30 results about "Memory effect" patented technology

The invention discloses a scanning point imaging encryption transmission method based on a GaN heterojunction device, and belongs to the technical field of semiconductor photoelectronics. According to the method, the same device is switched between a high-speed'detection mode 'and a'synaptic mode' with a memory effect, encryption is realized by utilizing response difference of the device in two working modes, and an encryption key of the device is an analog and time-varying bias protocol instead of a simple digital sequence. The characteristics of the physical unclonable function endow the physical unclonable function with extremely high encryption strength, so that the physical unclonable function is difficult to crack by a conventional algorithm and has higher security.

The invention relates to the field of force-electromagnetic coupling measurement, and discloses a low-voltage electric energy metering box internal arrangement method and system based on a modular mutual inductor plugging structure, and the method comprises the steps: synchronously collecting three-phase module magnetic flux density, base strain field and pre-tightening force time sequence data through deploying a multi-source sensor; a directed weighted hypergraph is constructed to represent a magnetoelastic coupling topology, the topology is converted into a time-varying creep rate matrix through a viscoelastic creep memory operator, a compensation strategy is generated in combination with phase space division and a magnetostriction response tree, and finally an optimization scheme of module spacing and interface rigidity is reversely mapped. By accurately capturing the time-varying mechanical law of the strain field and the pre-tightening force, the problem of time-varying coupling drift of the modular mutual inductor caused by structure-electromagnetic coupling and viscoelastic memory effect is solved, the limitation of traditional static independent calibration is broken through, and long-term stability of three-phase metering precision is achieved.

This invention relates to the field of force-electromagnetic coupling measurement, and discloses a method and system for the internal arrangement of a low-voltage power metering box based on a modular transformer plug-in structure. This method deploys multi-source sensors to synchronously collect time-series data on the magnetic flux density of the three-phase modules, the strain field of the base, and the preload. A directed weighted hypergraph is constructed to represent the magnetoelastic coupling topology, which is then converted into a time-varying creep rate matrix using a viscoelastic creep memory operator. Combined with phase space partitioning and a magnetostrictive response tree generation compensation strategy, the final result is an inverse mapping to an optimized scheme for module spacing and interface stiffness. This invention solves the problem of time-varying coupling drift caused by structure-electromagnetic coupling and viscoelastic memory effects in modular transformers by accurately capturing the time-varying mechanical laws of the strain field and preload, overcoming the limitations of traditional static independent calibration, and achieving long-term stability of three-phase metering accuracy.

It is an object to provide a semiconductor having a novel structure. In the semiconductor device, a plurality of memory elements are connected in series and each of the plurality of memory elements includes first to third transistors thus forming a memory circuit. A source or a drain of a first transistor which includes an oxide semiconductor layer is in electrical contact with a gate of one of a second and a third transistor. The extremely low off current of a first transistor containing the oxide semiconductor layer allows storing, for long periods of time, electrical charges in the gate electrode of one of the second and the third transistor, whereby a substantially permanent memory effect can be obtained. The second and the third transistors which do not contain an oxide semiconductor layer allow high-speed operations when using the memory circuit.

The utility model relates to a sealing assembly for sealing a liquid injection hole of a battery cell. The sealing assembly comprises a sealing piece and a memory metal piece, the sealing element is matched with the battery cell liquid injection hole to realize sealing; the memory metal part has a heating state and a cooling state in the application process; when the memory metal piece is in a heating state, one end of the memory metal piece can penetrate through the sealing piece and extend to the outside of the liquid injection hole, and the other end of the memory metal piece is used for clamping and limiting the sealing piece; and when the memory metal piece is in a cooling state, one end, extending to the outside of the liquid injection hole, of the memory metal piece is deformed so as to be attached to the inner surface of the battery cell cover plate. By utilizing the shape memory effect of the memory metal part and the shape change in two different states of heating and cooling, the liquid injection hole can be detachably sealed by the sealing assembly.

An amplification system can be operated by providing an input signal to an amplifying transistor, amplifying the input signal with the amplifying transistor to generate an amplified signal, and monitoring one or more of a supply current for the amplifying transistor, an injection voltage point of the amplifying transistor, forward power at an output of the amplifying transistor, and temperature of the amplifying transistor. Operation of the amplification system can further include generating a control signal based on monitored information for correcting either or both of a memory effect of the amplifying transistor and a current collapse effect of the amplifying transistor.

