| In recent years,a new low-temperature sintering technique called“cold sintering process”has attracted widespread attention.This technique can reduce the sintering temperature of ceramics to temperatures below 350 oC by introducing a transient liquid phase and uniaxial pressure.This green and energy-saving sintering method has broad application prospects,which not only avoids overfiring of ceramic particles in high-temperature sintering,but also provides a feasible route for densification of numerous materials prone to pyrolysis.Cold sintering is generally considered to achieve densification by inducing a process of“dissolution-precipitation”for ceramic particles.This mechanism requires a certain solubility of the material in the liquid phase,but many ceramics are insoluble,leading to a limited range of applicable material systems for current cold sintering technique.Therefore,it is crucial to break through this limitation for the development of cold sintering technique.To address this problem,three universal ideas are proposed in this thesis to realize the cold sintering densification of three types of material systems and to expand the applicable material range.Hydroxyapatite,several oxides and ABO3-type perovskite ceramics were used as the study system,and cold sintering was carried out by introducing“additives”,“hydroxide precursors”and“synthesis reaction”.Dense ceramics or composites were prepared at low temperatures,the key factors affecting the cold sintering behavior were investigated,and the densification mechanism during cold sintering was revealed.The main results are as follows:(1)Densification of hydroxyapatite was achieved by cold sintering with the aid of the additives easy to densify.Hydroxyapatite-based composites with relative density over 90%were prepared using chitosan or CaHPO4·2H2O as additives at the conditions no more than150 oC and 600 MPa.The influence of the phase composition on the mechanical properties of the composites was clarified.The composites with an additive content of 30 wt.%exhibited the best mechanical properties,due to the balance between densification and hydroxyapatite content.The densification mechanism may be related to the strong hydrogen bonding formed between the additive and hydroxyapatite.This additive-assisted densification provides a simple and feasible technological route for the cold sintering of many materials.(2)A generalized low-temperature densification method for many oxides is proposed based on cold sintering with the aid of hydroxides.The densification of MgO,Cu O,Zn O and WO3 was achieved by cold sintering at temperatures not exceeding 450 oC with the introduction of the corresponding hydroxides.The cold sintering behavior of the samples was investigated.It was found that both the hydroxides and the generated oxides showed excellent sinterability.The densification mechanism was revealed.The decomposition of hydroxide induces material reorganization,the in situ release of water effectively promotes particle rearrangement,and the surface defects of generated oxides accelerate the diffusion of intercrystalline atoms,eventually leading to densification.The relationship among densification behavior-surface defects-oxide solubility was established.The surface atoms of highly soluble oxide are more capable of detaching from the crystal lattice to form defects in water,and thus its densification in water-assisted conventional cold sintering is more obvious(using oxide powders as raw materials).This approach greatly enriches the applicable material systems for cold sintering technique due to the presence of corresponding hydroxides for almost all oxides.(3)Combining cold sintering and low-temperature reactions,the idea of achieving the synthesis and densification of ceramics in a single step was proposed,and a new technique for the low-temperature preparation of ABO3 perovskite ceramics was developed.The one-step synthesis and densification of Ba Ti O3,BaZrO3,Sr Ti O3,Sr Zr O3 and BaxSr1-xTi O3 ceramics were accomplished at 300 oC using Ba(OH)2·8H2O,Sr(OH)2·8H2O,H2Ti O3 or Zr(OH)4 as raw materials.The key process parameters were optimized,including raw material A/B ratio(corresponding to ABO3),temperature and dwelling time.The mechanism of synthesis and densification was elucidated.The conversion of raw materials into products underwent“dissolution-reaction-precipitation”process.Due to the presence of pressure,the dissolved ions in this process tended to deposit in the pores,thus enabling simultaneous synthesis and densification.In addition,CaTi O3 and CaZr O3 were studied as target products,and the synthesis and densification regulation was summarized.The solubility of the hydroxides of Ba,Sr,and Cadecreases in turn,and thus the concentration of the reactants decreases,resulting in an increase in the difficulty of synthesis and densification of the corresponding perovskite ceramics in sequence.This reactive cold sintering method is not only expected to solve the low-temperature densification challenges of many ceramics,but also provides a simple and efficient strategy for ceramic fabrication.In conclusion,this thesis devotes to broaden the scope of application for cold sintering,proposes three expandable cold sintering routes,investigates the cold sintering behavior of three types of ceramic materials,reveals the densification mechanism,and is expected to promote the development and breakthrough of bottlenecks in cold sintering technique. |
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