The Preparation Of Metal Ceramics Heater And Regulation Of Temperature Coefficient Of Resistance

With the increasing application of electrothermal materials in civil heating and heat treatment of industrial products in recent years,they are required to satisfy higher needs,such as fast heating rate,small volume and corrosion resistance.The metal ceramics heater（MCH）,a new type of electrothermal materials,has been widely favored.As it is widely used in civil,medical and military and other fields,higher requirements are put forward on the heating rate and bonding strength.In this paper,in order to meet the requirements of high efficiency under the condition of constant voltage and high temperature,the temperature coefficient of resistance（TCR）of MCH was reduced by changing the composition of the metal paste to meet the heating efficiency of the product under the condition of constant pressure and high temperature.Besides,MCH of high bonding strength was prepared by adding inorganic oxides to the metal paste.It was through the XRF,XRD,SEM and other testing methods as well as the thermodynamics and kinetics knowledge to explore the mechanism of the decrease of TCR of the sample and the improvement of the bonding strength,which provided a theoretical basis for the research and development of the enterprise.The main research results were as follows:（1）In order to prepare MCH with low TCR,Mo,V,Ge,Te and X with different weight were added to the metal paste,whose elements were mainly W and Mn.The results showed that the TCR of samples A0 and B0 without other powders was about3800ppm/°C.When Mo with 7wt%was added into the metal paste,the TCR of sample A4 was about 2500ppm/°C.When the content of V,Ge,and Te were 5wt%respectively,the TCR of samples were significantly reduced,and the value of TCR was about 2600ppm/°C.When the content of X was 5wt%,the value of TCR of sample was reduced to a minimum of 1986ppm/°C.According to the thermodynamics and kinetics research and the microscopic test results,the decrease of TCR of samples A0 and B0 was caused by the W-Si solid solution,which was formed when most of the SiO₂ in the ceramic matrix was reduced to the elemental Si by the residual C.That is why the TCR of the samples was lower than that of the pure metal W.The W-Mo and W-V infinite solid solution were formed after adding Mo and V into the metal paste,leading to the TCR of samples decreased.At the same time,when adding Ge and Te into the metal paste,the liquid phase and W phase coexisted at the sintering temperature.The original cell structure of W was broken as a small amount of Ge or Te were dissolved in the W phase,which resulted in the decrease of TCR of samples.（2）As the content of MgO·Al2O3 spinel phase in the inorganic oxides at the high temperature increased,the maximum pull force of lead wires improved.When the inorganic oxides were all in MgO·Al₂O₃ spinel phase,the maximum pull force was32N.The microsteucture test showed that the MgO·Al2O3 spinel exised not only at the interface between the metal layer and the ceramic layer but in the metal layer itself.As the MgO·Al₂O₃ spinel at the junction interlocked with those in the metal layer and the ceramic layer,the bonding strength of the sample was dramatically improved.The bonding strength of the sample was not significantly increased when the added inorganic oxides were composed of liquid phase and mullite phase at a high temperature.The reason is that most of the inorganic oxide SiO₂ was reduced to elemental Si by residual C rather than formed the mullite phase.Thus,the bonding strength of the samples mainly depended on the mechanical bonding between the Al2O3 particles of the alumina ceramic layer and the W particles of the metal layer.When the bonding strength was weak,there was no evident improvement in the bonding strength of the samples.