The invention discloses a FeMnSi series alloy with a remarkable room-temperature shape memory effect for 3D printing. The FeMnSi series alloy is prepared from the following elements in percentage by mass: 16% of Mn, 7.5%-8.2% of Cr, 3.5%-4.2% of Ni, 5.5%-5.7% of Si and the balance of Fe. The invention further discloses a preparation method of the FeMnSi series alloy. According to the FeMnSi alloy, the proportion with the characteristics of low Cr, low Ni and medium Si is designed, the martensite phase transformation point temperature is effectively increased, meanwhile, the internal stacking fault and dislocation defect density of the FeMnSi alloy is increased through 3D printing, phase transformation from an fcc phase to an hcp phase in the room-temperature deformation process and inverse phase transformation after heating are promoted, the remarkable shape memory effect and tensile strength are achieved at the room temperature, and the high-strength and high-toughness FeMnSi alloy is obtained. The method is suitable for preparing impact-resistant, load-bearing and functional devices which can be repeatedly used in a room-temperature environment.

The invention discloses a QPSO (Quadrature Particle Swarm Optimization) algorithm-based method for determining a two-way memory alloy component ratio, which belongs to the field of memory alloy ratios and is used for determining the memory alloy component ratio and accurately regulating and controlling the temperature sensitivity and elongation error of a material so as to enable a spring to keep stable deformation performance in repeated thermal cycles and prolong the service life of a device. The method mainly comprises the following steps: determining a multi-target fitness function according to a two-way shape memory effect alloy element; the optimized parameter is the proportion of each element, a four-dimensional solution space is formed, a random particle population is generated, the number of particles is N, and the maximum number of iterations is stipulated as S; evaluating a fitness value of each particle based on a fitness function; updating particle positions; and calculating the fitness value of each particle at the new position, and updating the individual optimal position Pb of the particle and the global optimal position Gb of the particle swarm. The method is mainly used for matching the components of the two-way memory alloy.

Current steering digital-to-analog converters (DACs) are described. These DACs are suitable for high speed operation. Weight transistors conventionally used in DACs are replaced with resistors, resulting in a lower RC constant. Further, the resistors cause a smaller voltage drop, thus improving the voltage headroom of the DAC. Additionally, the current steering switches are biased in the triode region, as opposed to the saturation region as in conventional designs. Biasing the switches in the triode region results in a smaller drain-source voltage, which further improves the voltage headroom of the DAC. The triode operation further results in a substantially smaller output impedance, which leads to the output voltage being dictated primarily by the output transistor of the current path. Lastly, reset switches are added which reduce data-dependent memory effects that can otherwise produce distortion.

The invention relates to the technical field of semiconductors, and provides a preparation method of a silicon carbide epitaxial wafer, which comprises the following steps: S1, placing a silicon carbide substrate in a reaction chamber, and depositing a first doping layer on at least one side of the substrate; s2, at the temperature of 1610 DEG C to 1660 DEG C and the pressure of 300 mbar to 1000 mbar, carrying out purging etching on the graphite piece in the reaction cavity by adopting hydrogen or chlorine-containing gas; s3, depositing a second doping layer on one side, deviating from the silicon carbide substrate, of the first doping layer; one of the first doped layer and the second doped layer is a P-type epitaxial layer, and the other one is an N-type epitaxial layer. According to the method, the N-type epitaxial layer and the P-type epitaxial layer can be alternately grown in the same chamber, the memory effect of nitrogen atoms or aluminum atoms is weakened, and the problem that interface doping of the N-type epitaxial layer and the P-type epitaxial layer is fuzzy is solved.

