Investigation On Betavoltaic Micro-nuclear Batteries

The nuclear batteries?or radioisotope batteries?are the devices,which convert the decay energy of radioisotopes??particles,?particles or?rays?into the electrical energy.Along with the advance in the micro-electronic mechanical systems?MEMS?,the micro-nuclear batteries have been widely researched in the miniaturized low power devices.Moreover,compared with the micro-chemical cells and the micro-solar cells,the betavoltaic batteries have been become the promising micro-power sources for the micro-electronic mechanical systems?MEMS?.The advantages of betavoltaic batteries mainly include the easy integration in the small scale,the small mass,the high energy density,the long service lifetime,the miniaturized and integrated scale,the self-sustaining and the little maintenance.In addition,the output performance of a beta source is stable because of the little dependence on the temperature,the press and the other environmental factors.Moreover,the electrical performance and the energy conversion efficiency of a betavoltaic battery are determined by the radioisotope,the radiation transport properties of the beta particles in the target materials and the structure of the energy converter.These contents have attracted the interest of many researchers.Based on the interaction of the beta particles with the semiconductor material,both the self-absorption effect of a beta source and the radiation transport properties of the beta particles in the target material are simulated by using the Monte Carlo Method.On the basis of the simulated results,the investigations on the betavoltaic batteries are done.Then some valuable results are achieved in this study.First of all,the self-absorption effect of the beta source(⁶³Ni)is studied.Furthermore,the backscattering coefficients of the mono-energic electrons and the source(⁶³Ni)on the surface of the target materials?C and SiC?are researched,respectively.Moreover,the transport scale length of the radiation and the deposited energy distribution for the mono-energic electrons and the beta source(⁶³Ni)in the target materials?C and SiC?are also analyzed,respectively.Secondly,based on the energy converters with the p-n junction and the Schottky barrier diode,the theoretical calculated model on the performance of the betavoltaic batteries are done.Then the main factors,which affect the energy conversion efficiencies of the batteries,are achieved.Thirdly,the maximum electrical properties of the C-⁶³Ni cell and the 4H-SiC-⁶³Ni cell have been derived.Finally,some optimal designs on the corresponding structure of the energy converter in this study are presented.The main results are shown as follows:1.The self-absorption effect of the beta source(⁶³Ni)is studied.Based on the simulated results,it is obtained that when the thickness of source increases,both the apparent activity density and the apparent power density increase and then reach their saturation values,respectively.The self-absorption loss of source decreases with the thickness of source increasing.Moreover,the emitted energy spectra are different with the various thicknesses of sources.Finally,it is shown that when the thickness of source is thick enough,the average energy of the emitted energy spectrum increases and then it almost unchanges.These results are helpful to select the suitable thickness of source in the ⁶³Ni–based micro-betavoltaic batteries.2.The backscattering process of the mono-energic electrons and the source(⁶³Ni)in the target materials?C and SiC?are studied,respectively.In a betavoltaic battery,the backscattering energy is the significant energy loss.First of all,the backscattering coefficients of the mono-energic electrons are affected by the incident direction,the incident energy of electron and the thickness of target material.Furthermore,for the emitted beta particles from the source(⁶³Ni),when the thickness of source increases,the number backscattering coefficient and the energy backscattering coefficient in the target materials?C and SiC?are obtained,respectively.Based on the simulated results,it can be seen that the lower atomic number of the target material,the higher incident energy of particles,and the thinner target material layer result in the lower backscattering coefficients.Therefore,the target materials?C and SiC?are suitable to fabricate the betavoltaic batteries.The main reason is that the low atomic number of energy conversion material contributes to the low backscattering energy loss in a betavoltaic battery.3.The transport scale length of the radiation and the deposited energy distribution for the mono-energic electrons in the target materials?C and SiC?are researched,respectively.First of all,when the energy of mono-energic electrons decrease to 1/10 and 1/100 of the incident energy,the corresponding transport scale lengths of the radiation are obtained,respectively.Then the deposited energy distribution of the mono-energic electrons are also achieved.It is known that a beta source is the continuous energy spectrum.Therefore,the results are helpful to show the radiation ionizing regions of the beta particles with different energy in the target materials?C and SiC?.These results are important for the optimal design of the basic structure in a betavoltaic battery.4.The deposited energy distribution of the source(⁶³Ni)in the target materials?C and SiC?are obtained.Based on the simulated results,first of all,the total deposited energy in the target material is obtained.Secondly,for a given source,the main transport scale length of the radiation in the material and the deposited energy distribution are analyzed,respectively.Thirdly,the deposited energy percentage of the source(⁶³Ni)in the target materials?C and SiC?indicates that the energy distribution has little dependence on the thickness of source.The further calculations show that the energy deposition per unit length of a beta source?or the energy flow density?nearly exponentially decreases with the penetration depth of the beta particle increasing in the matter.Thus,the maximum energy deposition per unit length of a beta source?or the energy flow density?in the matter takes place at the surface of an energy converter.These results mean that the decay energy of a beta source is quickly absorbed in the thin region,which is close to the surface of an energy conversion device.On the basis of the source(⁶³Ni)and the target materials?C and SiC?in the betavoltaic batteries,these results are necessary to design the absorbed layer to match the main ionizing radiation region of a beta source in the energy converter.Moreover,the results are also important to calculate the theoretical performance of the relative batteries.5.Based on the source(⁶³Ni)and the target materials?C and SiC?,the theoretical calculations on the performance of the betavoltaic batteries are obtained.Then some designs on the structure of energy converter are presented.First of all,for the energy converter with the p-n junction or the Schottky barrier diode,the theoretical calculated model on the performance of a betavoltaic cell is presented.To be specific,the performance of a micro-cell mainly contains the short-circuit current,the open-circuit voltage,the current-voltage characteristics,the maximum output power,the fill factor and the energy conversion efficiencies.Secondly,the main factors,which affect the energy conversion efficiencies of a cell,are analyzed.Thirdly,for the energy converter with the p-n junction or the Schottky barrier diode,the theoretical calculation on the maximum electrical properties of the C-⁶³Ni cell or the 4H-SiC-⁶³Ni cell have been derived.For a betavoltaic battery,it is shown that the higher apparent power of source and the larger built-in voltage of the energy converter contribute to the better electrical performance of cell and the higher energy conversion efficiencies.Furthermore,some optimal designs on the structure of the energy converter are achieved.Finally,although the source(⁶³Ni)and the target materials?C and SiC?are used in this work,the simulated model and the calculated method can be extended to explore the other semiconductor materials and the beta sources in the p-n junction-based and the Schottky barrier diode-based betavoltaic batteries.