A Facile Strategy for Intrinsic Low-Dk and Low-Df Polyimides Enabled by Spirobifluorene Groups

Modified polyimides(MPIs)show great potential towards 5G communication applications,due to its excellent thermal stability,mechanical property and chemical stability as compared to most of polymers.Introducing fluoride groups or porous structure is favorable to ultra-low dielectric constant(D_(k))and dielectric loss(D_(f)).However,the cost of the fluorinated MPIs is high and their synthetic processes are complicated,and porous MPIs suffer poor mechanical properties.Also,increasing the fraction of free volume is a very effective way to lower D_(k)through introducing more ultra-low-D_(k)air component.However,most of this kind of MPIs lag far behind the fluorinated MPIs and the porous MPIs in terms of ultra-low D_(f),hindering the application of MPIs in high-speed communication devices.Thus,it is highly desirable to develop intrinsic ultra-low-D_(k)/D_(f)MPIs at high frequency with less fluoric groups and nonporous structure.Herein,we introduce a facile and effective strategy to lower D_(k)and D_(f)through introducing rigid and large sterically hindered aromatic groups into MPIs.On the one hand,their large steric hindrance effect leads to low D_(k)by increasing intrinsic free volume.On the other hand,the resulting highly stiff polymer chain and strong intermolecular interaction are favorable to reduce D_(f)by inhibiting dipole orientations.Based on this strategy,the spirobifluorene groups are preferred.The as-prepared MPIs show excellent dielectric performance with low D_(k)of 2.74–2.76 and low D_(f)of 0.00599 at 10 GHz,to some extent,exceeding the multiple fluorinated MPI with D_(k)/D_(f)of 2.67/0.00663 at 10 GHz.

Mechanism of vertical crack formation in Yb_(2)SiO_(5) coatings deposited via plasma spray-physical vapor deposition

Plasma spray-physical vapor deposition(i.e.,PS-PVD)is a promising method for obtaining advanced environmental barrier coatings(EBCs).The EBCs must meet some requirements in the application,in which the thermal cycle performance affects the service lifetime.The preparation of artificial vertical cracks in Yb_(2)SiO_(5) coatings is an effective approach for meeting the requirements above because vertical cracks provide a strain tolerance.To clarify the formation mechanism of vertical cracks during the PSPVD,the effects of coating thickness and substrate temperature on the formation of vertical cracks were investigated.In addition,the interactions of spray powder and plasma flame during coating deposition were also characterized by optical spectroscopy.It is indicated that vertical cracks are formed due to a thermal expansion mismatch between Yb_(2)SiO_(5) and mullite coating,transient cooling after deposition and the nucleation of evaporated Yb_(2)SiO_(5) as well.

Al-modification for PS-PVD 7YSZ TBCs to improve particle erosion and thermal cycle performances

Plasma spray-physical vapor deposition(PS-PVD)as a novel process was used to prepare feather-like columnar thermal barrier coatings(TBCs).This special microstructure shows good strain tolerance and non-line-of-sight(NLOS)deposition,giving great potential application in aero-engine.However,due to serious service environment of aero-engine,particle erosion performance is a weakness for PS-PVD 7YSZ TBCs.As a solution,an Al-modification approach was proposed in this investigation.Through in-situ reaction of Al and ZrO2,anα-Al2O3 overlay can be formed on the surface of 7YSZ columnar coating.The results demonstrate that this approach can improve particle erosion resistance since hardness improvement of Al-modified TBCs.Meanwhile,as another important performance of thermal cycle,it has a better optimization with 350-cycle water-quenching,compared with the as-sprayed TBCs.