The invention relates to the technical field of piezoelectric actuators, and particularly discloses a large-strain memory piezoelectric stack actuator and a working method.The large-strain memory piezoelectric stack actuator is characterized in that piezoelectric ceramic pieces and electrode pieces are alternately stacked to form a piezoelectric stack, the piezoelectric stack is arranged on the outer wall of a sleeve and positioned by the sleeve, zirconium oxide pieces are placed at the two ends of the piezoelectric stack, and a pre-tightening device is arranged in the sleeve; the whole body is fixed with the cover plate through a pre-tightening device, and pre-tightening force is applied through a pre-tightening screw; pins on the same side of the electrode plates are connected in series to lead out wires for external electrical connection. The actuator is based on a piezoelectric ceramic material with a strain memory effect, adopts a central radial electrode plate with low radial rigidity, and applies a pre-tightening force to a piezoelectric stack through an internal pre-tightening device to form an integral structure, so that the radial constraint on the stacked actuator is reduced, and a power-off strain memory function is realized. The actuator has a power-off strain memory function, the memory strain capacity is 4-8 times that of piezoelectric materials such as PZT-5H, and the actuator is suitable for application scenes such as spacecraft mirror surface shape adjustment and high-precision displacement keeping.

The utility model discloses a low -voltage power utilization safety real scene model demonstration system belongs to electric power safety demonstration equipment technical field, including model, simulation unit, alarm device, escape path guiding system, and simulation unit includes photovoltaic board module, charging stake module, electric vehicle module, fault simulation unit, and fault simulation unit includes atomizer, LED lamp, and atomizer can produce smoke, and LED lamp can simulate dynamic flame, and simulation unit can simulate fault disaster, and escape path guiding system includes embedded LED lamp area. Through the utility model, set up simulation unit, and photovoltaic board module, charging stake module, electric vehicle module, fault simulation unit in simulation unit can simulate fault disaster, can transform the abstract concept such as electric energy transmission, fault arc and fire spread into visual, audible and tactile perceptible simulation phenomenon, can intuitively understand the cause of failure, transmission path and consequence, thereby significantly improve the cognitive depth and memory effect.

The invention discloses an electrochromic film laminated glass intelligent control system and a control method thereof, and the control system comprises electrochromic film laminated glass, and also comprises a sensing unit, a control unit and an interaction unit. The sensing unit is used for sensing external environment and personnel conditions; the interaction unit is used for receiving a control signal; the sensing unit is electrically connected with the control unit, the control unit is electrically connected with the electrochromic film laminated glass, and the interaction unit is electrically connected with the control unit. According to the invention, through a multi-sensor fusion decision, balance between comfort and energy saving is automatically found; the electrochromism technology is extremely low in power consumption and has a memory effect, and the refrigeration energy consumption can be effectively reduced by combining solar radiation control; automatic operation and manual wireless remote control are supported, and the user experience is better.

The utility model relates to the related technical field of electric beds, which comprises an electric bed, the upper end of the electric bed is provided with a spring for buffering and damping, the outer ring of the spring is sleeved with a wrapping layer for enhancing the supporting performance and the springback speed, and the upper end of the electric bed is provided with a supporting strip for supporting. Elastic frameworks used for dispersing pressure are fixedly installed at the two ends of each supporting strip in a penetrating mode, the nickel-titanium memory alloy springs absorb dynamic loads through elastic deformation of the nickel-titanium memory alloy springs, the shape memory effect of the nickel-titanium memory alloy springs can automatically rebound and reset after being loaded and compressed, and longitudinal permanent deformation is restrained; the high-support memory foam wrapping layer which is spirally and tightly wrapped outside synchronously limits radial expansion of the spring through physical constraint, uniformly disperses pressure and improves the rebound speed, is tightly attached to the outer surface of the high-strength engineering plastic supporting strip, and is matched with the three-dimensional hexagonal honeycomb elastic framework to form rigid support; the pressure is uniformly dispersed through the hollow truss reinforcing ribs and the honeycomb structure.

This invention discloses a modified photoresponse material for memory and a modular dual-reaction system. The modified photoresponse material for memory is (BiVO4). y / Ag z / Fe x In 2‑x S3, where x is 0.18–1, y is 0.66–1.5, and z is 0.15–0.45, exhibits a memory effect, high charge separation and transfer efficiency, and good photocatalytic activity. This modular dual-reaction integrated treatment system employs multi-layer, multi-stage reaction tanks and utilizes the modified memory photoresponse material described in this invention to treat naphthalene-containing industrial wastewater. The operation process is simple, efficient, and effectively saves costs. Its tail gas absorption device avoids secondary pollution, making it environmentally friendly. Using this modified memory photoresponse material and the matching modular dual-reaction integrated treatment system to treat naphthalene-containing industrial wastewater, the primary COD degradation rate is over 65%.

