

In this study, laminates consisting of sintered alumina substrates and green Low Temperature Co‐fired Ceramics (LTCC) tapes have been produced via Cold Low Pressure Lamination which is based on adhesive tapes for joining of layers at room temperature and pressures <5 MPa. The influences of lamination parameters such as temperature, pressure, and time on the quality of the green and sintered multilayer stack have been determined. If the bottom LTCC layer of an alumina–LTCC–LTCC laminate is metallized by screen printing defects such as crack formation can occur due to stress formation caused by constrained sintering. By adapting the lamination parameters, these stresses can be avoided. Another defect observed is cavities which form along the printed circuit lines. This type of defect is caused by the shrinkage of the circuit line width during firing; by reducing the height of the conductor line during screen printing, the cavity size can be reduced. In addition, different screen‐printed metallization layouts have been tested to determine the influence of line and spaces on the quality of sintered laminates.     

Processing, Microstructure, and Properties of Laminated Silicon Nitride Stacks

By lamination of silicon nitride tapes, components with complex geometries can be produced. Unstructured tapes can be laminated by common thermal compression. Structured tapes, however, have to be joined by pressureless processes using e.g. pastes as lamination aids because deformation of the structures would occur. These pastes usually contain a binder for maintaining the mechanical contact between the tapes during processing. To prevent the high mass loss of typical organic binders during burnout, pre-ceramic polymers were used in this work. These ceramic precursors convert partly into an inorganic material during heat treatment with a significant reduced mass loss compared with common organic binders. Thus, the porosity in the interlayer of a laminated stack is strongly decreased, which should be favorable for the mechanical and thermal properties. This work discusses the resulting microstructure, strength, and thermal diffusivity data of stacks laminated with pastes containing various precursor contents. These results are compared with those obtained by samples prepared by compression of green tapes. It is found that except for some large pores, the microstructure of the precursor-derived interlayers is qualitatively the same as in the tape material. For stacks made by both lamination methods, strength measurements reveal that the properties parallel and perpendicular to the layers are different. It is shown that the same strength level can be obtained both by using the pressureless route and by the compression method. Unlike the strength, the thermal conductivity does not change with the direction of measurement.     

Structural,magnetic and electric properties of ZrO2 tapes decorated with magnetic nanoparticles

We produced ZrO₂ ceramic tape decorated with magnetic nanoparticles through tape casting technique. The green and sintered magnetic tapes were characterized by XRD, SEM, EDS, magnetic measurements, and I–V curves. We investigated the changes in the structural, magnetic and electrical properties, after the sintering process, and discussed the connections between them. The magnetic properties, performed in a wide range of external magnetic field and temperature, show magnetite phase for the magnetic nanoparticles governing the magnetic and electric properties of the green tape. On the other hand, for the sintered tape, the increase in the hematite phase led to remarkable changes in the magnetic and electrical properties. The electrical characterization reflects the observed changes in the structural properties after the sintering process. Additionally, the main advantages of the ceramic tapes decorated with magnetic nanoparticles reside in the possibility of producing functional thin ceramic materials that are easily moldable for electronic devices applications.     

Laminated Object Manufacturing of in-situ synthesized MAX-phase composites

In this work green tapes comprised of TiC and SiC were further processed by lamination, pyrolysis and liquid silicon infiltration. The in-situ synthesis of MAX phase Ti₃SiC₂ by silicon infiltration was investigated and discussed. The synthesis was supported by thermodynamic calculations. The mechanical and microstructural properties of the siliconized composites were studied. The processing route, in combination with Laminated Object Manufacturing (LOM) resulted in the successful fabrication of a three-dimensional gear. The gear showed a defect-free structure with a linear shrinkage of less than 3% relative to the green state. Thus, this approach can be considered as a near-net-shaping process of ceramic components with complex geometries.     

Aqueous tape casting of super-hard B4C laminates with rGO-enriched reinforcing interlayers

Superhard B₄C parts with microarchitectures constituted by ceramic layers and evenly-spaced rGO-enriched reinforcing interlayers were fabricated for the first time. To this end, a concentrated slurry of B₄C with its Ti-Al sintering additive was first prepared by aqueous colloidal processing, optimizing its total solids loading and content of both binder and plasticizer to obtain, by tape casting, handleable and flexible green tapes. A semi-dilute aqueous suspension of B₄C with Ti-Al and abundant GO was also prepared to dip-coat those green tapes with a GO-enriched layer, optimizing the withdrawal rate and the dipping time. The bare and GO-coated B₄C+Ti-Al tapes were then sequentially laminated, thus yielding green multilayered laminates that finally were appropriately debinded and densified by spark plasma sintering. Vickers indentation tests demonstrated that these multilayered laminates are superhard (～31 GPa), and that their rGO-enriched reinforcing interlayers are effective in arresting crack propagation.