The invention discloses a Fe-Mn-Si series alloy with a remarkable shape memory effect at room temperature and low temperature for 3D printing. The Fe-Mn-Si series alloy is composed of the following elements of, by mass, 16% of Mn, 8.0%-8.5% of Cr, 5.8%-6.2% of Ni, 5.5%-5.7% of Si and the balance Fe. The invention further discloses a preparation method and application of the Fe-Mn-Si alloy. According to the Fe-Mn-Si alloy, the composition proportion of low Cr, high Ni and medium Si is designed, the martensite phase transformation point temperature is controlled between the room temperature and the low temperature, the alloy can have high stacking and dislocation density through 3D printing, phase transformation from an fcc phase to an hcp phase in the room-temperature or low-temperature deformation process and inverse phase transformation after heating are promoted, and therefore the high-strength and high-toughness Fe-Mn-Si alloy is obtained. The material has obvious shape memory effect and tensile strength at room temperature and low temperature, and is suitable for preparing devices in the aerospace field.

The invention discloses an electrolytic bath hot spot temperature online estimation method and system based on fractional calculus, and belongs to the technical field of energy equipment state monitoring. The invention aims to solve the technical problem that a temperature estimation model in the prior art is large in calculation amount or insufficient in precision. According to the technical scheme, the method is characterized by comprising the steps that a multi-input single-output observer model is established, and the model takes the total input power and the outer surface temperature of the electrolytic cell as input and takes an estimated value of the internal hot spot temperature as output; decoupling the model into a heat production fast dynamic channel and a heat transfer slow dynamic channel; and mathematical description is carried out on the one or two channels by adopting a fractional order transfer function so as to accurately describe the memory effect of the heat transfer process. The method further comprises the steps that model parameters are determined through system identification, and temperature estimation is conducted on line through the model. The method gives consideration to calculation efficiency and estimation precision, and can be used for guaranteeing safe and efficient operation of the electrolytic cell.

The invention relates to the technical field of new energy vehicles, and particularly discloses a new energy vehicle SOC estimation system and method based on a fractional order neural network, and the method depends on the fractional order calculus characteristics of the fractional order neural network, a fractional order operator has a memory effect, and can accurately capture the historical change correlation of voltage, temperature and voltage drop in the charging and discharging process of a battery. According to the method, the fitting precision of the dynamic attenuation process of the battery after long-term circulation can be improved, error accumulation caused by neglecting a historical state is avoided, meanwhile, the lagging response characteristic of the battery can be quickly responded, and the adaptability to time-varying nonlinearity is higher through dynamic adaptation of fractional order numbers; after the fractional order neural network is iterated by the sparrow search optimization algorithm, the weight, the threshold and the order parameter can be accurately matched with the battery characteristics, a dynamic voltage drop characteristic weight adjustment mechanism is matched, the average absolute percentage error of SOC prediction is reduced, prediction precision reduction caused by battery aging is avoided, and the SOC estimation precision is improved.

Provided are a silicon-carbon composite material and a preparation method therefor, a secondary battery, and an electric device. The silicon-carbon composite material includes a core and a coating coated on the surface of the core, the core includes a carbon matrix and a silicon-based material located on the surface of the carbon matrix; the silicon-based material includes a silicon cluster and a silicon carbide cluster, the silicon carbide cluster forms a network structure, and the silicon cluster is located in a gap of the network structure. The silicon-carbon composite material can eliminate the memory effect of silicon and improve the capacity retention rate.

The invention discloses a manufacturing method of electrochromic glass capable of prolonging a memory effect. The manufacturing method comprises the following steps: 1, cleaning a glass substrate; 2, depositing a layer of ITO (Indium Tin Oxide) film on the substrate as a bottom layer ITO film, wherein the thickness of the bottom layer ITO film is 80-120nm; 3, the electrochromic layer is made of tungsten trioxide as a base material, molybdenum and titanium elements are doped into WO3, the doping concentration is that the atomic percent of Mo is 1-5%, the atomic percent of Ti is 0.5-3%, and the thickness of the electrochromic layer is 300-600 nm; 4, annealing treatment is conducted in the inert atmosphere, the annealing temperature ranges from 300 DEG C to 450 DEG C, and the annealing time ranges from 1 h to 3 h; 5, generating an electrolyte layer on the electrochromic layer, wherein the electrolyte layer adopts lithium tantalate as a solid electrolyte layer; 6, the ion storage layer is deposited and attached to the electrolyte layer by taking nickel oxide as a base material, and the vanadium element is doped into NiO to realize doping modification; 7, depositing a layer of ITO film on the ion storage layer as a top layer ITO film, wherein the thickness of the top layer ITO film is 80-240 nm; and 8, packaging the device in a dry environment to obtain a finished product.

The invention discloses a cascading failure searching method based on a power transmission line temperature accumulation effect, and the method comprises the steps: building a cascading failure triggering criterion based on electrical betweenness, and carrying out the searching to obtain a cascading failure triggering branch set; constructing a line load rate calculation formula, a load loss rate calculation formula and a line overheating tripping model; and according to the line load rate calculation formula, constructing a load loss rate calculation formula and a line overheating tripping model, and obtaining a fault chain of each triggering fault branch in the cascading fault triggering branch set. According to the invention, the line tripping model proposed based on the temperature accumulation effect of the power transmission line accurately simulates the real thermal physical operation condition of the line, the memory effect in the iteration process can more accurately reflect the action mechanism of the thermal relay, and the thermal relay response depends on the historical accumulation of the temperature rather than the instantaneous value; the cascading failure process generated by the model has a strict time sequence and clear correlation between failures, clearly shows how the failures are propagated according to the thermophysical law, and is convenient for deeply analyzing the propagation path and key links of the failures.

This invention discloses a separator, its preparation method, and its application, relating to the field of battery separator technology. By first layering a stretched multilayer nanoporous membrane and then subjecting it to aging treatment, the membrane thickness is reduced and physical constraints are removed after layering, resulting in a smoother and more responsive release path for residual stress within the membrane. This effectively alleviates the localized high stress concentration phenomenon caused by the combined effects of thickness accumulation and memory effect. Furthermore, this invention utilizes a roll-up mechanism with adhesive strips. The spacing between the adhesive strips provides physical space for the membrane to freely retract during aging, further reducing the additional internal stress induced by hindered retraction. This makes the membrane deformation more uniform and reversible, ultimately significantly suppressing the occurrence of permanent appearance defects such as waviness and sagging edges.

The application discloses a method for rapidly reducing Hf memory effect in ICP-MS determination, which comprises the following steps: sequentially cleaning the sampling system with HF solution, HF and HNO3 mixed solution and HF and H2O2 mixed solution, and repeating the cleaning steps until the Hf element signal is reduced to the required value; the concentration of the HF solution is 2%-5%, the concentration of each component in the HF and HNO3 mixed solution is 2%-5%, the concentration of HF in the HF and H2O2 mixed solution is 2%-5%, and the concentration of H2O2 is 20%-40%. According to the adsorption capacity of Hf residues in the ICP-MS sampling system, different solutions are used for sequential washing, the cleaning process is simple and easy to monitor, Hf in the sampling system can be rapidly removed in a short time, the memory effect of Hf in ICP-MS determination is effectively reduced, and the accuracy of ICP-MS detection of Hf and elements such as Yb, Lu, Ta, W, Rb, Sr, Y, Zr and Ba interfered by Hf is improved.

The application provides a method for re-toughening metal glass, and relates to the technical field of mechanical property improvement of amorphous alloy, and comprises the following steps: performing current treatment on the relaxed state metal glass, heating the relaxed state metal glass to a first set temperature, and then performing quenching treatment; wherein the first set temperature is lower than the glass transition temperature Tg of the metal glass. It should be noted that the current treatment is performed on the relaxed state (aged state) metal glass to rapidly heat the metal glass to a higher temperature, at this time, the memory effect can be activated, the free volume content in the metal glass is increased to a certain extent, the energy state of the metal glass is improved, and then the increased free volume is retained through rapid cooling by quenching, so that the aging of the metal glass is realized.

The present invention relates to the technical field of lithium-ion batteries. Specifically, provided in the present invention are a polyolefin resin-containing microporous separator for a lithium battery, and a preparation method therefor. The microporous separator for a lithium battery in the present invention has good puncture strength, a long memory effect duration and a low compression deformation rate.

This invention discloses a scanning imaging encrypted transmission method based on GaN heterojunction devices, belonging to the field of semiconductor optoelectronics technology. The method achieves encryption by switching the same device between a high-speed "probe mode" and a "synaptic mode" with a memory effect, utilizing the response differences between the two operating modes. Because its encryption key is an analog and time-varying bias protocol rather than a simple digital sequence, this "physically unclonable function" characteristic endows it with extremely high encryption strength, making it difficult to crack by conventional algorithms and providing higher security.

The application discloses a magnesium-zinc co-doped AlGaInP red light semiconductor laser device and a preparation method thereof, and belongs to the optical electronic technical field. 0.5 In 0.5 P lower transition layer, Al 0.5 In 0.5 P lower confinement layer, (Al x1 Ga 1‑x1 ) y1 In 1‑y1 P lower waveguide layer, Ga x2 In 1‑ x2 P first quantum well, (Al x3 Ga 1‑x3 ) y2 In 1‑y2 P barrier layer, Ga x4 In 1‑x4 P second quantum well, (Al x5 Ga 1‑x5 ) y3 In 1‑y3 P upper waveguide layer, Al 0.5 In 0.5 P upper confinement layer-1, Al 0.5 In 0.5 P upper confinement layer-2, Ga x6 In 1‑x6 P etching stop layer, Al 0.5 In 0.5 P upper confinement layer-3, Ga 0.5 In 0.5 P upper transition layer and a GaAs cap layer. The waveguide layer is low-doped with Zn to reduce absorption loss, the confinement layer is co-doped with Mg and Zn to reduce the memory effect of Mg doping, the high-doping of the confinement layer is realized to reduce carrier overflow, waste heat generation is reduced, and high-temperature working reliability is improved.

The invention discloses a gate valve micro-channel leakage prevention and control device and a prevention and control method based on lattice Boltzmann. The gate valve micro-channel leakage prevention and control device comprises a plurality of sealing blocks and heat insulation blocks which are evenly distributed in a staggered mode in the circumferential direction of one inner side wall face of a micro-channel. Each sealing block comprises a piezoelectric ceramic block, an SMA, a temperature sensor and a heating assembly; the bottom surface of the piezoelectric ceramic block is arranged on the inner side wall surface of the micro-channel; the SMA is arranged on the top surface of the piezoelectric ceramic block; the temperature sensor is used for monitoring the temperature of the SMA in real time; the heating assembly can heat the SMA. The SMA and piezoelectric ceramic combined structure is adopted, a wear area is identified through piezoelectric signals, the SMA is heated through a PID temperature control strategy to trigger the shape memory effect, compensation displacement is generated to recover the sealing state in real time, wear monitoring and closed-loop compensation are achieved, and therefore self-repairing after leakage is caused by surface wear of the microgap of the gate valve sealing ring can be achieved, and the service life of the gate valve sealing ring is prolonged. The anti-leakage gate valve sealing structure has the advantages of being good in anti-leakage effect, high in control precision, high in response speed and the like.

The invention provides a sensing regulation and control integrated multifunctional memory metal stranded wire and a preparation method and application thereof, and belongs to the technical field of metal materials. The Ni-Ti-Zr alloy has a stable linear resistance-strain relationship, so that the strain state of the concrete can be judged based on the resistance change degree of the Ni-Ti-Zr alloy; the Fe-Mn-Si-Cr-Ni alloy has a stable shape memory effect and a stable phase change characteristic, when the Fe-Mn-Si-Cr-Ni alloy is stressed at the room temperature, the Fe-Mn-Si-Cr-Ni alloy can be converted into a martensite phase from an austenite phase, the process is reversible, when the alloy is heated to the temperature above the austenite initial temperature, the martensite phase can be converted into the austenite phase again, and the material is recovered to the initial shape; when the material is physically limited during shape recovery, recovery stress can be generated in the material, and the recovery stress can form pressure stress in the concrete structure, so that strain regulation and control and intelligent repair of the cracked concrete structure are realized.

Provided are a silicon-carbon composite material and a preparation method therefor, a secondary battery, and an electric device. The silicon-carbon composite material includes a core and a coating coated on the surface of the core, the core includes a carbon matrix and a silicon-based material located on the surface of the carbon matrix; the silicon-based material includes a silicon cluster and a silicon carbide cluster, the silicon carbide cluster forms a network structure, and the silicon cluster is located in a gap of the network structure. The silicon-carbon composite material can eliminate the memory effect of silicon and improve the capacity retention rate.